Setting up for Differential Gene Expression Analysis

Last updated: 2021-07-02

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Summary

On this page, we will prepare the data for the differential gene expression analysis. This involves:

Importing in the gene counts and sample metadata.
Obtaining gene annotations for the genes detected in this experiment.
Creating a DGEList object to store the gene counts, sample metadata, and gene annotations in a single object.
Dimension reduction to check how similar samples are, as well as whether there are any mislabeled samples.
Export out DGEList for use in the differential gene expression analysis.

Data Description

The dataset we will be using in this analysis comes from a recent study published in Cell.
The data has been uploaded onto the public repository GEO with the GSE171110 accession number, and I’ve copied the description of the data the authors gave there below:

The identification of COVID-19 patients with high-risk of severe disease is a challenge in routine care. We performed blood RNA-seq gene expression analyses in severe hospitalized patients compared to healthy donors. Supervised and unsupervised analyses revealed a high abundance of CD177, a specific neutrophil activation marker, contributing to the clustering of severe patients. Gene abundance correlated with high serum levels of CD177 in severe patients. These results highlight neutrophil activation as a hallmark of severe disease and CD177 assessment as a reliable prognostic marker for routine care.

The data was pre-processed by the authors of the Cell paper, by aligning .fastq files to the human hg18 transcriptome using Salmon. We will not be covering the pre-processing in this workshop due to time constraints, although I would highly encourage those interested to bookmark the Salmon documentation which goes through the commands needed to run Salmon on .fastq files yourself.

Load R Packages

The following code chunk contains the R packages that are required to run through the analysis on this page:

# Working with data:
library(dplyr)
library(magrittr)
library(readr)
library(tibble)
library(reshape2)

# Visualisation:
library(kableExtra)
library(ggplot2)
library(ggbiplot)
library(ggrepel)
library(grid)
# Set ggplot theme
theme_set(theme_bw())

# Other packages:
library(here)
library(export)

# Bioconductor packages:
library(AnnotationHub)
library(edgeR)
library(limma)
library(Glimma)

Import Data

The first step is to import the gene counts and sample metadata data into R.

Gene count matrix

The raw gene counts after RNA-seq data pre-processing were downloaded from GEO GSE171110 under the “Supplementary Files”. The GSE171110_Data_RNAseq_raw_counts_geo.txt.gz supplementary file was downloaded and uncompressed into the data directory, and the resulting tab-delimited text file (containing gene counts) is imported below.

counts <- readr::read_tsv(here("data", 
                               "GSE171110_Data_RNAseq_raw_counts_geo.txt"), 
                          skip = 2, 
                          col_names = TRUE) %>%
  as.data.frame %>%
  tibble::column_to_rownames("Code_Patient")

By previewing the counts object below, we can see that each row is a gene and each column is a sample. The counts object is a type of object in R called a data.frame which you can basically think of as being like a spreadsheet but in R.

# Preview first 5 rows and columns
counts[1:5,1:5]

          507-V    1189-V   1406-V   1918-V   1951-V
A1BG  133.14360 110.14223 94.68670 89.60004 85.81497
A1CF    9.00000   8.00000  1.00000 10.00020  3.00000
A2M    65.00001  31.72058 36.63567 19.22508 27.99998
A2ML1  29.28124  26.30470 20.57935 13.00000  5.00000
A2MP1 262.00034  83.00004 98.99997 55.99998 41.00000

The nrow and ncol functions can be used to check how many rows and columns are in a data.frame object:

nrow(counts)

[1] 30185

ncol(counts)

[1] 54

Questions

How many genes are in this dataset?
How many Healthy and Covid-19 samples are in this dataset?

Sample Metadata Table

We import the sample metadata from the table stored in the CSV file at data/sample_table.csv and store it in an object called samples:

samples <- readr::read_csv(here("data", "sample_table.csv")) %>%
  as.data.frame


── Column specification ────────────────────────────────────────────────────────
cols(
  patient_code = col_character(),
  age = col_double(),
  sex = col_character(),
  group = col_character(),
  geo_accession = col_character()
)

As we can see in the preview below, each row in the samples data.frame is a sample, and the columns represent different characteristics/information about the samples, including age, sex, and group (Healthy or Covid-19).

samples %>%
  kable %>%
  kable_styling()

patient_code	age	sex	group	geo_accession
507-V	53	M	Healthy	GSM5218990
1189-V	57	F	Healthy	GSM5218991
1406-V	61	M	Healthy	GSM5218992
1918-V	56	M	Healthy	GSM5218993
1951-V	57	F	Healthy	GSM5218994
1995-V	55	F	Healthy	GSM5218995
2064-V	55	M	Healthy	GSM5218996
3054-V	60	M	Healthy	GSM5218997
3136-V	51	F	Healthy	GSM5218998
3538-V	60	M	Healthy	GSM5218999
002-0001	51	M	Covid19	GSM5219000
002-0002	60	M	Covid19	GSM5219001
002-0003	53	F	Covid19	GSM5219002
002-0004	57	F	Covid19	GSM5219003
002-0011	58	F	Covid19	GSM5219004
002-0006	55	M	Covid19	GSM5219005
002-0031	53	M	Covid19	GSM5219006
001-0021	52	M	Covid19	GSM5219007
001-0033	60	M	Covid19	GSM5219008
001-0024	61	M	Covid19	GSM5219009
001-0040	58	M	Covid19	GSM5219010
001-0032	59	M	Covid19	GSM5219011
001-0029	61	M	Covid19	GSM5219012
002-0035	51	M	Covid19	GSM5219013
002-0037	55	M	Covid19	GSM5219014
001-0036	59	F	Covid19	GSM5219015
002-0040	55	F	Covid19	GSM5219016
002-0041	57	F	Covid19	GSM5219017
002-0046	60	M	Covid19	GSM5219018
002-0053	55	M	Covid19	GSM5219019
001-0068	56	M	Covid19	GSM5219020
002-0057	51	M	Covid19	GSM5219021
002-0058	57	M	Covid19	GSM5219022
002-0060	53	M	Covid19	GSM5219023
001-0094	60	M	Covid19	GSM5219024
002-0062	55	M	Covid19	GSM5219025
002-0061	57	M	Covid19	GSM5219026
002-0068	53	M	Covid19	GSM5219027
002-0067	55	M	Covid19	GSM5219028
002-0066	51	M	Covid19	GSM5219029
002-0064	52	M	Covid19	GSM5219030
002-0070	58	M	Covid19	GSM5219031
002-0072	57	M	Covid19	GSM5219032
001-0129	56	M	Covid19	GSM5219033
002-0078	55	M	Covid19	GSM5219034
002-0080	55	M	Covid19	GSM5219035
002-0079	59	M	Covid19	GSM5219036
002-0083	54	M	Covid19	GSM5219037
002-0084	59	M	Covid19	GSM5219038
001-0142	53	M	Covid19	GSM5219039
002-0088	61	M	Covid19	GSM5219040
002-0101	58	M	Covid19	GSM5219041
002-0104	58	M	Covid19	GSM5219042
051-0001	54	M	Covid19	GSM5219043

Readable sample names

You may have noticed when previewing the gene counts matrix stored in counts using head(counts) earlier that the sample names are not very descriptive (eg. 507-V, 1189-V etc.). These correspond to the patient_code column in the sample metadata table above.
Below, we will replace these patient_code names in the gene counts with readable sample names so that the rest of the analysis is easier to interpret, especially when visualising the data.
First, we create a new column in the samples metadata table, called sample, that will contain readable sample names (the group they are in followed by a number):

samples <- samples %>%
  dplyr::mutate(sample = paste0(group, "_", c(1:10, 1:44))) 

rownames(samples) <- samples$sample

# Preview the samples table with the added column:
samples %>%
  kable %>%
  kable_styling()

	patient_code	age	sex	group	geo_accession	sample
Healthy_1	507-V	53	M	Healthy	GSM5218990	Healthy_1
Healthy_2	1189-V	57	F	Healthy	GSM5218991	Healthy_2
Healthy_3	1406-V	61	M	Healthy	GSM5218992	Healthy_3
Healthy_4	1918-V	56	M	Healthy	GSM5218993	Healthy_4
Healthy_5	1951-V	57	F	Healthy	GSM5218994	Healthy_5
Healthy_6	1995-V	55	F	Healthy	GSM5218995	Healthy_6
Healthy_7	2064-V	55	M	Healthy	GSM5218996	Healthy_7
Healthy_8	3054-V	60	M	Healthy	GSM5218997	Healthy_8
Healthy_9	3136-V	51	F	Healthy	GSM5218998	Healthy_9
Healthy_10	3538-V	60	M	Healthy	GSM5218999	Healthy_10
Covid19_1	002-0001	51	M	Covid19	GSM5219000	Covid19_1
Covid19_2	002-0002	60	M	Covid19	GSM5219001	Covid19_2
Covid19_3	002-0003	53	F	Covid19	GSM5219002	Covid19_3
Covid19_4	002-0004	57	F	Covid19	GSM5219003	Covid19_4
Covid19_5	002-0011	58	F	Covid19	GSM5219004	Covid19_5
Covid19_6	002-0006	55	M	Covid19	GSM5219005	Covid19_6
Covid19_7	002-0031	53	M	Covid19	GSM5219006	Covid19_7
Covid19_8	001-0021	52	M	Covid19	GSM5219007	Covid19_8
Covid19_9	001-0033	60	M	Covid19	GSM5219008	Covid19_9
Covid19_10	001-0024	61	M	Covid19	GSM5219009	Covid19_10
Covid19_11	001-0040	58	M	Covid19	GSM5219010	Covid19_11
Covid19_12	001-0032	59	M	Covid19	GSM5219011	Covid19_12
Covid19_13	001-0029	61	M	Covid19	GSM5219012	Covid19_13
Covid19_14	002-0035	51	M	Covid19	GSM5219013	Covid19_14
Covid19_15	002-0037	55	M	Covid19	GSM5219014	Covid19_15
Covid19_16	001-0036	59	F	Covid19	GSM5219015	Covid19_16
Covid19_17	002-0040	55	F	Covid19	GSM5219016	Covid19_17
Covid19_18	002-0041	57	F	Covid19	GSM5219017	Covid19_18
Covid19_19	002-0046	60	M	Covid19	GSM5219018	Covid19_19
Covid19_20	002-0053	55	M	Covid19	GSM5219019	Covid19_20
Covid19_21	001-0068	56	M	Covid19	GSM5219020	Covid19_21
Covid19_22	002-0057	51	M	Covid19	GSM5219021	Covid19_22
Covid19_23	002-0058	57	M	Covid19	GSM5219022	Covid19_23
Covid19_24	002-0060	53	M	Covid19	GSM5219023	Covid19_24
Covid19_25	001-0094	60	M	Covid19	GSM5219024	Covid19_25
Covid19_26	002-0062	55	M	Covid19	GSM5219025	Covid19_26
Covid19_27	002-0061	57	M	Covid19	GSM5219026	Covid19_27
Covid19_28	002-0068	53	M	Covid19	GSM5219027	Covid19_28
Covid19_29	002-0067	55	M	Covid19	GSM5219028	Covid19_29
Covid19_30	002-0066	51	M	Covid19	GSM5219029	Covid19_30
Covid19_31	002-0064	52	M	Covid19	GSM5219030	Covid19_31
Covid19_32	002-0070	58	M	Covid19	GSM5219031	Covid19_32
Covid19_33	002-0072	57	M	Covid19	GSM5219032	Covid19_33
Covid19_34	001-0129	56	M	Covid19	GSM5219033	Covid19_34
Covid19_35	002-0078	55	M	Covid19	GSM5219034	Covid19_35
Covid19_36	002-0080	55	M	Covid19	GSM5219035	Covid19_36
Covid19_37	002-0079	59	M	Covid19	GSM5219036	Covid19_37
Covid19_38	002-0083	54	M	Covid19	GSM5219037	Covid19_38
Covid19_39	002-0084	59	M	Covid19	GSM5219038	Covid19_39
Covid19_40	001-0142	53	M	Covid19	GSM5219039	Covid19_40
Covid19_41	002-0088	61	M	Covid19	GSM5219040	Covid19_41
Covid19_42	002-0101	58	M	Covid19	GSM5219041	Covid19_42
Covid19_43	002-0104	58	M	Covid19	GSM5219042	Covid19_43
Covid19_44	051-0001	54	M	Covid19	GSM5219043	Covid19_44

We will then set the column names in counts (corresponding to sample names) to the new readable sample names:

# Reorder the columns in `counts` so they are in the same order as the rows in the samples table
counts <- counts[, samples$patient_code] 

# Set the colnames to the readable sample names
colnames(counts) <- samples$sample 

head(counts)

        Healthy_1 Healthy_2 Healthy_3 Healthy_4 Healthy_5 Healthy_6 Healthy_7
A1BG    133.14360 110.14223  94.68670  89.60004  85.81497  59.03335  77.18392
A1CF      9.00000   8.00000   1.00000  10.00020   3.00000   6.00000   4.00000
A2M      65.00001  31.72058  36.63567  19.22508  27.99998  40.60307  34.00003
A2ML1    29.28124  26.30470  20.57935  13.00000   5.00000  39.47160   8.13204
A2MP1   262.00034  83.00004  98.99997  55.99998  41.00000 124.99995  49.00002
A3GALT2  12.00000   1.00000   5.00000   7.00000   0.00000   1.00000   5.00000
        Healthy_8 Healthy_9 Healthy_10 Covid19_1 Covid19_2 Covid19_3 Covid19_4
A1BG     64.63351 118.31709   79.67834  83.26880 137.55230  68.16603  78.88510
A1CF      5.00000   6.00000    0.00000  16.00040   5.00324  12.00600   6.00000
A2M      31.56285  24.00001   28.00001  26.99995  38.00004  46.00000  21.99862
A2ML1    20.04058  31.02818   51.08296  57.80256  30.37114  33.68818  22.34460
A2MP1    56.99999  97.00004   45.00000   3.00000  56.00000  20.00000  22.00000
A3GALT2   0.00000   4.00000    0.00000   9.00000   5.00000  29.00000   0.00000
        Covid19_5 Covid19_6 Covid19_7 Covid19_8 Covid19_9 Covid19_10 Covid19_11
A1BG    114.23320  64.64760  53.95959 83.450900  74.57600   64.24903   92.80190
A1CF     11.00000   8.00000   6.00013  2.999998   6.00000    4.00000    5.00007
A2M      45.00001  67.00005  29.99999 25.999970  41.99999   39.00004   37.39638
A2ML1    45.79115  21.32390  31.66436 37.455300  42.54337   28.32695   29.74667
A2MP1    18.00004  22.99996  26.99999 44.000048  40.99998   25.00004  105.99995
A3GALT2  11.00000   1.00000   2.00000  2.000000   3.00000    5.00000    3.00000
        Covid19_12 Covid19_13 Covid19_14 Covid19_15 Covid19_16 Covid19_17
A1BG     59.465400   73.44665   47.97373   57.19031   64.57860   89.05430
A1CF      3.000027    6.00187    6.00000    1.00107   10.00000    2.00727
A2M      45.000016   30.00002   51.99996  129.00001   46.00005   84.00001
A2ML1    16.849970   20.70310   30.61882   25.00652   29.80826   69.83409
A2MP1    88.999980   56.00000   35.99997   12.00000   54.99995   14.00000
A3GALT2   0.000000    5.00000    5.00000    1.00000    7.00000    9.00000
        Covid19_18 Covid19_19 Covid19_20 Covid19_21 Covid19_22 Covid19_23
A1BG      48.00094  79.154480   71.01173   50.83940   85.36085   93.90937
A1CF      11.00000   8.003561   10.00780    3.00000    1.00000    5.00607
A2M       60.49292  29.999997   48.00000   38.99996   27.00002   53.00000
A2ML1     31.87888  33.128400   29.33900   21.56825   31.68098   13.08699
A2MP1     38.00002  40.999970   18.00001    7.00000    6.00000   33.00003
A3GALT2    2.00000   1.000000    6.00000   10.00000    4.00000    6.00000
        Covid19_24 Covid19_25 Covid19_26 Covid19_27 Covid19_28 Covid19_29
A1BG      73.99996   88.82890   75.26563   71.68710  124.19999   79.64403
A1CF       5.00323    1.00124    5.00000    7.00000    6.00007    5.00369
A2M       28.00001   99.99995   60.99995   47.99997   47.99996   73.00000
A2ML1     67.26220   18.74255   44.07126   73.17947   16.01840   56.59880
A2MP1     77.99996   33.00003   15.99998   12.99996   19.99999   14.00000
A3GALT2   16.00000    0.00000    7.00000    9.00000    3.00000   17.00000
        Covid19_30 Covid19_31 Covid19_32 Covid19_33 Covid19_34 Covid19_35
A1BG      60.10780   64.65766   61.99567   71.27866   63.87714   54.11832
A1CF       6.00000    7.00000   15.00040    3.00501    2.00048    6.00473
A2M       17.25811   20.00000   32.99999   27.99999   28.99995   16.99998
A2ML1     84.14297   29.39331   37.07064   19.60490   36.41428   33.22690
A2MP1     26.00000   15.00000   11.00001   41.00004   12.00000    8.00000
A3GALT2   71.00000    7.00000   17.00000   18.00000   11.00000   49.00000
        Covid19_36 Covid19_37 Covid19_38 Covid19_39 Covid19_40 Covid19_41
A1BG      61.43464   90.23180  116.96380  104.56376  134.22198   51.22791
A1CF      13.00020    2.00002   13.00490    9.00014   16.00240    6.00000
A2M       29.00002   57.00001   35.99996   57.00004   25.00001   62.99998
A2ML1     54.13332   65.73852   75.60080   51.43706   66.83647   30.94580
A2MP1     12.00000   29.99997   96.00001   29.99998   66.99996   20.99998
A3GALT2   14.00000   13.00000   40.00000   44.00000   44.00000   30.00000
        Covid19_42 Covid19_43 Covid19_44
A1BG     71.184132   79.81150   78.14628
A1CF      8.002895    5.00104    8.00000
A2M      21.000028   12.00000   19.99997
A2ML1    33.123070   38.82977   49.44860
A2MP1     6.000000   14.99997   21.99996
A3GALT2   5.000000   21.00000    6.00000

Gene annotations

Gene annotations are useful for later interpreting the results of the differential gene expression analysis. They give information about genes, including genomic location (e.g. start and end coordinates), descriptions, type of gene (e.g. whether it’s a non-coding or protein-coding gene) etc.
Below, we retrieve gene annotation metadata from the Ensembl database (this step can take a while to run if running AnnotationHub for the first time, so I have saved out the genes object as an R object).

# Below is not run in this RMarkdown document:

# Load annotation hub
ah <- AnnotationHub()

# Search for the correct Ensembl gene annotation
ah %>%
  subset(grepl("sapiens", species)) %>%
  subset(rdataclass == "EnsDb")

ensDb <- ah[["AH78783"]]

# Get gene annotations
genes <- genes(ensDb) %>%
  as.data.frame

# genes %>% saveRDS(here("data", "R", "genes.rds"))

# I load in a saved version below since the above code can take a while to run 
# if AnnotationHub objects are not already pre-downloaded. 
genes <- readRDS(here("data", "R", "genes.rds"))

The genes data.frame is prepared so that the row.names (corresponding to Ensembl gene IDs) are in the same order as for the gene count matrix stored in counts.

genes <- data.frame(gene = rownames(counts)) %>%
  left_join(genes %>% as.data.frame, 
            by = c("gene"="symbol")) %>%
  dplyr::distinct(gene, .keep_all=TRUE) 

rownames(genes) <- genes$gene

From previewing the gene annotations below, we can see that each row is a gene and each column contains information about the gene.

genes %>%
  head(50) %>%
  kable %>%
  kable_styling %>%
  scroll_box(height = "600px")

	gene	seqnames	start	end	width	gene_id	gene_name	gene_biotype	seq_coord_system	description	gene_id_version	entrezid
A1BG	A1BG	19	58345178	58353492	8315	ENSG00000121410	A1BG	protein_coding	chromosome	alpha-1-B glycoprotein [Source:HGNC Symbol;Acc:HGNC:5]	ENSG00000121410.12	1
A1CF	A1CF	10	50799409	50885675	86267	ENSG00000148584	A1CF	protein_coding	chromosome	APOBEC1 complementation factor [Source:HGNC Symbol;Acc:HGNC:24086]	ENSG00000148584.15	29974
A2M	A2M	12	9067664	9116229	48566	ENSG00000175899	A2M	protein_coding	chromosome	alpha-2-macroglobulin [Source:HGNC Symbol;Acc:HGNC:7]	ENSG00000175899.14	2
A2ML1	A2ML1	12	8822621	8887001	64381	ENSG00000166535	A2ML1	protein_coding	chromosome	alpha-2-macroglobulin like 1 [Source:HGNC Symbol;Acc:HGNC:23336]	ENSG00000166535.20	144568
A2MP1	A2MP1	12	9228533	9275817	47285	ENSG00000256069	A2MP1	transcribed_unprocessed_pseudogene	chromosome	alpha-2-macroglobulin pseudogene 1 [Source:HGNC Symbol;Acc:HGNC:8]	ENSG00000256069.7	NA
A3GALT2	A3GALT2	1	33306766	33321098	14333	ENSG00000184389	A3GALT2	protein_coding	chromosome	alpha 1,3-galactosyltransferase 2 [Source:HGNC Symbol;Acc:HGNC:30005]	ENSG00000184389.9	127550
A4GALT	A4GALT	22	42692121	42721298	29178	ENSG00000128274	A4GALT	protein_coding	chromosome	alpha 1,4-galactosyltransferase (P blood group) [Source:HGNC Symbol;Acc:HGNC:18149]	ENSG00000128274.17	53947
A4GNT	A4GNT	3	138123713	138132390	8678	ENSG00000118017	A4GNT	protein_coding	chromosome	alpha-1,4-N-acetylglucosaminyltransferase [Source:HGNC Symbol;Acc:HGNC:17968]	ENSG00000118017.4	51146
AAAS	AAAS	12	53307456	53324864	17409	ENSG00000094914	AAAS	protein_coding	chromosome	aladin WD repeat nucleoporin [Source:HGNC Symbol;Acc:HGNC:13666]	ENSG00000094914.14	8086
AACS	AACS	12	125065434	125143333	77900	ENSG00000081760	AACS	protein_coding	chromosome	acetoacetyl-CoA synthetase [Source:HGNC Symbol;Acc:HGNC:21298]	ENSG00000081760.17	65985
AACSP1	AACSP1	5	178764861	178818435	53575	ENSG00000250420	AACSP1	transcribed_unprocessed_pseudogene	chromosome	acetoacetyl-CoA synthetase pseudogene 1 [Source:HGNC Symbol;Acc:HGNC:18226]	ENSG00000250420.8	NA
AADAC	AADAC	3	151814073	151828488	14416	ENSG00000114771	AADAC	protein_coding	chromosome	arylacetamide deacetylase [Source:HGNC Symbol;Acc:HGNC:17]	ENSG00000114771.14	13
AADACL2	AADACL2	3	151733916	151761339	27424	ENSG00000197953	AADACL2	protein_coding	chromosome	arylacetamide deacetylase like 2 [Source:HGNC Symbol;Acc:HGNC:24427]	ENSG00000197953.6	344752
AADACL3	AADACL3	1	12716110	12728760	12651	ENSG00000188984	AADACL3	protein_coding	chromosome	arylacetamide deacetylase like 3 [Source:HGNC Symbol;Acc:HGNC:32037]	ENSG00000188984.12	126767
AADACL4	AADACL4	1	12644547	12667086	22540	ENSG00000204518	AADACL4	protein_coding	chromosome	arylacetamide deacetylase like 4 [Source:HGNC Symbol;Acc:HGNC:32038]	ENSG00000204518.2	343066
AADACP1	AADACP1	3	151770428	151784894	14467	ENSG00000240602	AADACP1	transcribed_unprocessed_pseudogene	chromosome	arylacetamide deacetylase pseudogene 1 [Source:HGNC Symbol;Acc:HGNC:50305]	ENSG00000240602.7	NA
AADAT	AADAT	4	170060222	170091699	31478	ENSG00000109576	AADAT	protein_coding	chromosome	aminoadipate aminotransferase [Source:HGNC Symbol;Acc:HGNC:17929]	ENSG00000109576.14	51166
AAGAB	AAGAB	15	67200667	67255195	54529	ENSG00000103591	AAGAB	protein_coding	chromosome	alpha and gamma adaptin binding protein [Source:HGNC Symbol;Acc:HGNC:25662]	ENSG00000103591.13	79719
AAK1	AAK1	2	69457997	69674349	216353	ENSG00000115977	AAK1	protein_coding	chromosome	AP2 associated kinase 1 [Source:HGNC Symbol;Acc:HGNC:19679]	ENSG00000115977.19	22848
AAMDC	AAMDC	11	77821109	77918432	97324	ENSG00000087884	AAMDC	protein_coding	chromosome	adipogenesis associated Mth938 domain containing [Source:HGNC Symbol;Acc:HGNC:30205]	ENSG00000087884.14	28971
AAMP	AAMP	2	218264123	218270257	6135	ENSG00000127837	AAMP	protein_coding	chromosome	angio associated migratory cell protein [Source:HGNC Symbol;Acc:HGNC:18]	ENSG00000127837.9	14
AANAT	AANAT	17	76453351	76470117	16767	ENSG00000129673	AANAT	protein_coding	chromosome	aralkylamine N-acetyltransferase [Source:HGNC Symbol;Acc:HGNC:19]	ENSG00000129673.9	15
AAR2	AAR2	20	36236459	36270918	34460	ENSG00000131043	AAR2	protein_coding	chromosome	AAR2 splicing factor [Source:HGNC Symbol;Acc:HGNC:15886]	ENSG00000131043.12	25980
AARD	AARD	8	116938207	116944487	6281	ENSG00000205002	AARD	protein_coding	chromosome	alanine and arginine rich domain containing protein [Source:HGNC Symbol;Acc:HGNC:33842]	ENSG00000205002.4	441376
AARS1	AARS1	16	70252295	70289543	37249	ENSG00000090861	AARS1	protein_coding	chromosome	alanyl-tRNA synthetase 1 [Source:HGNC Symbol;Acc:HGNC:20]	ENSG00000090861.15	16
AARS2	AARS2	6	44298731	44313347	14617	ENSG00000124608	AARS2	protein_coding	chromosome	alanyl-tRNA synthetase 2, mitochondrial [Source:HGNC Symbol;Acc:HGNC:21022]	ENSG00000124608.5	57505
AARSD1	AARSD1	17	42950526	42964498	13973	ENSG00000266967	AARSD1	protein_coding	chromosome	alanyl-tRNA synthetase domain containing 1 [Source:HGNC Symbol;Acc:HGNC:28417]	ENSG00000266967.7	80755
AASDH	AASDH	4	56338287	56387508	49222	ENSG00000157426	AASDH	protein_coding	chromosome	aminoadipate-semialdehyde dehydrogenase [Source:HGNC Symbol;Acc:HGNC:23993]	ENSG00000157426.14	132949
AASDHPPT	AASDHPPT	11	106075501	106098699	23199	ENSG00000149313	AASDHPPT	protein_coding	chromosome	aminoadipate-semialdehyde dehydrogenase-phosphopantetheinyl transferase [Source:HGNC Symbol;Acc:HGNC:14235]	ENSG00000149313.11	60496
AASS	AASS	7	122073549	122144255	70707	ENSG00000008311	AASS	protein_coding	chromosome	aminoadipate-semialdehyde synthase [Source:HGNC Symbol;Acc:HGNC:17366]	ENSG00000008311.15	10157
AATF	AATF	17	36948954	37056871	107918	ENSG00000275700	AATF	protein_coding	chromosome	apoptosis antagonizing transcription factor [Source:HGNC Symbol;Acc:HGNC:19235]	ENSG00000275700.5	26574
AATK	AATK	17	81110487	81166221	55735	ENSG00000181409	AATK	protein_coding	chromosome	apoptosis associated tyrosine kinase [Source:HGNC Symbol;Acc:HGNC:21]	ENSG00000181409.14	9625
ABAT	ABAT	16	8674596	8784575	109980	ENSG00000183044	ABAT	protein_coding	chromosome	4-aminobutyrate aminotransferase [Source:HGNC Symbol;Acc:HGNC:23]	ENSG00000183044.12	18
ABCA1	ABCA1	9	104781006	104928155	147150	ENSG00000165029	ABCA1	protein_coding	chromosome	ATP binding cassette subfamily A member 1 [Source:HGNC Symbol;Acc:HGNC:29]	ENSG00000165029.16	19
ABCA10	ABCA10	17	69147214	69244846	97633	ENSG00000154263	ABCA10	protein_coding	chromosome	ATP binding cassette subfamily A member 10 [Source:HGNC Symbol;Acc:HGNC:30]	ENSG00000154263.17	10349
ABCA11P	ABCA11P	4	425435	474129	48695	ENSG00000251595	ABCA11P	transcribed_processed_pseudogene	chromosome	ATP binding cassette subfamily A member 11, pseudogene [Source:HGNC Symbol;Acc:HGNC:31]	ENSG00000251595.7	NA
ABCA12	ABCA12	2	214931542	215138626	207085	ENSG00000144452	ABCA12	protein_coding	chromosome	ATP binding cassette subfamily A member 12 [Source:HGNC Symbol;Acc:HGNC:14637]	ENSG00000144452.15	26154
ABCA13	ABCA13	7	48171458	48647497	476040	ENSG00000179869	ABCA13	protein_coding	chromosome	ATP binding cassette subfamily A member 13 [Source:HGNC Symbol;Acc:HGNC:14638]	ENSG00000179869.15	154664
ABCA17P	ABCA17P	16	2339150	2426699	87550	ENSG00000238098	ABCA17P	transcribed_unitary_pseudogene	chromosome	ATP binding cassette subfamily A member 17, pseudogene [Source:HGNC Symbol;Acc:HGNC:32972]	ENSG00000238098.9	NA
ABCA2	ABCA2	9	137007234	137028922	21689	ENSG00000107331	ABCA2	protein_coding	chromosome	ATP binding cassette subfamily A member 2 [Source:HGNC Symbol;Acc:HGNC:32]	ENSG00000107331.17	20
ABCA3	ABCA3	16	2275881	2340746	64866	ENSG00000167972	ABCA3	protein_coding	chromosome	ATP binding cassette subfamily A member 3 [Source:HGNC Symbol;Acc:HGNC:33]	ENSG00000167972.14	21
ABCA4	ABCA4	1	93992834	94121148	128315	ENSG00000198691	ABCA4	protein_coding	chromosome	ATP binding cassette subfamily A member 4 [Source:HGNC Symbol;Acc:HGNC:34]	ENSG00000198691.13	24
ABCA5	ABCA5	17	69244311	69327244	82934	ENSG00000154265	ABCA5	protein_coding	chromosome	ATP binding cassette subfamily A member 5 [Source:HGNC Symbol;Acc:HGNC:35]	ENSG00000154265.16	23461
ABCA6	ABCA6	17	69078702	69141895	63194	ENSG00000154262	ABCA6	protein_coding	chromosome	ATP binding cassette subfamily A member 6 [Source:HGNC Symbol;Acc:HGNC:36]	ENSG00000154262.13	23460
ABCA7	ABCA7	19	1039997	1065572	25576	ENSG00000064687	ABCA7	protein_coding	chromosome	ATP binding cassette subfamily A member 7 [Source:HGNC Symbol;Acc:HGNC:37]	ENSG00000064687.13	10347
ABCA8	ABCA8	17	68867289	68955392	88104	ENSG00000141338	ABCA8	protein_coding	chromosome	ATP binding cassette subfamily A member 8 [Source:HGNC Symbol;Acc:HGNC:38]	ENSG00000141338.14	10351
ABCA9	ABCA9	17	68974488	69060949	86462	ENSG00000154258	ABCA9	protein_coding	chromosome	ATP binding cassette subfamily A member 9 [Source:HGNC Symbol;Acc:HGNC:39]	ENSG00000154258.17	10350
ABCB1	ABCB1	7	87503017	87713323	210307	ENSG00000085563	ABCB1	protein_coding	chromosome	ATP binding cassette subfamily B member 1 [Source:HGNC Symbol;Acc:HGNC:40]	ENSG00000085563.15	5243
ABCB10	ABCB10	1	229516582	229558707	42126	ENSG00000135776	ABCB10	protein_coding	chromosome	ATP binding cassette subfamily B member 10 [Source:HGNC Symbol;Acc:HGNC:41]	ENSG00000135776.5	23456
ABCB10P3	ABCB10P3	15	28402868	28405033	2166	ENSG00000261524	ABCB10P3	processed_pseudogene	chromosome	ABCB10 pseudogene 3 [Source:HGNC Symbol;Acc:HGNC:31129]	ENSG00000261524.1	NA

The columns of information about the genes include:

seqnames: Chromosome the gene is located on
start: Start coordinate
end: End coordinate
width: Length of gene (bp)
strand Strand the gene is located on
gene_id: Ensembl gene identifier
gene_name: Gene symbol
gene_biotype: Type of gene
seq_coord_system: Whether the gene is located on a chromosome or scaffold
description: Short description of the gene / what it encodes
gene_id_version: Version of the gene model in Ensembl
entrezid: Entrez gene identifier

This information can help to put the differential gene expression analysis results in context later. For example, in some analyses, people may wish to focus on looking at particular gene_biotypes (e.g. protein-coding genes).

By using the table function, we can count the number of genes belonging to each gene_biotype in this particular dataset.

genes$gene_biotype %>%
  table

.
                         IG_C_gene                    IG_C_pseudogene 
                                14                                  9 
                         IG_D_gene                          IG_J_gene 
                                 8                                 18 
                   IG_J_pseudogene                          IG_V_gene 
                                 3                                153 
                   IG_V_pseudogene                             lncRNA 
                               165                                149 
                          LRG_gene                              miRNA 
                               143                                  1 
            polymorphic_pseudogene               processed_pseudogene 
                                35                               7259 
                    protein_coding                         pseudogene 
                             18761                                 21 
                               TEC                          TR_C_gene 
                                17                                  6 
                         TR_J_gene                    TR_J_pseudogene 
                                79                                  4 
                         TR_V_gene                    TR_V_pseudogene 
                               111                                 30 
  transcribed_processed_pseudogene     transcribed_unitary_pseudogene 
                               433                                 83 
transcribed_unprocessed_pseudogene    translated_processed_pseudogene 
                               729                                  1 
 translated_unprocessed_pseudogene                 unitary_pseudogene 
                                 1                                 50 
            unprocessed_pseudogene 
                              1639

Questions

What percentage of genes in this dataset are protein_coding?

Create DGEList object

We will store the gene counts, sample information, and gene information in a DGEList (Digital Gene Expression) object called dge, which simplifies plotting and interacting with this information. It will also be used for the differential gene expression analysis later on.
The DGEList object is prepared as follows. Here, we remove all genes that have 0 counts across all samples, and apply Trimmed Mean of M-values (TMM) normalisation to normalise counts according to library size differences between samples.

dge <- DGEList(counts = counts, 
               genes = genes,
               samples = samples,
               remove.zeros = TRUE) %>%  
  calcNormFactors("TMM")

Removing 883 rows with all zero counts

# Preview the DGEList object
dge

An object of class "DGEList"
$counts
      Healthy_1 Healthy_2 Healthy_3 Healthy_4 Healthy_5 Healthy_6 Healthy_7
A1BG  133.14360 110.14223  94.68670  89.60004  85.81497  59.03335  77.18392
A1CF    9.00000   8.00000   1.00000  10.00020   3.00000   6.00000   4.00000
A2M    65.00001  31.72058  36.63567  19.22508  27.99998  40.60307  34.00003
A2ML1  29.28124  26.30470  20.57935  13.00000   5.00000  39.47160   8.13204
A2MP1 262.00034  83.00004  98.99997  55.99998  41.00000 124.99995  49.00002
      Healthy_8 Healthy_9 Healthy_10 Covid19_1 Covid19_2 Covid19_3 Covid19_4
A1BG   64.63351 118.31709   79.67834  83.26880 137.55230  68.16603  78.88510
A1CF    5.00000   6.00000    0.00000  16.00040   5.00324  12.00600   6.00000
A2M    31.56285  24.00001   28.00001  26.99995  38.00004  46.00000  21.99862
A2ML1  20.04058  31.02818   51.08296  57.80256  30.37114  33.68818  22.34460
A2MP1  56.99999  97.00004   45.00000   3.00000  56.00000  20.00000  22.00000
      Covid19_5 Covid19_6 Covid19_7 Covid19_8 Covid19_9 Covid19_10 Covid19_11
A1BG  114.23320  64.64760  53.95959 83.450900  74.57600   64.24903   92.80190
A1CF   11.00000   8.00000   6.00013  2.999998   6.00000    4.00000    5.00007
A2M    45.00001  67.00005  29.99999 25.999970  41.99999   39.00004   37.39638
A2ML1  45.79115  21.32390  31.66436 37.455300  42.54337   28.32695   29.74667
A2MP1  18.00004  22.99996  26.99999 44.000048  40.99998   25.00004  105.99995
      Covid19_12 Covid19_13 Covid19_14 Covid19_15 Covid19_16 Covid19_17
A1BG   59.465400   73.44665   47.97373   57.19031   64.57860   89.05430
A1CF    3.000027    6.00187    6.00000    1.00107   10.00000    2.00727
A2M    45.000016   30.00002   51.99996  129.00001   46.00005   84.00001
A2ML1  16.849970   20.70310   30.61882   25.00652   29.80826   69.83409
A2MP1  88.999980   56.00000   35.99997   12.00000   54.99995   14.00000
      Covid19_18 Covid19_19 Covid19_20 Covid19_21 Covid19_22 Covid19_23
A1BG    48.00094  79.154480   71.01173   50.83940   85.36085   93.90937
A1CF    11.00000   8.003561   10.00780    3.00000    1.00000    5.00607
A2M     60.49292  29.999997   48.00000   38.99996   27.00002   53.00000
A2ML1   31.87888  33.128400   29.33900   21.56825   31.68098   13.08699
A2MP1   38.00002  40.999970   18.00001    7.00000    6.00000   33.00003
      Covid19_24 Covid19_25 Covid19_26 Covid19_27 Covid19_28 Covid19_29
A1BG    73.99996   88.82890   75.26563   71.68710  124.19999   79.64403
A1CF     5.00323    1.00124    5.00000    7.00000    6.00007    5.00369
A2M     28.00001   99.99995   60.99995   47.99997   47.99996   73.00000
A2ML1   67.26220   18.74255   44.07126   73.17947   16.01840   56.59880
A2MP1   77.99996   33.00003   15.99998   12.99996   19.99999   14.00000
      Covid19_30 Covid19_31 Covid19_32 Covid19_33 Covid19_34 Covid19_35
A1BG    60.10780   64.65766   61.99567   71.27866   63.87714   54.11832
A1CF     6.00000    7.00000   15.00040    3.00501    2.00048    6.00473
A2M     17.25811   20.00000   32.99999   27.99999   28.99995   16.99998
A2ML1   84.14297   29.39331   37.07064   19.60490   36.41428   33.22690
A2MP1   26.00000   15.00000   11.00001   41.00004   12.00000    8.00000
      Covid19_36 Covid19_37 Covid19_38 Covid19_39 Covid19_40 Covid19_41
A1BG    61.43464   90.23180  116.96380  104.56376  134.22198   51.22791
A1CF    13.00020    2.00002   13.00490    9.00014   16.00240    6.00000
A2M     29.00002   57.00001   35.99996   57.00004   25.00001   62.99998
A2ML1   54.13332   65.73852   75.60080   51.43706   66.83647   30.94580
A2MP1   12.00000   29.99997   96.00001   29.99998   66.99996   20.99998
      Covid19_42 Covid19_43 Covid19_44
A1BG   71.184132   79.81150   78.14628
A1CF    8.002895    5.00104    8.00000
A2M    21.000028   12.00000   19.99997
A2ML1  33.123070   38.82977   49.44860
A2MP1   6.000000   14.99997   21.99996
29297 more rows ...

$samples
            group lib.size norm.factors patient_code age sex geo_accession
Healthy_1 Healthy 40594184     1.139493        507-V  53   M    GSM5218990
Healthy_2 Healthy 36139025     1.127571       1189-V  57   F    GSM5218991
Healthy_3 Healthy 29466261     1.180684       1406-V  61   M    GSM5218992
Healthy_4 Healthy 33691447     1.080104       1918-V  56   M    GSM5218993
Healthy_5 Healthy 33097717     1.012251       1951-V  57   F    GSM5218994
             sample
Healthy_1 Healthy_1
Healthy_2 Healthy_2
Healthy_3 Healthy_3
Healthy_4 Healthy_4
Healthy_5 Healthy_5
49 more rows ...

$genes
       gene seqnames    start      end width strand         gene_id gene_name
A1BG   A1BG       19 58345178 58353492  8315      - ENSG00000121410      A1BG
A1CF   A1CF       10 50799409 50885675 86267      - ENSG00000148584      A1CF
A2M     A2M       12  9067664  9116229 48566      - ENSG00000175899       A2M
A2ML1 A2ML1       12  8822621  8887001 64381      + ENSG00000166535     A2ML1
A2MP1 A2MP1       12  9228533  9275817 47285      - ENSG00000256069     A2MP1
                            gene_biotype seq_coord_system
A1BG                      protein_coding       chromosome
A1CF                      protein_coding       chromosome
A2M                       protein_coding       chromosome
A2ML1                     protein_coding       chromosome
A2MP1 transcribed_unprocessed_pseudogene       chromosome
                                                             description
A1BG              alpha-1-B glycoprotein [Source:HGNC Symbol;Acc:HGNC:5]
A1CF  APOBEC1 complementation factor [Source:HGNC Symbol;Acc:HGNC:24086]
A2M                alpha-2-macroglobulin [Source:HGNC Symbol;Acc:HGNC:7]
A2ML1   alpha-2-macroglobulin like 1 [Source:HGNC Symbol;Acc:HGNC:23336]
A2MP1 alpha-2-macroglobulin pseudogene 1 [Source:HGNC Symbol;Acc:HGNC:8]
         gene_id_version entrezid
A1BG  ENSG00000121410.12        1
A1CF  ENSG00000148584.15    29974
A2M   ENSG00000175899.14        2
A2ML1 ENSG00000166535.20   144568
A2MP1  ENSG00000256069.7       NA
29297 more rows ...

The counts, genes or samples can be accessed from the dge object using the $ operator. This is exactly the same information as that stored in the original counts, samples and genes objects, but putting them together in the dge object makes it easier to do certain plots/analysis later.

# Preview first 5 rows and columns of the counts stored in `dge`:
dge$counts[1:5, 1:5]

      Healthy_1 Healthy_2 Healthy_3 Healthy_4 Healthy_5
A1BG  133.14360 110.14223  94.68670  89.60004  85.81497
A1CF    9.00000   8.00000   1.00000  10.00020   3.00000
A2M    65.00001  31.72058  36.63567  19.22508  27.99998
A2ML1  29.28124  26.30470  20.57935  13.00000   5.00000
A2MP1 262.00034  83.00004  98.99997  55.99998  41.00000

# Preview first 5 rows and columns of the samples stored in `dge`:
dge$samples[1:5, 1:5]

            group lib.size norm.factors patient_code age
Healthy_1 Healthy 40594184     1.139493        507-V  53
Healthy_2 Healthy 36139025     1.127571       1189-V  57
Healthy_3 Healthy 29466261     1.180684       1406-V  61
Healthy_4 Healthy 33691447     1.080104       1918-V  56
Healthy_5 Healthy 33097717     1.012251       1951-V  57

# Preview first 5 rows and columns of the genes stored in `dge`:
dge$genes[1:5, 1:5]

       gene seqnames    start      end width
A1BG   A1BG       19 58345178 58353492  8315
A1CF   A1CF       10 50799409 50885675 86267
A2M     A2M       12  9067664  9116229 48566
A2ML1 A2ML1       12  8822621  8887001 64381
A2MP1 A2MP1       12  9228533  9275817 47285

Quality Checks

In this section we will do some basic quality checks to see if any samples are outliers or potentially mislabeled.

Library Sizes of samples

Overall, the library sizes of samples are shown. Sometimes if there is an outlier sample with an abnormally large or small library size, it is worth keeping an eye on in case it is a failed sample (e.g. library prep went wrong). We can see that certain samples have a larger library size below, but overall they are quite comparable.

dge$samples %>%
  ggplot(aes(x = sample, y = lib.size, fill = group)) +
  geom_bar(stat = "identity") +
  labs(y = "Library size (total number of mapped and quantified reads)",
       x = "Sample", fill = "Group") +
  coord_flip()

Version	Author	Date
f62ae4f	Nhi Hin	2021-06-30

Dimension Reduction

Multi Dimensional Scaling (MDS)

Multi Dimensional Scaling (MDS) is a dimension reduction technique that summarises expression of the top 500 most variable genes expressed in each sample down to 2 dimensions, known as Dimension 1 and Dimension 2. Dimension 1 represents the largest amount of variance that can be explained in the data. Plotting these principal components can help us visualise which samples show overall similar gene expression to each other – similar samples will be situated closer to each other on the plot.
Because our information is stored in a DGEList type of object, we can make use of the glMDSPlot function from the Glimma package to make an interactive MDS plot. The MDS plot produced using the function below can be viewed by navigating to the glimma-plots directory and clicking on the MDS-Plot.html file, or by clicking here.

glMDSPlot(dge, 
          labels = dge$samples$sample,
          groups = dge$samples$group)

Principal Component Analysis (PCA)

Principal component analysis (PCA) is similar to MDS, but it uses all genes (29302 genes for this particular dataset) to summarise down into a few principal components. Plotting these in the same way as for an MDS plot can also give us information about which samples are similar to each other. In most cases, the MDS and PCA plots will look very similar.
Below, the PCA plot created using the ggbiplot function shows that overall, samples belonging to the same group tend to have similar gene expression patterns, and there is quite a large difference between Healthy and Covid-19 samples. There do not appear to be any obvious outlier samples, although there is quite a bit of variation in gene expression amongst Covid-19 samples.
We will not remove any samples in this particular dataset as there is no indication so far that any of the samples are obvious outliers.

# Perform PCA analysis:
pca_analysis <- prcomp(t(cpm(dge, log=TRUE)))
summary(pca_analysis)

Importance of components:
                           PC1      PC2     PC3     PC4      PC5      PC6
Standard deviation     54.2313 33.64178 30.5264 24.6467 22.04485 21.31052
Proportion of Variance  0.2213  0.08514  0.0701  0.0457  0.03656  0.03416
Cumulative Proportion   0.2213  0.30639  0.3765  0.4222  0.45875  0.49292
                           PC7      PC8      PC9     PC10     PC11     PC12
Standard deviation     18.1218 16.98897 16.60252 15.78163 15.01215 14.43417
Proportion of Variance  0.0247  0.02171  0.02074  0.01874  0.01695  0.01567
Cumulative Proportion   0.5176  0.53933  0.56007  0.57881  0.59576  0.61143
                           PC13     PC14     PC15     PC16     PC17     PC18
Standard deviation     14.21691 13.67726 13.32318 13.17782 12.95002 12.75765
Proportion of Variance  0.01521  0.01407  0.01335  0.01306  0.01262  0.01224
Cumulative Proportion   0.62664  0.64071  0.65407  0.66713  0.67975  0.69199
                           PC19    PC20     PC21     PC22     PC23    PC24
Standard deviation     12.58172 12.3644 12.16441 12.14183 12.05517 11.9261
Proportion of Variance  0.01191  0.0115  0.01113  0.01109  0.01093  0.0107
Cumulative Proportion   0.70390  0.7154  0.72653  0.73762  0.74855  0.7592
                           PC25     PC26     PC27     PC28     PC29     PC30
Standard deviation     11.65802 11.62907 11.57784 11.35174 11.28734 11.22792
Proportion of Variance  0.01022  0.01017  0.01008  0.00969  0.00958  0.00948
Cumulative Proportion   0.76948  0.77965  0.78974  0.79943  0.80901  0.81850
                           PC31    PC32     PC33     PC34     PC35    PC36
Standard deviation     11.13608 11.0569 10.95973 10.88569 10.71282 10.6322
Proportion of Variance  0.00933  0.0092  0.00904  0.00891  0.00863  0.0085
Cumulative Proportion   0.82783  0.8370  0.84606  0.85498  0.86361  0.8721
                           PC37     PC38     PC39     PC40     PC41     PC42
Standard deviation     10.58408 10.54698 10.51209 10.33571 10.27164 10.20218
Proportion of Variance  0.00843  0.00837  0.00831  0.00804  0.00794  0.00783
Cumulative Proportion   0.88054  0.88891  0.89722  0.90526  0.91320  0.92103
                           PC43     PC44    PC45    PC46   PC47   PC48   PC49
Standard deviation     10.15098 10.08666 10.0534 9.90851 9.8527 9.7849 9.6467
Proportion of Variance  0.00775  0.00765  0.0076 0.00739 0.0073 0.0072 0.0070
Cumulative Proportion   0.92878  0.93643  0.9440 0.95142 0.9587 0.9659 0.9729
                          PC50    PC51    PC52   PC53      PC54
Standard deviation     9.59333 9.52176 9.45986 9.3645 8.843e-14
Proportion of Variance 0.00692 0.00682 0.00673 0.0066 0.000e+00
Cumulative Proportion  0.97985 0.98667 0.99340 1.0000 1.000e+00

# Create the plot:
pca_plot <- ggbiplot::ggbiplot(pca_analysis, 
                   groups = dge$samples$group, 
                   ellipse = TRUE,
                   var.axes = FALSE)

pca_plot

Version	Author	Date
f62ae4f	Nhi Hin	2021-06-30

We also have other sample metadata including age and sex of the samples in the dge$samples table. These variables can also be plotted on the PCA plot by changing what is specified in the groups argument below, so that we can see whether samples are grouping by age or sex. Doing this is useful as it can tell us whether these variables are confounding variables that have the potential to obscure real biological effects.
Below, we can see that the ages of patients from which samples were derived ranges from 51 to 61. It does appear that some of the older patients’ samples are closer to the top of the plot.

ggbiplot::ggbiplot(pca_analysis, 
                   groups = dge$samples$age, 
                   ellipse = FALSE,
                   var.axes = FALSE)

Version	Author	Date
66b2c36	Nhi Hin	2021-07-02
f62ae4f	Nhi Hin	2021-06-30

Below, we can see that sex of the samples does not seem to really influence where the points lie on the plot, suggesting that sex does not have a noticeable effect on gene expression of the samples here.

ggbiplot::ggbiplot(pca_analysis, 
                   groups = dge$samples$sex, 
                   ellipse = FALSE,
                   var.axes = FALSE)

Version	Author	Date
66b2c36	Nhi Hin	2021-07-02

Questions

Should we treat male and female samples differently in the analysis, based on the information in the MDS/PCA plots?
Should we do anything about the older patients’ samples, based on the information in the MDS/PCA plots?

Next Step - Differential gene expression analysis

Overall the data quality looks good and there do not appear to be outlier samples or mislabeled samples.
We will export the DGEList object created in this document for the next step of the analysis, differential gene expression analysis. The dge object is exported into the data/R directory.

r.dir <- here("data", "R")

ifelse(!dir.exists(r.dir),
       dir.create(r.dir), 
       FALSE)

dge %>% saveRDS(file.path(r.dir, "dge.rds"))

sessionInfo()

R version 4.0.3 (2020-10-10)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Mojave 10.14.6

Matrix products: default
BLAS:   /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRblas.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib

locale:
[1] en_AU.UTF-8/en_AU.UTF-8/en_AU.UTF-8/C/en_AU.UTF-8/en_AU.UTF-8

attached base packages:
[1] parallel  grid      stats     graphics  grDevices utils     datasets 
[8] methods   base     

other attached packages:
 [1] Glimma_2.0.0         edgeR_3.32.0         limma_3.46.0        
 [4] AnnotationHub_2.22.0 BiocFileCache_1.14.0 dbplyr_2.1.1        
 [7] BiocGenerics_0.36.0  export_0.3.0         here_1.0.0          
[10] ggrepel_0.8.2        ggbiplot_0.55        scales_1.1.1        
[13] plyr_1.8.6           ggplot2_3.3.3        kableExtra_1.3.2    
[16] reshape2_1.4.4       tibble_3.1.1         readr_1.4.0         
[19] magrittr_2.0.1       dplyr_1.0.5          workflowr_1.6.2     

loaded via a namespace (and not attached):
  [1] uuid_0.1-4                    backports_1.2.0              
  [3] systemfonts_0.3.2             splines_4.0.3                
  [5] BiocParallel_1.24.1           crosstalk_1.1.0.1            
  [7] GenomeInfoDb_1.26.1           digest_0.6.27                
  [9] htmltools_0.5.1.1             fansi_0.4.1                  
 [11] memoise_1.1.0                 openxlsx_4.2.3               
 [13] annotate_1.68.0               matrixStats_0.57.0           
 [15] officer_0.3.15                colorspace_2.0-0             
 [17] blob_1.2.1                    rvest_1.0.0                  
 [19] rappdirs_0.3.1                xfun_0.23                    
 [21] crayon_1.4.1                  RCurl_1.98-1.2               
 [23] jsonlite_1.7.2                genefilter_1.72.1            
 [25] survival_3.2-7                glue_1.4.2                   
 [27] rvg_0.2.5                     gtable_0.3.0                 
 [29] zlibbioc_1.36.0               XVector_0.30.0               
 [31] webshot_0.5.2                 DelayedArray_0.16.0          
 [33] DBI_1.1.0                     miniUI_0.1.1.1               
 [35] Rcpp_1.0.5                    viridisLite_0.3.0            
 [37] xtable_1.8-4                  bit_4.0.4                    
 [39] stats4_4.0.3                  htmlwidgets_1.5.2            
 [41] httr_1.4.2                    RColorBrewer_1.1-2           
 [43] ellipsis_0.3.1                farver_2.0.3                 
 [45] pkgconfig_2.0.3               XML_3.99-0.5                 
 [47] sass_0.3.1                    locfit_1.5-9.4               
 [49] utf8_1.1.4                    labeling_0.4.2               
 [51] tidyselect_1.1.0              rlang_0.4.10                 
 [53] manipulateWidget_0.10.1       later_1.1.0.1                
 [55] AnnotationDbi_1.52.0          munsell_0.5.0                
 [57] BiocVersion_3.12.0            tools_4.0.3                  
 [59] cli_2.5.0                     generics_0.1.0               
 [61] RSQLite_2.2.1                 devEMF_4.0-2                 
 [63] broom_0.7.6                   evaluate_0.14                
 [65] stringr_1.4.0                 fastmap_1.0.1                
 [67] yaml_2.2.1                    knitr_1.30                   
 [69] bit64_4.0.5                   fs_1.5.0                     
 [71] zip_2.1.1                     rgl_0.103.5                  
 [73] purrr_0.3.4                   whisker_0.4                  
 [75] mime_0.9                      xml2_1.3.2                   
 [77] compiler_4.0.3                rstudioapi_0.13              
 [79] curl_4.3                      interactiveDisplayBase_1.28.0
 [81] geneplotter_1.68.0            bslib_0.2.4                  
 [83] stringi_1.5.3                 highr_0.8                    
 [85] gdtools_0.2.2                 stargazer_5.2.2              
 [87] lattice_0.20-41               Matrix_1.2-18                
 [89] vctrs_0.3.7                   pillar_1.6.0                 
 [91] lifecycle_1.0.0               BiocManager_1.30.10          
 [93] jquerylib_0.1.3               data.table_1.13.2            
 [95] bitops_1.0-6                  flextable_0.6.1              
 [97] httpuv_1.5.4                  GenomicRanges_1.42.0         
 [99] R6_2.5.0                      promises_1.1.1               
[101] IRanges_2.24.0                assertthat_0.2.1             
[103] SummarizedExperiment_1.20.0   DESeq2_1.30.0                
[105] rprojroot_2.0.2               withr_2.3.0                  
[107] S4Vectors_0.28.0              GenomeInfoDbData_1.2.4       
[109] hms_1.0.0                     tidyr_1.1.3                  
[111] rmarkdown_2.8                 MatrixGenerics_1.2.0         
[113] git2r_0.27.1                  Biobase_2.50.0               
[115] shiny_1.6.0                   base64enc_0.1-3

Setting up for Differential Gene Expression Analysis

Nhi Hin

2021-06-28

Summary

Data Description

Load R Packages

Import Data

Gene count matrix

Questions

Sample Metadata Table

Readable sample names

Gene annotations

Questions

Create DGEList object

Quality Checks

Library Sizes of samples

Dimension Reduction

Multi Dimensional Scaling (MDS)

Principal Component Analysis (PCA)

Questions

Next Step - Differential gene expression analysis