Tuxedo Suite For Splice Variant Analysis and Identifying Novel Transcripts II 2014
- Dhivya Arasappan
All these steps have already been run. We'll be spending time looking at the commands and output. Let's get set up.
Get to the results
cds cd my_rnaseq_course cp -r /corral-repl/utexas/BioITeam/rnaseq_course/cufflinks_results . cd cufflinks_results
Step 1: Tophat
We've already gone over how tophat results look here so let's move on to step 2.
Step 2: Cufflinks
HOW WAS IT RUN?
#If you've copied it over successfully: cat run_commands/commands.cufflinks #If you don't have a local copy, you can read it from the source: cat /corral-repl/utexas/BioITeam/rnaseq_course/cufflinks_results/run_commands/commands.cufflinks
HOW DOES THE OUTPUT LOOK?
Take a look at output for one of our samples, C1_R1. The important file is transcripts.gtf, which contains Tophat's assembled junctions for C1_R1.
#If you have a local copy: ls -l C1_R1_clout #If you don't have a local copy: ls -l /corral-repl/utexas/BioITeam/rnaseq_course/cufflinks_results/C1_R1_clout -rw------- 1 daras G-801020 627673 May 17 16:58 genes.fpkm_tracking -rw------- 1 daras G-801020 1021025 May 17 16:58 isoforms.fpkm_tracking -rw------- 1 daras G-801020 0 May 17 16:50 skipped.gtf -rw------- 1 daras G-801020 14784740 May 17 16:58 transcripts.gtf
DESCRIPTION OF TRANSCRIPTS.GTF FILE
Each row corresponds to one transcript or an exon of a transcript. First 7 columns are standard gff/gtf columns, followed by some attributes added by cufflinks.
| Column number | Column name | Example | Description |
| 1 | seqname | chrX | Chromosome or contig name |
| 2 | source | Cufflinks | The name of the program that generated this file (always 'Cufflinks') |
| 3 | feature | exon | The type of record (always either "transcript" or "exon".) |
| 4 | start | 77696957 | The leftmost coordinate of this record. |
| 5 | end | 77712009 | The rightmost coordinate of this record, inclusive. |
| 6 | score | 77712009 | Score that tells you how abundant this isoform is compared to other isoforms of the gene |
| 7 | strand | + | Cufflinks' guess for which strand the isoform came from. Always one of "+", "-", "." |
| 7 | frame | . | Not used |
| 8 | attributes | ... | See below. |
- Each GTF record is decorated with the following attributes:
| Attribute | Example | Description |
| gene_id | CUFF.1 | Cufflinks gene id |
| transcript_id | CUFF.1.1 | Cufflinks transcript id |
| FPKM | 101.267 | Isoform-level relative abundance in FPKM |
| frac | 0.7647 | Reserved. Please ignore. |
| cov | 100.765 | Estimate for the absolute depth of read coverage across the whole transcript |
| full_read_support | yes | When RABT assembly is used, this attribute reports whether or not all introns and internal exons were fully covered by reads from the data. |
Exercise: Find out how many entries in the transcripts.gtf file are from novel transcripts and how many from annotated transcripts.
The secret lies in the gene_id column. #For counting novel entries grep 'CUFF' C1_R1_clout/transcripts.gtf |wc -l 54847 #For counting entries corresponding to annotated genes grep -v 'CUFF' C1_R1_clout/transcripts.gtf |wc -l 88644 What do you think grep -v does?
Step 3: Cuffcompare
HOW WAS IT RUN?
cat run_commands/commands.cuffcompare
HOW DOES THE OUTPUT LOOK?
cuffcmp.tracking is the most important file to pay attention to. It tells you how the assembled transcript are related to known transcripts (from genes.gtf file)
#If you have a local copy: ls -l cmp_out -rw-rw-r-- 1 daras G-801020 26041012 May 21 01:34 cuffcmp.combined.gtf -rw-rw-r-- 1 daras G-801020 2544325 May 21 01:34 cuffcmp.loci -rw-rw-r-- 1 daras G-801020 6234 May 21 01:34 cuffcmp.stats -rw-rw-r-- 1 daras G-801020 10816986 May 21 01:34 cuffcmp.tracking
DESCRIPTION OF *TRACKING FILE CLASS CODES
| Column name | Example | Description | |
| 1 | Cufflinks transfrag id | TCONS_00000045 | A unique internal id for the transfrag |
| 2 | Cufflinks locus id | XLOC_000023 | A unique internal id for the locus |
| 3 | Reference gene id | Tcea | The gene_name attribute of the reference GTF record for this transcript, or '-' if no reference transcript overlaps this Cufflinks transcript |
| 4 | Reference transcript id | uc007afj.1 | The transcript_id attribute of the reference GTF record for this transcript, or '-' if no reference transcript overlaps this Cufflinks transcript |
| 5 | Class code | c | The type of match between the Cufflinks transcripts in column 6 and the reference transcript. See below for class code info |
CLASS CODES
| Priority | Code | Description | |
| 1 | = | Complete match of intron chain | |
| 2 | c | Contained | |
| 3 | j | Potentially novel isoform (fragment): at least one splice junction is shared with a reference transcript | |
| 4 | e | Single exon transfrag overlapping a reference exon and at least 10 bp of a reference intron, indicating a possible pre-mRNA fragment. | |
| 5 | i | A transfrag falling entirely within a reference intron | |
| 6 | o | Generic exonic overlap with a reference transcript | |
| 7 | p | Possible polymerase run-on fragment (within 2Kbases of a reference transcript) | |
| 8 | r | Repeat. Currently determined by looking at the soft-masked reference sequence and applied to transcripts where at least 50% of the bases are lower case | |
| 9 | u | Unknown, intergenic transcript | |
| 10 | x | Exonic overlap with reference on the opposite strand | |
| 11 | s | An intron of the transfrag overlaps a reference intron on the opposite strand (likely due to read mapping errors) |
Exercise 2: Count how many potentially novel fragments we have in our data
cat cmp_out/cuffcmp.tracking |awk '{if ($4 == "j")print}'|wc -l
556
Why did we choose awk here instead of grep?
Step 4: Cuffmerge
HOW WAS IT RUN?
We first created a file listing the paths of all per-sample transcripts.gtf files so far, then pass that to cuffmerge:
find . -name transcripts.gtf > assembly_list.txt #If you have a local copy: cat assembly_list.txt
cat run_commands/commands.cuffmerge
cuffmerge -g reference/genes.exons.gtf assembly_list.txt
HOW DOES THE OUTPUT LOOK?
The most important file is merged.gif, which contains the consensus transcriptome annotations cuffmerge has calculated. This is the input for the next step.
#If you have a local copy: ls -l merged_asm -rwxrwxr-x 1 daras G-803889 1571816 Aug 16 2012 genes.fpkm_tracking -rwxrwxr-x 1 daras G-803889 2281319 Aug 16 2012 isoforms.fpkm_tracking drwxrwxr-x 2 daras G-803889 32768 Aug 16 2012 logs -r-xrwxr-x 1 daras G-803889 32090408 Aug 16 2012 merged.gtf -rwxrwxr-x 1 daras G-803889 0 Aug 16 2012 skipped.gtf drwxrwxr-x 2 daras G-803889 32768 Aug 16 2012 tmp -rwxrwxr-x 1 daras G-803889 34844830 Aug 16 2012 transcripts.gtf
Step 5: Cuffdiff
HOW WAS IT RUN?
#If you have a local copy: cuffdiff -o diff_out -b reference/genome.fa -p 8 -L C1,C2 -u merged_asm/merged.gtf C1_R1_thout/accepted_hits.bam,C1_R2_thout/accepted_hits.bam,C1_R3_thout/accepted_hits.bam C2_R1_thout/accepted_hits.bam,C2_R2_thout/accepted_hits.bam,C2_R3_thout/accepted_hits.bam
#If you have a local copy: ls -l diff_out -rwxr-x--- 1 daras G-801020 2691192 Aug 21 12:20 isoform_exp.diff : Differential expression testing for transcripts -rwxr-x--- 1 daras G-801020 1483520 Aug 21 12:20 gene_exp.diff : Differential expression testing for genes -rwxr-x--- 1 daras G-801020 1729831 Aug 21 12:20 tss_group_exp.diff: Differential expression testing for primary transcripts -rwxr-x--- 1 daras G-801020 1369451 Aug 21 12:20 cds_exp.diff : Differential expression testing for coding sequences -rwxr-x--- 1 daras G-801020 3277177 Aug 21 12:20 isoforms.fpkm_tracking -rwxr-x--- 1 daras G-801020 1628659 Aug 21 12:20 genes.fpkm_tracking -rwxr-x--- 1 daras G-801020 1885773 Aug 21 12:20 tss_groups.fpkm_tracking -rwxr-x--- 1 daras G-801020 1477492 Aug 21 12:20 cds.fpkm_tracking -rwxr-x--- 1 daras G-801020 1349574 Aug 21 12:20 splicing.diff : Differential splicing tests -rwxr-x--- 1 daras G-801020 1158560 Aug 21 12:20 promoters.diff : Differential promoter usage -rwxr-x--- 1 daras G-801020 919690 Aug 21 12:20 cds.diff : Differential coding output
PARSING CUFFDIFF OUTPUT
Exercise 3: What is the status column in our gene_exp.diff files? and what values does it have?
cat diff_out/gene_exp.diff|cut -f 7|sort|uniq
Here is a basic command useful for parsing/sorting the gene_exp.diff or isoform_exp.diff files:
cat diff_out/isoform_exp.diff | awk '{print $10 "\t" $4}' | sort -n -r | head
Exercise 3: Find the 10 most up-regulated genes, by fold change that are classified as significantly changed.
cat diff_out/gene_exp.diff |grep 'yes'|sort -k10nr,10|head #Lets pull out just gene names cat diff_out/gene_exp.diff |grep 'yes'|sort -k10nr,10|head|cut -f 3
Exercise 4: Find the 10 most down-regulated genes, by fold change that are classified as significantly changed.
cat diff_out/gene_exp.diff |grep 'yes'|sort -k10n,10|head|cut -f 3
Exercise 5: Find the 10 most up-regulated isoforms, by fold change that are classified as significantly changed. What genes do they belong to?
cat diff_out/isoform_exp.diff |grep 'yes'|sort -k10nr,10|head #To pull out gene names: cat diff_out/isoform_exp.diff |grep 'yes'|sort -k10nr,10|head
Exercise 6: Pull out and count the significantly differentially expressed genes do we have (pvalue <=0.05 and abs fold change > 2 or log2 fold change > 1)?
cat diff_out/gene_exp.diff |grep 'yes' |awk '{if (($10 >= 1)||($10<=1)) print }' > DEG
wc -l DEG