QC text summary

The text summary is saved in two versions:

• Summary_QC.txt – formatted for viewing in Excel.
• Summary_QC_R.txt – formatted for easy import in R.

The number of BAM files

The nbams QC metric counts the number of BAM files. This cumulative metric is calculated for total of 42 BAMs.

cat("Number of BAMs:", qc$nbams)

## Number of BAMs: 42

Total number of reads in the BAM file(s)

The reads.total QC metric counts the number of reads scanned in the BAM file(s). This includes reads later excluded due to low alignment score or as duplicates.
The 42 BAMs contain 2.46 billion reads.

cat("Reads total:", qc$reads.total)

## Reads total: 2464441416

Number of reads aligned to the reference genome

The reads.aligned QC metric indicates the number of reads that were successfully aligned to the reference genome.
The number of aligned reads is only 2% smaller, 2.42 billion.

{
cat("Reads aligned:", qc$reads.aligned, "\n")
cat("This is ", qc$reads.aligned / qc$reads.total * 100,
    "% of all reads", sep="")
}

## Reads aligned: 2422356052 
## This is 98.2923% of all reads

Number of reads that passed minimum score filter

At step 1 of RaMWAS, reads are filtered by the scoretag parameter, which is usually either the “MAPQ” field or “AS” tag in the BAM file. Reads with scores below minscore are excluded.

The reads.recorded QC metric counts the number of reads that passed the score threshold.
Almost of 2.2 billion reads passed the score threshold.

{
cat("Reads recorded:",qc$reads.recorded,"\n")
cat("This is ", qc$reads.recorded / qc$reads.aligned * 100,
    "% of aligned reads", sep="")
}

## Reads recorded: 2197642600 
## This is 90.72335% of aligned reads

Number of reads after removal of duplicate reads

Reads that start at the same nucleotide positions and aligned to the same strand are called duplicate reads. When sequencing a whole genome, duplicate-reads often arise from template preparation or amplification artifacts. In the context of sequencing an enriched genomic fraction, duplicate-reads are increasingly likely to occur because reads align to a much smaller fraction of the genome. RaMWAS allows the user to define a threshold for the number of reads starting at the same position and limits the read count to this threshold (implicitly assuming that an excess of reads are tagging the same clonal fragment).

The threshold is set by maxrepeats parameter with the default value 3.

When there are mulptiple reads with the same start position, we suspect them to be falsely duplicated.

The reads.recorded.no.repeats QC metric records the total number of reads after removal of duplicates.

In our example, 10% of reads are removed as duplicates.

{
cat("Reads without duplicates:", qc$reads.recorded.no.repeats, "\n")
cat("This is ", qc$reads.recorded.no.repeats / qc$reads.recorded * 100,
    "% of aligned reads", "\n", sep="")
}

## Reads without duplicates: 1987137468 
## This is 90.42132% of aligned reads

Number of recorded reads aligned to each strand

The frwrev QC metric records two values – the number of reads aligned to the forward and reverse strands respectively, after filtering reads by alignment score.

The frwrev.no.repeats is similar to frwrev, but it excludes duplicate reads.

For these measures, the qcmean function returns the fraction of reads on the forward strand. Normally, the number of reads on the forward and reverse strands are very close, so qcmean should give a number close to 0.5.

{
    cat("Excluding duplicate reads", "\n")
    cat("Reads on forward strand:", qc$frwrev.no.repeats[1], "\n")
    cat("Reads on reverse strand:", qc$frwrev.no.repeats[2], "\n")
    cat("Fraction of reads on forward strand:",
        qcmean(qc$frwrev.no.repeats), "\n")
}

## Excluding duplicate reads 
## Reads on forward strand: 993398396 
## Reads on reverse strand: 993739072 
## Fraction of reads on forward strand: 0.4999143

Clearly before removal of duplicate reads the fraction of reads on the forward strand was further away from 0.5.

{
    cat("Not excluding duplicate reads", "\n")
    cat("Reads on forward strand:", qc$frwrev[1], "\n")
    cat("Reads on reverse strand:", qc$frwrev[2], "\n")
    cat("Fraction of reads on forward strand (before QC):",
        qcmean(qc$frwrev), "\n")
}

## Not excluding duplicate reads 
## Reads on forward strand: 1098313225 
## Reads on reverse strand: 1099329375 
## Fraction of reads on forward strand (before QC): 0.4997688

Distribution of the alignment scores

The QC metrics bf.hist.score1 and hist.score1 record the distribution of the alignment scores before and after the filter. The score is defined by the scoretag parameter. While hist.score1 contain the distribution for reads that passed the filter, bf.hist.score1 has the distribution for all reads. The qcmean function for these QC measures returns the average score for the respective group. The first element of the vector qc$hist.score1 contains the number of reads with score of 0, the second with score of 1, and so on. Negative scores (if any) are ignored.

{
cat("Average alignment score, after filter:", qcmean(qc$hist.score1),    "\n")
cat("Average alignment score, no filter:   ", qcmean(qc$bf.hist.score1), "\n")
par(mfrow=c(1,2))
plot(qc$hist.score1)
plot(qc$bf.hist.score1)
}

## Average alignment score, after filter: 144.1565 
## Average alignment score, no filter:    137.8565

Distribution of the length of the aligned part of the reads

The hist.length.matched QC metrix records the distribution of the length of the aligned part of the reads. The length of the aligned part of a read is calculated from the CIGAR string in the BAM file using the cigarWidthAlongQuerySpace function. The vector hist.length.matched has the distribution for reads that passed the filter, bf.hist.length.matched – for all reads. The qcmean function for these QC measures returns the average length The first element of the vector contains the number of reads with 1 aligned basepair, the second with 2, and so on.

{
cat("Average aligned length, after filter:", 
    qcmean(qc$hist.length.matched),    "\n")
cat("Average aligned length, no filter:   ",
    qcmean(qc$bf.hist.length.matched), "\n")
par(mfrow = c(1,2))
plot(qc$hist.length.matched)
plot(qc$bf.hist.length.matched)
}

## Average aligned length, after filter: 74.22412 
## Average aligned length, no filter:    72.60318

Distribution of edit distance

The hist.edit.dist1 QC metricrecords the distribution of the number of mismatches between the aligned part of the read and the reference genome. The mismatches are caused by base call errors and genetic variation. The metric is calculated from the NM tag in BAM files.

The bf.hist.edit.dist1 QC metric records the distribution before read filtering, while hist.edit.dist1 after. The first element of the vector contains the number of reads with 0 edit distance (perfect match), the second with edit distance 1, and so on.

The qcmean function for this QC metric returns the average edit distance.

{
cat("Average edit distance, after filter:", 
    qcmean(qc$hist.edit.dist1),    "\n")
cat("Average edit distance, no filter:   ", 
    qcmean(qc$bf.hist.edit.dist1), "\n")
par(mfrow = c(1,2))
plot(qc$hist.edit.dist1)
plot(qc$bf.hist.edit.dist1)
}

## Average edit distance, after filter: 0.7361304 
## Average edit distance, no filter:    1.220491

Number of reads away from CpGs

MBD-seq detects CpG methylation, such that reads aligning to loci that are at least maxfragmentsize away from any CpGs represent “noise”. These reads occur due to alignment errors or imperfect enrichment leading to sequencing of non-methylated fragments).

The cnt.nonCpG.reads QC metric contains the number of non-CpG reads in it’s first element and the total number of reads in the second.

The qcmean function for this QC metric returns the percent of non-CpG reads out of all reads.

For our data there is less than 1% of non-CpG reads, which is consistent with low level of noise.

{
cat("Non-CpG reads:", qc$cnt.nonCpG.reads[1], "\n")
cat("This is ", qcmean(qc$cnt.nonCpG.reads)*100, "% of recorded reads", sep="")
}

## Non-CpG reads: 10764340 
## This is 0.5669618% of recorded reads

Average CpG score for CpGs and non-CpGs

The avg.cpg.coverage and avg.noncpg.coverage QC metrics record the average CpG score (fragment coverage) for all CpGs and for all non-CpGs (locations away from CpGs).

For successfull enrichment, the average CpG score should be much larger that average non-CpG score The ratio of these metrics, gives us a lower bound on the enrichment level (higher is better). The opposite ratio measures the noise level (lower is better).

{
cat("Summed across", qc$nbams, "bams", "\n")
cat("Average     CpG coverage:", qc$avg.cpg.coverage, "\n")
cat("Average non-CpG coverage:", qc$avg.noncpg.coverage, "\n")
cat("Enrichment ratio:", qc$avg.cpg.coverage / qc$avg.noncpg.coverage, "\n")
cat("Noise level:", qc$avg.noncpg.coverage / qc$avg.cpg.coverage)
}

## Summed across 42 bams 
## Average     CpG coverage: 255.8143 
## Average non-CpG coverage: 1.766378 
## Enrichment ratio: 144.8242 
## Noise level: 0.006904925

Average CpG score vs. CpG density

Enrichment profiles are not only affected by the total amount of methylation of the DNA fragments, which is a function of the number of CpGs and how many of them are methylated, but also by variability in the laboratory protocol. To capture this variability, the avg.coverage.by.density QC metric records the dependence of average CpG score as a function of CpG density. The qcmean function returns the square root of the CpG density where maximum average CpG score is achieved.

This ‘sqrt peak’ sensitivity measure can be used as a covariate in downstream analyses to regress out variability in enrichment profiles across samples caused by lab-technical factors.

{
cat("Highest coverage is observed at CpG density of",
    qcmean(qc$avg.coverage.by.density)^2)
plot(qc$avg.coverage.by.density)
}

## Highest coverage is observed at CpG density of 8.4681

Coverage around isolated CpGs

A CpG is called isolated if there are no other CpGs within a sufficient distance from it. The distance is usually the longest possible fragment size.

The distribution of reads around isolated CpGs also contains information about the enrichment profile. Because the total amount of methylation in fragments containing isolated CpGs is small, this distribution reflects the sensitivity of the enrichment. That is, a sensitive assay would be characterized by a pattern where most reads start close to the isolated CpG. The read counts should decrease further away from the CpG until they eventually stabilize to a “noise” level at distances larger than the maximum fragment size. This pattern of read counts around isolated CpGs also form the basis of the fragment size estimation needed to calculate CpG score for single end sequencing data.

The hist.isolated.dist1 QC metric records the distribution of distances from read start sites to isolated CpGs. In our example, as we expected, there are more reads starting closer to the isolated CpGs.

plot(qc$hist.isolated.dist1)

Fraction of reads from chrX and chrY

The fractions of reads from chromosome X and chromosome Y can be used to test whether the investigated biosamples have the same sex as recorded in the phenotype data. Mismatches often indicate swapped biosamples or phenotypes information. They can also indicate contamination across biosamples.

The chrX.count QC metric records the number of chromosome X reads in its first element and the total number of reads in the second. The qcmean function returns the percent of chromosome X reads out of the total.

The chrY.count QC metric is defined analogously for chromosome Y.

{
cat("ChrX reads: ", qc$chrX.count[1], ", which is ",
    qcmean(qc$chrX.count)*100, "% of total", sep="", "\n")
cat("ChrY reads: ", qc$chrY.count[1], ", which is ",
    qcmean(qc$chrY.count)*100, "% of total", sep="", "\n")
}

## ChrX reads: 58392794, which is 2.939042% of total
## ChrY reads: 15592584, which is 0.7848102% of total