Cited by 140Genomics and Phylogenetic StudiesSingle-cell and spatial transcriptomicsRNA Research and Splicing

The DOI matches its Crossref record; 29 of the 29 references it cites were found in a registry or an index; 25 of the 29 sources its claims cite were checked for support.

Sources of bias in measures of allele-specific expression derived from RNA-seq data aligned to a single reference genome

Open access

Abstract

Background: RNA-seq can be used to measure allele-specific expression (ASE) by assigning sequence reads to individual alleles; however, relative ASE is systematically biased when sequence reads are aligned to a single reference genome. Aligning sequence reads to both parental genomes can eliminate this bias, but this approach is not always practical, especially for non-model organisms. To improve accuracy of ASE measured using a single reference genome, we identified properties of differentiating sites responsible for biased measures of relative ASE. Results: We found that clusters of differentiating sites prevented sequence reads from an alternate allele from aligning to the reference genome, causing a bias in relative ASE favoring the reference allele. This bias increased with greater sequence divergence between alleles. Increasing the number of mismatches allowed when aligning sequence reads to the reference genome and restricting analysis to genomic regions with fewer differentiating sites than the number of mismatches allowed almost completely eliminated this systematic bias. Accuracy of allelic abundance was increased further by excluding differentiating sites within sequence reads that could not be aligned uniquely within the genome (imperfect mappability) and reads that overlapped one or more insertions or deletions (indels) between alleles. Conclusions: After aligning sequence reads to a single reference genome, excluding differentiating sites with at least as many neighboring differentiating sites as the number of mismatches allowed, imperfect mappability, and/or an indel(s) nearby resulted in measures of allelic abundance comparable to those derived from aligning sequence reads to both parental genomes.

Overview

AI snapshot

Written from the paper’s full text.

TL;DR

When RNA-seq reads are aligned to a single reference genome, clusters of differentiating sites and nearby indels cause reads from the alternate allele to misalign, biasing relative allele-specific expression toward the reference allele. Excluding differentiating sites with as many or more neighboring differentiating sites than mismatches allowed, imperfect mappability, and/or nearby indels yields allelic abundance measures comparable to aligning reads to both parental genomes.

Key findings

  • In simulated 36-base reads from two *D. melanogaster* genotypes aligned with one mismatch, 50.9% of differentiating sites had unequal allelic abundance, 99.3% of which were biased toward the reference allele; among sites with at least one neighboring differentiating site, 99.8% favored the reference allele. Allowing two or three mismatches increased equal abundance to 80.2% and 91.9%, while aligning to both parental genomes with zero mismatches gave 99.0% equal abundance.
  • Bias increased with sequence divergence: the interspecific *D. melanogaster*/*D. simulans* simulation (1,130,435 differentiating sites) showed stronger reference bias than the intraspecific comparison, but parental-genome alignment made divergence impact negligible. Longer 50-base reads were more biased at the same mismatch allowance (40.6%, 73.0%, and 88.9% equal at one, two, or three mismatches), but matching the mismatch ratio (three for 36-base, four for 50-base) gave 91.9% and 92.1% equal abundance.
  • Imperfect mappability and nearby indels also caused inaccurate measures: for interspecific sites with perfect mappability, ~94% showed equal abundance when aligned to a single reference genome versus 99.9% when aligned to both parental genomes. In real F1 hybrid genomic DNA reads, equal allelic abundance was 70.4%, 88.9%, and 93.3% with one, two, or three mismatches to a single genome and 96.9% with parental genomes; excluding problematic sites raised single-reference measures to 96.3–96.6%, comparable to parental alignment.
  • Filtering retained 46.6–86.9% of differentiating sites in the intraspecific simulation and 8.3–50.5% in the interspecific simulation, but 81.8–91.8% and 66.3–95.2% of exons, respectively, still contained at least one reliable differentiating site.

Methods

  • Simulated equal numbers of 36- and 50-base reads from constitutive *D. melanogaster* exons for the dm3 reference allele versus a DGRP line_40 alternative allele (93,781 altered coding sites; 93,395,272 36-base reads, ~3.4 Gb). Reads were aligned with Bowtie v0.12.7 uniquely allowing 0–3 mismatches to a single reference genome and to both allele-specific genomes, then quantified with SAMtools and custom Perl/R scripts.
  • Interspecific simulation and real-data validation used 60,040 orthologous *D. melanogaster*/*D. simulans* exons (1,130,435 differentiating sites) and published 36-base Illumina genomic DNA reads from female F1 hybrids. Reads were aligned to the single *D. melanogaster* exome or both parental exomes; binomial exact tests with a 0.05 false discovery rate were applied to sites with ≥20 overlapping reads.
  • Neighboring differentiating sites were counted as the maximum number of differences in any overlapping read; mappability was calculated with the GEM library; indels were identified from FSA pairwise alignments.

Limitations

  • The paper states that true relative ASE is rarely known, so real-data validation used genomic DNA from F1 hybrids, where maternal and paternal alleles are expected in equal amounts, rather than RNA. It also states that filtering is expected to preferentially eliminate the most rapidly evolving exons.
  • The paper notes that aligning reads to both parental genomes is not always practical, especially for non-model organisms, and the single-reference correction depends on knowing differentiating sites and the mismatch allowance used.
  • Evident from the text: analyses are limited to *Drosophila* simulations and one *D. melanogaster*/*D. simulans* F1 hybrid dataset; performance in other taxa, at other polymorphism levels, and with newer aligners or read lengths is not tested.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1186/1471-2164-14-536

Cites

29 of 29 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Figures

Figure 1 Simulating an allele-specific RNA-seq experiment. Reads were generated from the “reference” D. melanogaster (dm3) allele (blue) and from an “alternative” allele (red) that contained all homozygous single nucleotide variants found in the DGRP strain “line_40”. For each exon, one read (arrow) was generated starting at each position for each allele from 1 to n-k, where n is the length of the exon and k is the length of the read, both in bases. This process was repeated for the reverse complement of each exon. The black arrows indicate reads with no allele-specific information.
Figure 1 Simulating an allele-specific RNA-seq experiment. Reads were generated from the “reference” D. melanogaster (dm3) allele (blue) and from an “alternative” allele (red) that contained all homozygous single nucleotide variants found in the DGRP strain “line_40”. For each exon, one read (arrow) was generated starting at each position for each allele from 1 to n-k, where n is the length of the exon and k is the length of the read, both in bases. This process was repeated for the reverse complement of each exon. The black arrows indicate reads with no allele-specific information.CC BY · PubMed Central
Figure 2 The density of differentiating sites affects relative allelic abundance when simulated reads are mapped to only one genome. Relative allelic abundance was measured using the 36-base (A-D) and 50-base (E-H) reads simulated from the two D. melanogaster genotypes as well as using the 36-base reads simulated from D. melanogaster and D. simulans(I-L) aligned to a single reference genome, allowing either one mismatch (A, E, I), two mismatches (B, F, J), or three mismatches (C, G, K), as well as by aligning reads to both allele-specific genomes allowing no mismatches (D, H, L). The number of neighboring differentiating sites is shown on the x-axis of each panel for each differentiating site and describes the maximum number of other sites that differ between the two alleles in any potential read overlapping the focal differentiating site. The y-axis shows the proportion of reads that were assigned to the reference allele for each differentiating site, summarized in box plots where the width of each box is proportional to the number of sites in that class. A proportion of 0.5 (indicated with a red dotted line in each panel) is expected if all reads overlapping a differentiating site are correctly assigned to alleles. The pie chart inset in each panel shows the total number of differentiating sites with equal (white) and unequal (grey) abundance of reads assigned to each allele.
Figure 2 The density of differentiating sites affects relative allelic abundance when simulated reads are mapped to only one genome. Relative allelic abundance was measured using the 36-base (A-D) and 50-base (E-H) reads simulated from the two D. melanogaster genotypes as well as using the 36-base reads simulated from D. melanogaster and D. simulans(I-L) aligned to a single reference genome, allowing either one mismatch (A, E, I), two mismatches (B, F, J), or three mismatches (C, G, K), as well as by aligning reads to both allele-specific genomes allowing no mismatches (D, H, L). The number of neighboring differentiating sites is shown on the x-axis of each panel for each differentiating site and describes the maximum number of other sites that differ between the two alleles in any potential read overlapping the focal differentiating site. The y-axis shows the proportion of reads that were assigned to the reference allele for each differentiating site, summarized in box plots where the width of each box is proportional to the number of sites in that class. A proportion of 0.5 (indicated with a red dotted line in each panel) is expected if all reads overlapping a differentiating site are correctly assigned to alleles. The pie chart inset in each panel shows the total number of differentiating sites with equal (white) and unequal (grey) abundance of reads assigned to each allele.CC BY · PubMed Central
Figure 3 Imperfect mappability causes inaccurate measures of relative allelic abundance. For unbiased differentiating sites (i.e., those with fewer neighboring differentiating sites than the number of mismatches allowed) with either perfect (white) or imperfect (grey) mappability, the distribution of relative allelic abundance (measured as the proportion of mapped reads assigned to the reference allele) is shown for the 36-base (A-D) and 50-base (E-H) reads simulated from the two D. melanogaster genotypes as well as for the 36-base reads simulated from D. melanogaster and D. simulans(I-L) aligned to a single genome, allowing one (A, E, I), two (B, F, J), or three (C, G, K) mismatches. The distribution of relative allelic abundance for unbiased differentiating sites with perfect (white) and imperfect (grey) mappability is also shown for all three simulated datasets after aligning reads to both the reference and alternative genomes, allowing no mismatches (D, H, L).
Figure 3 Imperfect mappability causes inaccurate measures of relative allelic abundance. For unbiased differentiating sites (i.e., those with fewer neighboring differentiating sites than the number of mismatches allowed) with either perfect (white) or imperfect (grey) mappability, the distribution of relative allelic abundance (measured as the proportion of mapped reads assigned to the reference allele) is shown for the 36-base (A-D) and 50-base (E-H) reads simulated from the two D. melanogaster genotypes as well as for the 36-base reads simulated from D. melanogaster and D. simulans(I-L) aligned to a single genome, allowing one (A, E, I), two (B, F, J), or three (C, G, K) mismatches. The distribution of relative allelic abundance for unbiased differentiating sites with perfect (white) and imperfect (grey) mappability is also shown for all three simulated datasets after aligning reads to both the reference and alternative genomes, allowing no mismatches (D, H, L).CC BY · PubMed Central
Figure 4 Insertions and deletions (indels) cause biased allele-specific assignment when reads are aligned to a single reference genome. For differentiating sites with perfect mappability and fewer neighboring differentiating sites than the number of mismatches allowed, the distributions of relative allelic abundance are shown for differentiating sites with (grey) and without (white) one or more nearby indel(s) after aligning the 36-base reads simulated from D. melanogaster and D. simulans to either the D. melanogaster genome with one (A), two (B), or three (C) mismatches allowed or to both the D. melanogaster and D. simulans genomes with no mismatches allowed (D).
Figure 4 Insertions and deletions (indels) cause biased allele-specific assignment when reads are aligned to a single reference genome. For differentiating sites with perfect mappability and fewer neighboring differentiating sites than the number of mismatches allowed, the distributions of relative allelic abundance are shown for differentiating sites with (grey) and without (white) one or more nearby indel(s) after aligning the 36-base reads simulated from D. melanogaster and D. simulans to either the D. melanogaster genome with one (A), two (B), or three (C) mismatches allowed or to both the D. melanogaster and D. simulans genomes with no mismatches allowed (D).CC BY · PubMed Central
Figure 5 Real reads aligned to a single reference genome produce reliable measures of allelic abundance after excluding problematic differentiating sites. (A) The relative proportions of sites with an excess of neighboring differentiating sites (cyan), imperfect mappability (magenta), an indel(s) nearby (yellow), or more than one of these properties are shown for the simulated 36-base intra- (mel-mel) and interspecific (mel-sim) datasets allowing one (1 mm), two (2 mm), or 3 (3 mm) mismatches during alignment to a single reference genome. (B) The proportion of differentiating sites with no statistically significant difference in relative allelic expression is shown for the real reads from F1 hybrids between D. melanogaster and D. simulans after aligning to either a single reference genome with one, two, or three mismatches allowed or to both the maternal and paternal genomes with zero mismatches allowed before excluding any sites (grey) and after sequentially excluding differentiating sites with an excess of neighboring differentiating sties (cyan), imperfect mappability (magenta), or an indel(s) nearby (yellow). (C-E) For each differentiating site retained after filtering based on neighboring differentiating sites, mappability, and indels, the proportion of reads assigned to the reference allele is plotted after aligning reads to a single reference genome (y-axis) or to separate allele-specific genomes (x-axis), allowing one (C), two (D), or three (E) mismatches. The pie chart insets reflect the total number of differentiating sites that showed either no statistically significant difference in relative allelic abundance using either alignment strategy (grey), a statistically significant difference when reads were aligned to either a single reference genome (blue) or both the maternal and paternal genomes (red), or a significant difference with both alignment methods (purple). Binomial exact tests and a false discovery rate of 0.05 were used to assess statistical significance in all cases.
Figure 5 Real reads aligned to a single reference genome produce reliable measures of allelic abundance after excluding problematic differentiating sites. (A) The relative proportions of sites with an excess of neighboring differentiating sites (cyan), imperfect mappability (magenta), an indel(s) nearby (yellow), or more than one of these properties are shown for the simulated 36-base intra- (mel-mel) and interspecific (mel-sim) datasets allowing one (1 mm), two (2 mm), or 3 (3 mm) mismatches during alignment to a single reference genome. (B) The proportion of differentiating sites with no statistically significant difference in relative allelic expression is shown for the real reads from F1 hybrids between D. melanogaster and D. simulans after aligning to either a single reference genome with one, two, or three mismatches allowed or to both the maternal and paternal genomes with zero mismatches allowed before excluding any sites (grey) and after sequentially excluding differentiating sites with an excess of neighboring differentiating sties (cyan), imperfect mappability (magenta), or an indel(s) nearby (yellow). (C-E) For each differentiating site retained after filtering based on neighboring differentiating sites, mappability, and indels, the proportion of reads assigned to the reference allele is plotted after aligning reads to a single reference genome (y-axis) or to separate allele-specific genomes (x-axis), allowing one (C), two (D), or three (E) mismatches. The pie chart insets reflect the total number of differentiating sites that showed either no statistically significant difference in relative allelic abundance using either alignment strategy (grey), a statistically significant difference when reads were aligned to either a single reference genome (blue) or both the maternal and paternal genomes (red), or a significant difference with both alignment methods (purple). Binomial exact tests and a false discovery rate of 0.05 were used to assess statistical significance in all cases.CC BY · PubMed Central
Figure 6 Relative allelic abundance can be estimated for most exons after excluding sites problematic sites. The proportion of differentiating sites (blue) and exons with at least one differentiating site (red) suitable for quantifying ASE after excluding sites with an excess of neighboring differentiating sites, imperfect mappability (black) and an indel(s) nearby (grey) are shown for the 36-base reads simulated from the two D. melanogaster genotypes (left) and from the D. melanogaster and D. simulans exomes (right). Each pair of bars results from aligning reads to either a single reference genome (Ref) or both the maternal and paternal genomes (M + P) with zero (0), one (1), two (2), or three (3) mismatches allowed. The two D. melanogaster genotypes compared did not include any indels, as described in the main text.
Figure 6 Relative allelic abundance can be estimated for most exons after excluding sites problematic sites. The proportion of differentiating sites (blue) and exons with at least one differentiating site (red) suitable for quantifying ASE after excluding sites with an excess of neighboring differentiating sites, imperfect mappability (black) and an indel(s) nearby (grey) are shown for the 36-base reads simulated from the two D. melanogaster genotypes (left) and from the D. melanogaster and D. simulans exomes (right). Each pair of bars results from aligning reads to either a single reference genome (Ref) or both the maternal and paternal genomes (M + P) with zero (0), one (1), two (2), or three (3) mismatches allowed. The two D. melanogaster genotypes compared did not include any indels, as described in the main text.CC BY · PubMed Central

Verification

Claims the paper makes

43 Own result · 9 Own interpretation · 6 Partly supported · 5 Supported · 5 Own method · 5 No basis given · 1 Supported so far · 1 Not supported · 1 No full text of the sources · accuracy not yet measured

Show all 53 claims

  • Probably not a substantial claim. 13
  • It may state two separate claims as one. 12
  • The quote only partly states this claim. 2
23 of 23
  • The authors state this as an assumption.

    The number of differentiating sites with allele-specific mappability differences is similar between alleles.

    “Assuming the number of such differentiating sites is similar between alleles, differences in allele-specific mappability should not systematically favor one allele or the other, but will still cause errors in relative ASE.”
  • The paper relies on this without support.

    In F1 hybrids, all maternal and paternal alleles are present in equal amounts.

    “we used sequence data that was collected in a comparable manner from genomic DNA extracted from F1 hybrids, in which all maternal and paternal alleles are expected to be present in equal amounts.”
  • The paper relies on this without support.

    The true value of relative allele-specific expression is rarely known.

    “Assessing the accuracy of relative ASE measurements derived from RNA-seq data is challenging because the true value of relative ASE is rarely known.”
  • The paper relies on this without support.

    Independent empirical methods for measuring relative allele-specific expression are not suitable for quantifying relative allele-specific expression on a genomic scale.

    “Independent empirical methods for measuring relative ASE such as Pyrosequencing and qPCR can be used to validate RNA-seq data for individual genes, but they are not suitable for quantifying relative ASE on a genomic scale.”
  • The authors state this as an assumption.

    Reads aligning uniquely to one or the other allele-specific genome are evidence that the read is allele-specific, while reads aligning equally well to both genomes are not.

    “We posited that reads aligning uniquely to one or the other allele-specific genome was evidence that that read was allele-specific, while reads aligning equally well to both genomes was not.”
  • The paper relies on this without support.

    Sequence divergence is lower within D. melanogaster than between D. melanogaster and D. simulans.

    “is consistent with the lower sequence divergence within than between species.”
6 of 6

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

4 of 4

Citations

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Metadata

Authors
Stevenson, K. R., Coolon, J. D., & Wittkopp, P. J.
Affiliations
University of Michigan (US)
Corresponding author
Wittkopp, Patricia J
Venue
BMC Genomics
Published
2013-08-07
Volume
14
Issue
1
Pages
536-536
ISSN
1471-2164 (ISSN-L)
OpenAlex
W2164158485
Cited by
140
References
29
Citations by year
2026: 3 · 2025: 7 · 2024: 7 · 2023: 8 · 2022: 4 · 2021: 15 · 2020: 14 · 2019: 12 · 2018: 9 · 2017: 9 · 2016: 14 · 2015: 16 · 2014: 19 · 2013: 2
FWCI
6.86
Type
journal article
Language
en
Open access
Licence
cc-by
Locations
  • BMC Genomics (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • Deep Blue (University of Michigan) (submittedVersion)
  • Europe PMC (PubMed Central) (submittedVersion, cc-by)
  • PubMed Central (submittedVersion, cc-by)
Funders
  • National Science Foundation (1021398, 0903629)
  • National Institutes of Health (1f32gm089009-01a1)
  • National Institute of General Medical Sciences (1F32-GM089009-01A1)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Genomics and Phylogenetic Studies
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Single-cell and spatial transcriptomics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › RNA Research and Splicing
Keywords
allele-specific expression, RNA-seq, reference genome bias, indels, non-model organisms, read mapping
MeSH terms
Alleles, Animals, Drosophila, Exons, Genome, Sequence Analysis, RNA

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