In this ASHG 2020 PacBio Workshop Emily Farrow of Children’s Mercy Kansas City, shares how the incorporation of long-read sequencing into the Genomic Answers for Kids research study is increasing…
As they migrated out of Africa and into Europe and Asia, anatomically modern humans interbred with archaic hominins, such as Neanderthals and Denisovans. The result of this genetic introgression on the recipient populations has been of considerable interest, especially in cases of selection for specific archaic genetic variants. Hsieh et al. characterized adaptive structural variants and copy number variants that are likely targets of positive selection in Melanesians. Focusing on population-specific regions of the genome that carry duplicated genes and show an excess of amino acid replacements provides evidence for one of the mechanisms by which genetic novelty can arise and result in differentiation between human genomes.Science, this issue p. eaax2083INTRODUCTIONCharacterizing genetic variants underlying local adaptations in human populations is one of the central goals of evolutionary research. Most studies have focused on adaptive single-nucleotide variants that either arose as new beneficial mutations or were introduced after interbreeding with our now-extinct relatives, including Neanderthals and Denisovans. The adaptive role of copy number variants (CNVs), another well-known form of genomic variation generated through deletions or duplications that affect more base pairs in the genome, is less well understood, despite evidence that such mutations are subject to stronger selective pressures.RATIONALEThis study focuses on the discovery of introgressed and adaptive CNVs that have become enriched in specific human populations. We combine whole-genome CNV calling and population genetic inference methods to discover CNVs and then assess signals of selection after controlling for demographic history. We examine 266 publicly available modern human genomes from the Simons Genome Diversity Project and genomes of three ancient homininstextemdasha Denisovan, a Neanderthal from the Altai Mountains in Siberia, and a Neanderthal from Croatia. We apply long-read sequencing methods to sequence-resolve complex CNVs of interest specifically in the Melanesianstextemdashan Oceanian population distributed from Papua New Guinea to as far east as the islands of Fiji and known to harbor some of the greatest amounts of Neanderthal and Denisovan ancestry.RESULTSConsistent with the hypothesis of archaic introgression outside Africa, we find a significant excess of CNV sharing between modern non-African populations and archaic hominins (P = 0.039). Among Melanesians, we observe an enrichment of CNVs with potential signals of positive selection (n = 37 CNVs), of which 19 CNVs likely introgressed from archaic hominins. We show that Melanesian-stratified CNVs are significantly associated with signals of positive selection (P = 0.0323). Many map near or within genes associated with metabolism (e.g., ACOT1 and ACOT2), development and cell cycle or signaling (e.g., TNFRSF10D and CDK11A and CDK11B), or immune response (e.g., IFNLR1). We characterize two of the largest and most complex CNVs on chromosomes 16p11.2 and 8p21.3 that introgressed from Denisovans and Neanderthals, respectively, and are absent from most other human populations. At chromosome 16p11.2, we sequence-resolve a large duplication of >383 thousand base pairs (kbp) that originated from Denisovans and introgressed into the ancestral Melanesian population 60,000 to 170,000 years ago. This large duplication occurs at high frequency (>79%) in diverse Melanesian groups, shows signatures of positive selection, and maps adjacent to Homo sapienstextendashspecific duplications that predispose to rearrangements associated with autism. On chromosome 8p21.3, we identify a Melanesian haplotype that carries two CNVs, a ~6-kbp deletion, and a ~38-kbp duplication, with a Neanderthal origin and that introgressed into non-Africans 40,000 to 120,000 years ago. This CNV haplotype occurs at high frequency (44%) and shows signals consistent with a partial selective sweep in Melanesians. Using long-read sequencing genomic and transcriptomic data, we reconstruct the structure and complex evolutionary history for these two CNVs and discover previously undescribed duplicated genes (TNFRSF10D1, TNFRSF10D2, and NPIPB16) that show an excess of amino acid replacements consistent with the action of positive selection.CONCLUSIONOur results suggest that large CNVs originating in archaic hominins and introgressed into modern humans have played an important role in local population adaptation and represent an insufficiently studied source of large-scale genetic variation that is absent from current reference genomes.Large adaptive-introgressed CNVs at chromosomes 8p21.3 and 16p11.2 in Melanesians.The magnifying glasses highlight structural differences between the archaic (top) and reference (bottom) genomes. Neanderthal (red) and Denisovan (blue) haplotypes encompassing large CNVs occur at high frequencies in Melanesians (44 and 79%, respectively) but are absent (black) in all non-Melanesians. These CNVs create positively selected genes (TNFRSF10D1, TNFRSF10D2, and NPIPB16) that are absent from the reference genome.Copy number variants (CNVs) are subject to stronger selective pressure than single-nucleotide variants, but their roles in archaic introgression and adaptation have not been systematically investigated. We show that stratified CNVs are significantly associated with signatures of positive selection in Melanesians and provide evidence for adaptive introgression of large CNVs at chromosomes 16p11.2 and 8p21.3 from Denisovans and Neanderthals, respectively. Using long-read sequence data, we reconstruct the structure and complex evolutionary history of these polymorphisms and show that both encode positively selected genes absent from most human populations. Our results collectively suggest that large CNVs originating in archaic hominins and introgressed into modern humans have played an important role in local population adaptation and represent an insufficiently studied source of large-scale genetic variation.
The discovery of mutations associated with human genetic dis- ease is an exercise in comparative genomics (see Glossary). Although there are many different strategies and approaches, the central premise is that affected persons harbor a significant excess of pathogenic DNA variants as com- pared with a group of unaffected persons (controls) that is either clinically defined1 or established by surveying large swaths of the general population.2 The more exclu- sive the variant is to the disease, the greater its penetrance, the larger its effect size, and the more relevant it becomes to both disease diagnosis and future therapeutic investigation. The most popular approach used by researchers in human genetics is the case–control design, but there are others that can be used to track variants and disease in a family context or that consider the probability of different classes of mutations based on evolutionary patterns of divergence or de novo mutational change.3,4 Although the approaches may be straightforward, the discovery of patho- genic variation and its mechanism of action often is less trivial, and decades of research can be required in order to identify the variants underlying both mendelian and complex genetic traits.
We report a family with progressive myoclonic epilepsy who underwent whole-exome sequencing but was negative for pathogenic variants. Similar clinical courses of a devastating neurodegenerative phenotype of two affected siblings were highly suggestive of a genetic etiology, which indicates that the survey of genetic variation by whole-exome sequencing was not comprehensive. To investigate the presence of a variant that remained unrecognized by standard genetic testing, PacBio long-read sequencing was performed. Structural variant (SV) detection using low-coverage (6×) whole-genome sequencing called 17,165 SVs (7,216 deletions and 9,949 insertions). Our SV selection narrowed down potential candidates to only five SVs (two deletions and three insertions) on the genes tagged with autosomal recessive phenotypes. Among them, a 12.4-kb deletion involving the CLN6 gene was the top candidate because its homozygous abnormalities cause neuronal ceroid lipofuscinosis. This deletion included the initiation codon and was found in a GC-rich region containing multiple repetitive elements. These results indicate the presence of a causal variant in a difficult-to-sequence region and suggest that such variants that remain enigmatic after the application of current whole-exome sequencing technology could be uncovered by unbiased application of long-read whole-genome sequencing.
Current diagnostic testing for genetic disorders involves serial use of specialized assays spanning multiple technologies. In principle, genome sequencing (GS) can detect all genomic pathogenic variant types on a single platform. Here we evaluate copy-number variant (CNV) calling as part of a clinically accredited GS test.We performed analytical validation of CNV calling on 17 reference samples, compared the sensitivity of GS-based variants with those from a clinical microarray, and set a bound on precision using orthogonal technologies. We developed a protocol for family-based analysis of GS-based CNV calls, and deployed this across a clinical cohort of 79 rare and undiagnosed cases.We found that CNV calls from GS are at least as sensitive as those from microarrays, while only creating a modest increase in the number of variants interpreted (~10 CNVs per case). We identified clinically significant CNVs in 15% of the first 79 cases analyzed, all of which were confirmed by an orthogonal approach. The pipeline also enabled discovery of a uniparental disomy (UPD) and a 50% mosaic trisomy 14. Directed analysis of select CNVs enabled breakpoint level resolution of genomic rearrangements and phasing of de novo CNVs.Robust identification of CNVs by GS is possible within a clinical testing environment.
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