Biology subjects

Sasani, T. A.

Publications and source records attributed to Sasani, T. A..

14 recordsLinked to original sources

bioRxiv2026

Inherited DNA damage generates multi-allelic mutations in C. elegans

Exogenous and endogenous mutagens generate a wide variety of DNA lesions, including bulky adducts, chemical modifications, and single- or double-stranded breaks. A phenomenon called "lesion segregation," in which lesions evade repair and persist for multiple cell divisions, has recently been documented in tumors and healthy somatic tissues from mice and humans, respectively. Persistent lesions can generate multi-allelic variants (MAVs) by serving as templates for multiple rounds of error-prone replication. By reanalyzing data from a large C. elegans mutagenesis experiment, we observed robust evidence for MAVs at a small fraction (~0.2%) of mutated sites in the offspring of strains treated with alkylating agents. Because these sequencing data were derived from the progeny of a single F1 animal -- itself the offspring of a mutagenized P0 -- all mutations should be biallelic. The presence of multi-allelic variation implies that some DNA lesions are transmitted to the F1 zygote, evade repair, and are repeatedly bypassed by error-prone polymerases during embryogenesis. We suspect that many more lesions are inherited than is suggested by MAV prevalence, and that a large fraction of biallelic mutations are also caused by inherited lesions. Our results demonstrate that DNA lesions serve as durable, transgenerational templates for mutagenesis in C. elegans . We speculate that lesion segregation in the early embryo may be a source of mosaicism and genetic diversity in humans, as well.

genetics↗

bioRxiv2026

A family portrait of the genomic factors shaping tandem repeat mutagenesis

Tandem repeats (TRs) are among the most mutable loci in the human genome, but the genomic determinants of TR mutagenesis remain mysterious. We used PacBio HiFi long-read sequencing to profile nearly eight million TR loci in 28 members of a large, four-generation CEPH/Utah family designated K1463. We identified 1,270 de novo TR expansions and contractions across 20 children in the pedigree. De novo mutations (DNMs) were more likely to occur at loci that were longer, composed of uninterrupted motif sequences, and heterozygous in the parental germline. Children born to older fathers also exhibited more de novo mutations at short tandem repeats (STRs). A total of 43 TR loci were hyper-mutable in K1463, expanding or contracting up to twelve times across the pedigree. Among hyper-mutable loci that comprised multiple motifs (i.e., "complex" loci), specific motifs expanded and contracted more often than others; for example, all ten DNMs at a complex, hyper-mutable locus near the non-coding RNA LINC03021 involved the same 19bp motif. The mutability of particular motifs may be attributable to allele length, as 95% of DNMs at complex loci were expansions and contractions of the most abundant motif on a parental haplotype. However, future work will be required to disentangle the effects of nucleotide content and allele length on motif-specific mutability, especially at hyper-mutable TRs. Overall, this study combines long-read sequencing technologies with new software tools to comprehensively investigate the factors that influence TR mutagenesis.

genomics↗

bioRxiv2025

AVITI sequencing of a four-generation CEPH/Utah pedigree confirms low mutation rates at homopolymer loci despite their low sequence complexity

BackgroundShort tandem repeats (STRs) and homopolymers are among the most mutable loci in the human genome. Despite their presumed mutability owing to replication slippage, homopolymer loci exhibit lower mutation rates and minimal paternal age effects compared to other STRs. This paradox questions if technical limitations, rather than biological mechanisms, explain these observations. ResultsWe used the Element Biosciences AVITI platform to sequence the genomes of a 48-member, four-generation CEPH/Utah pedigree. As the AVITI platform reduces error rates at repetitive sequences compared to Illumina, this design enabled accurate mutation discovery at 90% of assayed homopolymers and a 1.7-fold increase in discoverable mutations compared to Illumina. We identified a median of 35 de novo homopolymer mutations per trio and a mutation rate of 5.28 x 10-5 DNMs per locus per generation, confirming a lower rate than dinucleotides (1.94 x 10-4). Most DNMs were single base-pair expansions or contractions. Despite comprising <1% of homopolymer loci, G/C homopolymers showed 18-fold higher mutation rates than A/T homopolymers; in contrast, the high dinucleotide mutation rate is not driven by a particular motif class. Parent-of-origin analysis revealed 78% of homopolymer mutations are paternal in origin, but no significant paternal age effect was observed. ConclusionsThis study confirms that homopolymers exhibit lower mutation rates and lack strong paternal age effects compared to other STRs, likely owing to the combination of a lower propensity to form slippage-causing secondary structures and more efficient mismatch repair. Our set of high-quality mutations suggest these phenomena are biological rather than technical in nature. Finally, we demonstrate that AVITI sequencing unlocks previously intractable regions of the genome and will be a powerful tool for continued investigation of repeat mutation.

genomics↗

bioRxiv2024

A familial, telomere-to-telomere reference for human de novo mutation and recombination from a four-generation pedigree

Using five complementary short- and long-read sequencing technologies, we phased and assembled >95% of each diploid human genome in a four-generation, 28-member family (CEPH 1463) allowing us to systematically assess de novo mutations (DNMs) and recombination. From this family, we estimate an average of 192 DNMs per generation, including 75.5 de novo single-nucleotide variants (SNVs), 7.4 non-tandem repeat indels, 79.6 de novo indels or structural variants (SVs) originating from tandem repeats, 7.7 centromeric de novo SVs and SNVs, and 12.4 de novo Y chromosome events per generation. STRs and VNTRs are the most mutable with 32 loci exhibiting recurrent mutation through the generations. We accurately assemble 288 centromeres and six Y chromosomes across the generations, documenting de novo SVs, and demonstrate that the DNM rate varies by an order of magnitude depending on repeat content, length, and sequence identity. We show a strong paternal bias (75-81%) for all forms of germline DNM, yet we estimate that 17% of de novo SNVs are postzygotic in origin with no paternal bias. We place all this variation in the context of a high-resolution recombination map ([~]3.5 kbp breakpoint resolution). We observe a strong maternal recombination bias (1.36 maternal:paternal ratio) with a consistent reduction in the number of crossovers with increasing paternal (r=0.85) and maternal (r=0.65) age. However, we observe no correlation between meiotic crossover locations and de novo SVs, arguing against non-allelic homologous recombination as a predominant mechanism. The use of multiple orthogonal technologies, near-telomere-to-telomere phased genome assemblies, and a multi-generation family to assess transmission has created the most comprehensive, publicly available "truth set" of all classes of genomic variants. The resource can be used to test and benchmark new algorithms and technologies to understand the most fundamental processes underlying human genetic variation.

genomics↗

bioRxiv2024

Decoding chromosome organization using CheC-PLS: chromosome conformation by proximity labeling and long-read sequencing

Genomic approaches have provided detailed insight into chromosome architecture. However, commonly deployed techniques do not preserve connectivity-based information, leaving large-scale genome organization poorly characterized. Here, we developed CheC-PLS: a proximity-labeling technique that indelibly marks, and then decodes, protein-associated sites. CheC-PLS tethers dam methyltransferase to a protein of interest, followed by Nanopore sequencing to identify methylated bases - indicative of in vivo proximity - along reads >100kb. As proof-of-concept we analyzed, in budding yeast, a cohesin-based meiotic backbone that organizes chromatin into an array of loops. Our data recapitulates previously obtained association patterns, and, importantly, exposes variability between cells. Single read data reveals cohesin translocation on DNA and, by anchoring reads onto unique regions, we define the internal organization of the ribosomal DNA locus. Our versatile technique, which we also deployed on isolated nuclei with nanobodies, promises to illuminate diverse chromosomal processes by describing the in vivo conformations of single chromosomes.

genomics↗

bioRxiv2023

Epistasis between mutator alleles contributes to germline mutation rate variability in laboratory mice

Maintaining germline genome integrity is essential and enormously complex. Although many proteins are involved in DNA replication, proofreading, and repair [1], mutator alleles have largely eluded detection in mammals. DNA replication and repair proteins often recognize sequence motifs or excise lesions at specific nucleotides. Thus, we might expect that the spectrum of de novo mutations -- the frequencies of C>T, A>G, etc. -- will differ between genomes that harbor either a mutator or wild-type allele. Previously, we used quantitative trait locus mapping to discover candidate mutator alleles in the DNA repair gene Mutyh that increased the C>A germline mutation rate in a family of inbred mice known as the BXDs [2,3]. In this study we developed a new method to detect alleles associated with mutation spectrum variation and applied it to mutation data from the BXDs. We discovered an additional C>A mutator locus on chromosome 6 that overlaps Ogg1, a DNA glycosylase involved in the same base-excision repair network as Mutyh [4]. Its effect depended on the presence of a mutator allele near Mutyh, and BXDs with mutator alleles at both loci had greater numbers of C>A mutations than those with mutator alleles at either locus alone. Our new methods for analyzing mutation spectra reveal evidence of epistasis between germline mutator alleles and may be applicable to mutation data from humans and other model organisms.

genetics↗

bioRxiv2022

Private and sub-family specific mutations of founder haplotypes in the BXD family reveal phenotypic consequences relevant to health and disease

The BXD family of recombinant inbred mice were developed by crossing and inbreeding progeny of C57BL/6J and DBA/2J strains. This family is the largest and most extensively phenotyped mammalian experimental genetic resource. Although used in genetics for 52 years, we do not yet have comprehensive data on DNA variants segregating in the BXDs. Using linked-read whole-genome sequencing, we sequenced 152 members of the family at about 40X coverage and quantified most variants. We identified 6.25 million polymorphism segregating at a near-optimal minor allele frequency of 0.42. We also defined two other major variants: strain-specific de novo singleton mutations and epoch-specific de novo polymorphism shared among subfamilies of BXDs. We quantified per-generation mutation rates of de novo variants and demonstrate how founder-derived, strain-specific, and epoch-specific variants can be analyzed jointly to model genome-phenome causality. This integration enables forward and reverse genetics at scale, rapid production of any of more than 10,000 diallel F1 hybrid progeny to test predictions across diverse environments or treatments. Combined with five decades of phenome data, the BXD family and F1 hybrids are a major resource for systems genetics and experimental precision medicine.

genomics↗

bioRxiv2021

A wild-derived antimutator drives germline mutation spectrum differences in a genetically diverse murine family

Summary ParagraphAlthough germline mutation rates and spectra can vary within and between species, genetic modifiers of these traits have long eluded detection. In this study, we searched for loci that influence germline mutagenesis using a uniquely powerful resource: a panel of recombinant inbred mouse lines known as the BXD, descended from the laboratory mouse strains C57BL/6J (B) and DBA/2J (D). Each BXD lineage has been maintained by brother-sister mating in the near absence of natural selection, accumulating de novo mutations for up to 50 years on a known genetic background that is a unique linear mosaic of B and D haplotypes. We show that mice inheriting D haplotypes at a quantitative trait locus (QTL) on chromosome 4 accumulate C>A germline mutations at a 50% higher rate than those inheriting B haplotypes, primarily due to the activity of a C>A-dominated mutational signature known as SBS18. The B and D QTL haplotypes encode different alleles of the DNA repair gene Mutyh, which underlies the heritable colorectal cancer syndrome in which SBS18 was first identified. The B and D Mutyh alleles are present in wild populations of Mus musculus domesticus, providing evidence that common genetic variation modulates germline mutagenesis in a model mammalian species.

genetics↗

bioRxiv2021

Unfazed: parent-of-origin detection for large and small de novo variants

SummaryUnfazed is a command-line tool to determine the parental gamete of origin for de novo mutations from paired-end Illumina DNA sequencing reads. Unfazed uses variant information for a sequenced trio to identify the parental gamete of origin by linking phase-informative inherited variants to de novo mutations using read-based phasing. It achieves a high success rate by chaining reads into haplotype groups, thus increasing the search space for informative sites. Unfazed provides a simple command-line interface and scales well to large inputs, determining parent-of-origin for nearly 30,000 de novo variants in under 60 hours. AvailabilityUnfazed is available at https://github.com/ibelyeu/unfazed.

genomics↗

bioRxiv2020

Poxviruses capture host genes by LINE-1 retrotransposition

Horizontal gene transfer (HGT) provides a major source of genetic variation. Many viruses, including poxviruses, encode genes with crucial functions directly gained by gene transfer from hosts. The mechanism of transfer to poxvirus genomes is unknown. Using genome analysis and experimental screens of infected cells, we discovered a central role for Long Interspersed Nuclear Element-1 (LINE-1) retrotransposition in HGT to virus genomes. The process recapitulates processed pseudogene generation, but with host messenger RNA directed into virus genomes. Intriguingly, hallmark features of retrotransposition appear to favor virus adaption through rapid duplication of captured host genes on arrival. Our study reveals a previously unrecognized conduit of genetic traffic with fundamental implications for the evolution of many virus classes and their hosts. SummaryActive selfish genetic elements in infected cells aid virus adaptation by catalyzing transfer of host genes to virus genomes.

evolutionary biology↗

bioRxiv2019

Large, three-generation CEPH families reveal post-zygotic mosaicism and variability in germline mutation accumulation

The number of de novo mutations (DNMs) found in an offsprings genome is known to increase with both paternal and maternal age. But does the rate of mutation accumulation in parental gametes differ across families? To answer this question, we analyzed DNMs in 33 large, three-generation families collected in Utah by the Centre dEtude du Polymorphisme Humain (CEPH) consortium. We observed significant variability in parental age effects on DNM counts across families, ranging from 0.24 to 3.33 additional DNMs per year. Using up to 14 grandchildren in these families, we find that 3% of DNMs originated following primordial germ cell specification (PGCS) in a parent, and differ from non-mosaic germline DNMs in their mutational spectra. We also identify a median of 3 gonosomal mutations per sample in the F1 generation, which, along with post-PGCS DNMs, occur at equivalent frequencies on the paternal and maternal haplotypes. These results demonstrate that the rate of germline mutation accumulation varies among families with similar ancestry, and confirm that parental mosaicism is a substantial source of de novo mutations in children. Data and code availabilityCode used for statistical analysis and figure generation has been deposited on GitHub as a collection of annotated Jupyter Notebooks: https://github.com/quinlan-lab/ceph-dnm-manuscript. Data files containing germline de novo mutations, as well as the gonosomal and post-primordial germ cell specification (PGCS) mosaic mutations, are included with these Notebooks. To mitigate compatibility/version issues, we have also made all notebooks available in a Binder environment, accessible at the above GitHub repository.

genetics↗

bioRxiv2018

SV-plaudit: A cloud-based framework for manually curating thousands of structural variants

SV-plaudit is a framework for rapidly curating structural variant (SVs) predictions. For each SV, we generate an image that visualizes the coverage and alignment signals from a set of samples. Images are uploaded to our cloud framework where users assess the quality of each image using a client-side web application. Reports can then be generated as a tab-delimited file or annotated VCF. As a proof of principle, nine researchers collaborated for one hour to evaluate 1,350 SVs each. We anticipate that SV-plaudit will become a standard step in variant calling pipelines and the crowd-sourced curation of other biological results.\n\nCode available at https://github.com/jbelyeu/SV-plaudit\n\nDemonstration video available at https://www.youtube.com/watch?v=ono8kHMKxDs

bioinformatics↗

bioRxiv2018

Long read sequencing reveals poxvirus evolution through rapid homogenization of gene arrays

Large DNA viruses rapidly evolve to defeat host defenses. Poxvirus adaptation can involve combinations of recombination-driven gene copy number variation and beneficial single nucleotide variants (SNVs) at the same locus, yet how these distinct mechanisms of genetic diversification might simultaneously facilitate adaptation to immune blocks is unknown. We performed experimental evolution with a vaccinia virus population harboring a SNV in a gene actively undergoing copy number amplification. Comparisons of virus genomes using the Oxford Nanopore Technologies sequencing platform allowed us to phase SNVs within large gene copy arrays for the first time, and uncovered a mechanism of adaptive SNV homogenization reminiscent of gene conversion, which is actively driven by selection. Our work reveals a new mechanism for the fluid gain of beneficial mutations in genetic regions undergoing active recombination in viruses, and illustrates the value of long read sequencing technologies for investigating complex genome dynamics in diverse biological systems.

evolutionary biology↗

bioRxiv2017

Nanopore sequencing and assembly of a human genome with ultra-long reads

Nanopore sequencing is a promising technique for genome sequencing due to its portability, ability to sequence long reads from single molecules, and to simultaneously assay DNA methylation. However until recently nanopore sequencing has been mainly applied to small genomes, due to the limited output attainable. We present nanopore sequencing and assembly of the GM12878 Utah/Ceph human reference genome generated using the Oxford Nanopore MinION and R9.4 version chemistry. We generated 91.2 Gb of sequence data ([~]30x theoretical coverage) from 39 flowcells. De novo assembly yielded a highly complete and contiguous assembly (NG50 [~]3Mb). We observed considerable variability in homopolymeric tract resolution between different basecallers. The data permitted sensitive detection of both large structural variants and epigenetic modifications. Further we developed a new approach exploiting the long-read capability of this system and found that adding an additional 5x-coverage of ultra-long reads (read N50 of 99.7kb) more than doubled the assembly contiguity. Modelling the repeat structure of the human genome predicts extraordinarily contiguous assemblies may be possible using nanopore reads alone. Portable de novo sequencing of human genomes may be important for rapid point-of-care diagnosis of rare genetic diseases and cancer, and monitoring of cancer progression. The complete dataset including raw signal is available as an Amazon Web Services Open Dataset at: https://github.com/nanopore-wgs-consortium/NA12878.

genomics↗