Publication from the lab in Genome Research

posted Mar 7, 2017, 9:40 AM by Alexej Abyzov   [ updated Mar 7, 2017, 9:42 AM ]

One thousand somatic SNVs per skin fibroblast cell set baseline of mosaic mutational load with patterns that suggest proliferative origin
Few studies have been conducted to understand post-zygotic accumulation of mutations in cells of the healthy human body. We reprogrammed 32 skin fibroblast cells from families of donors into human induced pluripotent stem cell (hiPSC) lines. The clonal nature of hiPSC lines allows a high-resolution analysis of the genomes of the founder fibroblast cells without being confounded by the artifacts of single cell whole genome amplification. We estimate that on average a fibroblast cell in children has 1,035 mostly benign mosaic SNVs. On average, 235 SNVs could be directly confirmed in the original fibroblast population by ultra-deep sequencing, down to an allele frequency (AF) of 0.1%. More sensitive droplet digital PCR experiments confirmed more SNVs as mosaic with AF as low as 0.01%, suggesting that 1,035 mosaic SNVs per fibroblast cell is the true average. Similar analyses in adults revealed no significant increase in the number of SNVs per cell, suggesting that a major fraction of mosaic SNVs in fibroblasts arises during development. Mosaic SNVs were distributed uniformly across the genome and were enriched in a mutational signature previously observed in cancers and in de novo variants and which, we hypothesize, is a hallmark of normal cell proliferation. Finally, AF distribution of mosaic SNVs had distinct narrow peaks, which could be a characteristic of clonal cell selection, clonal expansion, or both. These findings reveal a large degree of somatic mosaicism in healthy human tissues, link de novo and cancer mutations to somatic mosaicism and couple somatic mosaicism with cell proliferation.

Publication from the lab in Genome Research

posted May 29, 2016, 5:47 PM by Taejeong Bae   [ updated May 29, 2016, 5:47 PM ]

Elevated variant density around SVs breakpoints in germline lineage lends support to error prone replication hypothesis.
Copy number variants (CNVs) are a class of structural variants that may involve complex genomic rearrangements (CGRs) and that are hypothesized to have additional mutations around their breakpoints. Understanding the mechanisms underlying CNV formation is fundamental for understanding the repair and mutation mechanisms in cells, thereby shedding light on evolution, genomic disorders, cancer, and complex human traits. In this study, we employ data from the 1000 Genomes Project, to analyze hundreds of loci harboring heterozygous germline deletions in the subjects NA12878 and NA19240. By utilizing synthetic long-read data (longer than 2 kbp) in combination with high coverage short-read data and, in parallel, by comparing with parental genomes, we interrogated the phasing of these deletions with the flanking tens of thousands of heterozygous SNPs and indels. We found, that the density of SNPs/indels flanking the breakpoints of deletions (in-phase variants) is approximately twice as high as the corresponding density for the variants on the haplotype without deletion (out-of-phase variants). This fold-change was even larger, for the subset of deletions with signatures of replication-based mechanism of formation. The allele frequency (AF) spectrum for deletions is enriched for rare events; and the AF spectrum for in-phase SNPs is shifted towards this deletion spectrum, thus offering evidence consistent with the concomitance of the in-phase SNPs/indels with the deletion events. These findings, therefore, lend support to the hypothesis that the mutational mechanisms underlying CNV formation are error prone. Our results could also be relevant for resolving mutation rate discrepancies in human and to explain kataegis.

New lab member!!!

posted Feb 17, 2016, 11:09 PM by Alexej Abyzov   [ updated Feb 17, 2016, 11:10 PM ]

Postdoctoral fellow Tanmoy Roychowdhury, Ph.D. joined the lab on February 8, 2016. Welcome!

Publication from the lab in Nature

posted Oct 1, 2015, 10:51 PM by Alexej Abyzov   [ updated Oct 1, 2015, 10:52 PM ]

Our collaborative work within the framework of the 1000 Genomes Project on discovery and analysis of genome structural variants has been published in Nature.

An integrated map of structural variation in 2,504 human genomes
Structural variants are implicated in numerous diseases and make up the majority of varying nucleotides among human genomes. Here we describe an integrated set of eight structural variant classes comprising both balanced and unbalanced variants, which we constructed using short-read DNA sequencing data and statistically phased onto haplotype blocks in 26 human populations. Analysing this set, we identify numerous gene-intersecting structural variants exhibiting population stratification and describe naturally occurring homozygous gene knockouts that suggest the dispensability of a variety of human genes. We demonstrate that structural variants are enriched on haplotypes identified by genome-wide association studies and exhibit enrichment for expression quantitative trait loci. Additionally, we uncover appreciable levels of structural variant complexity at different scales, including genic loci subject to clusters of repeated rearrangement and complex structural variants with multiple breakpoints likely to have formed through individual mutational events. Our catalogue will enhance future studies into structural variant demography, functional impact and disease association.

Publication from the lab in Cell

posted Jul 18, 2015, 7:18 AM by Alexej Abyzov   [ updated Jul 18, 2015, 7:22 AM ]

Our collaborative work with Vaccarino lab on iPSC-derived telencephalic organoids has been published in Cell. These organoids, also called "miniature brains" or "mini brains", reflect human midfetal telencephalic development and were developed to model autism.

FOXG1-Dependent Dysregulation of GABA/Glutamate Neuron Differentiation in Autism Spectrum Disorders
Autism spectrum disorder (ASD) is a disorder of brain development. Most cases lack a clear etiology or genetic basis, and the difficulty of re-enacting human brain development has precluded understanding of ASD pathophysiology. Here we use three-dimensional neural cultures (organoids) derived from induced pluripotent stem cells (iPSCs) to investigate neurodevelopmental alterations in individuals with severe idiopathic ASD. While no known underlying genomic mutation could be identified, transcriptome and gene network analyses revealed upregulation of genes involved in cell proliferation, neuronal differentiation, and synaptic assembly. ASD-derived organoids exhibit an accelerated cell cycle and overproduction of GABAergic inhibitory neurons. Using RNA interference, we show that overexpression of the transcription factor FOXG1 is responsible for the overproduction of GABAergic neurons. Altered expression of gene network modules and FOXG1 are positively correlated with symptom severity. Our data suggest that a shift toward GABAergic neuron fate caused by FOXG1 is a developmental precursor of ASD.

Publication from the lab in Nature Communications

posted Jun 2, 2015, 10:58 AM by Alexej Abyzov   [ updated Jun 2, 2015, 3:55 PM ]

Analysis of deletion breakpoints from 1,092 humans reveals details of mutation mechanisms
Investigating genomic structural variants at basepair resolution is crucial for understanding their formation mechanisms. We identify and analyse 8,943 deletion breakpoints in 1,092 samples from the 1000 Genomes Project. We find breakpoints have more nearby SNPs and indels than the genomic average, likely a consequence of relaxed selection. By investigating the correlation of breakpoints with DNA methylation, Hi–C interactions, and histone marks and the substitution patterns of nucleotides near them, we find that breakpoints with the signature of non-allelic homologous recombination (NAHR) are associated with open chromatin. We hypothesize that some NAHR deletions occur without DNA replication and cell division, in embryonic and germline cells. In contrast, breakpoints associated with non-homologous (NH) mechanisms often have sequence microinsertions, templated from later replicating genomic sites, spaced at two characteristic distances from the breakpoint. These microinsertions are consistent with template-switching events and suggest a particular spatiotemporal configuration for DNA during the events.

Discovery of the year ... we made it!

posted Feb 25, 2015, 5:42 PM by Alexej Abyzov

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