A Recombination Hotspot in a Schizophrenia-Associated Region of GABRB2

Background: Schizophrenia is a major disorder with complex genetic mechanisms. Earlier, population genetic studies revealed the occurrence of strong positive selection in the GABRB2 gene encoding the β2 subunit of GABAA receptors, within a segment of 3,551 bp harboring twenty-nine single nucleotide polymorphisms (SNPs) and containing schizophrenia-associated SNPs and haplotypes. Methodology/Principal Findings:In the present study, the possible occurrence of recombination in this 'S1-S29' segment was assessed. The occurrence of hotspot recombination was indicated by high resolution recombination rate estimation, haplotype diversity, abundance of rare haplotypes, recurrent mutations and torsos in haplotype networks, and experimental haplotyping of somatic and sperm DNA. The sub-segment distribution of relative recombination strength, measured by the ratio of haplotype diversity (Hd) over mutation rate (θ), was indicative of a human specific Alu-Yi6 insertion serving as a central recombining sequence facilitating homologous recombination. Local anomalous DNA conformation attributable to the Alu-Yi6 element, as suggested by enhanced DNase I sensitivity and obstruction to DNA sequencing, could be a contributing factor of the increased sequence diversity. Linkage disequilibrium (LD) analysis yielded prominent low LD points that supported ongoing recombination. LD contrast revealed significant dissimilarity between control and schizophrenic cohorts. Among the large array of inferred haplotypes, H26 and H73 were identified to be protective, and H19 and H81 risk-conferring, toward the development of schizophrenia. Conclusions/Significance: The co-occurrence of hotspot recombination and positive selection in the S1-S29 segment of GABRB2 has provided a plausible contribution to the molecular genetics mechanisms for schizophrenia. The present findings therefore suggest that genome regions characterized by the co-occurrence of positive selection and hotspot recombination, two interacting factors both affecting genetic diversity, merit close scrutiny with respect to the etiology of common complex disorders. © 2010 Ng et al

Pathogenicity and selective constraint in the non-coding genome

Gene regulation plays a central role in evolution, organismal development, and disease. Despite the critical importance of gene regulation throughout development, there have been few genetic variants in regulatory elements with large effects that have been robustly associated to disease. In this work, my overarching aim was to gain a better understanding of the contribution of genetic variation in regulatory elements to Mendelian disorders and attempted to approach this problem from three different perspectives. I first sought to assess the contribution of regulatory variation to severe developmental disorders using sequence data from 8,000 affected individuals and their parents and to identify individual elements with a high probability of harbouring pathogenic regulatory elements. Next, I used population genetic models and data from more than 28,000 whole genome sequenced individuals to examine the forces of selection operating on non-coding elements genome-wide. Finally, I conducted a pilot experiment to assay >50,000 different non-coding variants across more than 700 different non-coding elements, including variants observed in patients with developmental disorders in a massively parallel reporter assay (MPRA) and collaborated on an assessment of the impact of patient mutations in eleven different enhancers using mouse transgenesis assays. A few key results from the work are summarised below: - I provide evidence that de novo SNVs in non-coding elements contribute to severe developmental disorders, and estimate that they contribute in 1-3% of cases not harbouring a likely diagnostic coding variant. - These de novo SNVs reside primarily in highly evolutionarily conserved regulatory elements and I estimate that a large fraction of conserved non-coding elements (50-70%) are acting as enhancers and a smaller subset (10-15%) have a function related to alternative splicing. - Statistical modelling of the distribution of variants in developmental disorder patients suggests that a small fraction of bases (maximum likelihood estimate of 3%) within a disease-associated non-coding element are likely pathogenic with high penetrance when mutated. - I develop a new genome-wide mutation rate model that accounts for a variety of germline features including recombination rate, replication timing, sequence context, and histone marks which greatly outperforms models based on sequence-context alone. - I find evidence for widespread purifying selection in the non-coding genome that is correlated with nucleotide-level evolutionary conservation, even when the conserved nucleotides lie within otherwise poorly conserved sequence. - I show that the selective constraint on small insertions and deletions is likely greater than the selective constraint on SNVs. - I present data from a pilot experiment assessing more than 50,000 different non-coding variants in a massively parallel reporter assay conducted in both HeLa and Neuroblastoma cells.Wellcome Trus

Mammalian comparative genomics and epigenomics

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references.The human genome sequence can be thought of as an instruction manual for our species, written and rewritten over more than a billion of years of evolution. Taking a complete inventory of our genome, dissecting its genes and their functional components, and elucidating how these genes are selectively used to establish and maintain cell types with markedly different behaviors, are key challenges of modern biology. In this thesis we present contributions to our understanding of the structure, function and evolution of the human genome. We rely on two complementary approaches. First, we study signatures of evolutionary processes that have acted on the genome using comparative sequence analysis. We generate high quality draft genome sequences of the chimpanzee, the dog and the opossum. These species share a last common ancestor with humans approximately 6 million, 80 million and 140 million years ago, respectively, and therefore provide distinct perspectives on our evolutionary history. We apply computational methods to explore the functional organization of the genome and to identify genes that contribute to shared and species-specific traits. Second, we study how the genome is bound by proteins and packaged into chromatin in distinct cell types. We develop new methods to map protein-DNA interactions and DNA methylation using single-molecule based sequencing technology. We apply these methods to identify new functional sequence elements based on characteristic chromatin signatures, and to explore the relationship between DNA sequence, chromatin and cellular state.by Tarjei Sigurd Mikkelsen.Ph.D

Investigating genome wide patterns of natural selection in eukaryotes

Mutations are the ultimate source of new genetic information and they can be neutral, harmful or beneficial. The ultimate fate of all mutations is either to be lost or to eventually become fixed in a population. In this thesis I investigate genome wide traces of natural selection in eukaryotes. I focus on the most common type of mutations, point mutations, in protein coding genes. I investigated whether there is adaptive evolution in 11 plant species comparisons by applying an extension of the McDonald Kreitman (MK) test and found little evidence of adaptive evolution. However, most of the investigated plant species have low effective population sizes (Ne) and the rate of adaptive evolution is thought to be correlated to Ne. I therefore extended my study using additional data from mammals, drosophilids and yeast to investigate the relationship between the rate of adaptive evolution and Ne. I found a highly significant correlation between the rate of adaptive evolution relative to the rate of neutral evolution (!a) and Ne. It has been proposed that evidence of adaptive evolution can be an artifact of fluctuating selection. I simulated a model of fluctuating selection, in which the average strength of selection acting upon mutations is zero. Under this model adaptive evolution is inferred using MK-type tests. However, the mutations which become fixed are on average positively selected. The signal of adaptive evolution is therefore genuine. Ne can not only vary between species but also across genomes. However, how much variation there is, and whether this affects the efficiency of natural selection, is unknown. I analysed 10 species and show that variation in Ne is widespread. However, this variation is limited, amounting to a few fold variation in Ne between most genomic regions. This is never-the-less sufficient to cause variation in the efficiency of selection

Unravelling the determinants of the rate of adaptive evolution at the molecular level

Ever since Darwin presented natural selection as a driver of evolution, evolutionary biologists have thrived to understand how beneficial mutations shape species adaptation to their environment. Studying adaptation, however, requires an understanding of the complex dynamics between nucleotides, sequences, proteins, organisms, populations, and species. In other words, it requires assessing the interplay of evolutionary processes across systems. Here, I studied adaptation in such a way by exploring the frequency and nature of adaptive mutations within genes, within genomes, and between species. At the intramolecular level, this project revealed that the residue’s solvent accessibility acts as the primary determinant of rates of adaptive substitutions both in animals and in plants, where adaptive mutations are more frequent at the protein surface. These analyses further showed higher rates of adaptation for genes encoding proteins with central cellular functions, which are the ones usually targeted by pathogens during host infection. These findings, therefore, suggested that protein adaptive evolution proceeds through interactions between molecules, particularly at the interspecific level, where host-pathogen coevolution likely plays a central role. By taking a step back and looking at adaptation at different time-scales within the genome, this thesis revealed the role of young genes in adaptive evolution. As these genes are further away from their fitness optimum, these findings suggested that proteins adapt in an “adaptive walk” manner. This project further highlighted that the distribution of adaptive mutations across time follows a pattern of diminishing returns. Looking at an even broader scale by studying adaptation at the species level and considering the effect of intramolecular variation across several animal species, this thesis demonstrated a negative correlation between rates of adaptive substitutions and the effective population size (N_e). Despite the relatively weak signal, these findings contradict initial population genetics theory. Instead, they seem to agree with theoretical expectations at the phenotypic space. In turn, the results regarding negative selection confirm the N_e hypothesis, where the efficiency of selection is stronger in large-N_e species. This effect was well depicted in the differences of the distribution of fitness effects between buried and exposed residues, where the former accumulates comparatively more mild effect mutations in low-N_e species. This project further expanded our findings at the intramolecular level, by revealing the strong influence of the protein’s macromolecular structure on rates of molecular adaptation across several taxa. By assessing the interplay of adaptive mutations across distinct organizational levels, this thesis provided a more profound understanding of rates of adaptive evolution at the molecular level, thus delivering a comprehensive view of the molecular basis of adaptation

Positive Selection within the Schizophrenia-Associated GABA(A) Receptor β(2) Gene

The gamma-aminobutyric acid type-A (GABA(A)) receptor plays a major role in inhibitory neurotransmissions. Intronic SNPs and haplotypes in GABRB2, the gene for GABA(A) receptor β(2) subunit, are associated with schizophrenia and correlated with the expression of two alternatively spliced β(2) isoforms. In the present study, using chimpanzee as an ancestral reference, high frequencies were observed for the derived (D) alleles of the four SNPs rs6556547, rs187269, rs1816071 and rs1816072 in GABRB2, suggesting the occurrence of positive selection for these derived alleles. Coalescence-based simulation showed that the population frequency spectra and the frequencies of H56, the haplotype having all four D alleles, significantly deviated from neutral-evolution expectation in various demographic models. Haplotypes containing the derived allele of rs1816072 displayed significantly less diversity compared to haplotypes containing its ancestral allele, further supporting positive selection. The variations in DD-genotype frequencies in five human populations provided a snapshot of the evolutionary history, which suggested that the positive selections of the D alleles are recent and likely ongoing. The divergence between the DD-genotype profiles of schizophrenic and control samples pointed to the schizophrenia-relevance of positive selections, with the schizophrenic samples showing weakened selections compared to the controls. These DD-genotypes were previously found to increase the expression of β(2), especially its long isoform. Electrophysiological analysis showed that this long β(2) isoform favored by the positive selections is more sensitive than the short isoform to the inhibition of GABA(A) receptor function by energy depletion. These findings represent the first demonstration of positive selection in a schizophrenia-associated gene

Evolutionary genomics : statistical and computational methods

This open access book addresses the challenge of analyzing and understanding the evolutionary dynamics of complex biological systems at the genomic level, and elaborates on some promising strategies that would bring us closer to uncovering of the vital relationships between genotype and phenotype. After a few educational primers, the book continues with sections on sequence homology and alignment, phylogenetic methods to study genome evolution, methodologies for evaluating selective pressures on genomic sequences as well as genomic evolution in light of protein domain architecture and transposable elements, population genomics and other omics, and discussions of current bottlenecks in handling and analyzing genomic data. Written for the highly successful Methods in Molecular Biology series, chapters include the kind of detail and expert implementation advice that lead to the best results. Authoritative and comprehensive, Evolutionary Genomics: Statistical and Computational Methods, Second Edition aims to serve both novices in biology with strong statistics and computational skills, and molecular biologists with a good grasp of standard mathematical concepts, in moving this important field of study forward

CONSIDERING CYTONUCLEAR INTERACTIONS IN THE FACE OF HETEROPLASMY: EVIDENCE FROM DAUCUS CAROTA (APIACEAE), A GYNODIOECIOUS PLANT SPECIES

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