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May 1, 2013 (Vol. 33, No. 9)

Comparative Genomics: Insights into Disease Susceptibility

  • Comparative genomics is akin to comparing notes across different species. By comparing features at the genetic level—of completely sequenced (or nearly so) genomes—comparative sequence analyses facilitate the functional annotation of genomes and elucidate whole-genome approaches across species.

    Recent studies show the difficulty of detecting a disease-causing mutation within a large volume of rare variants in the human population, despite a large number of genomes available for study. Of the 7,098 complete mitochondrial genomes, researchers identified 6,110 single nucleotide variants (SNVs), but could not detect more than 18 known pathogenic mutations.

    Comparative genomics is a powerful approach for identifying and interpreting functional elements in the human genome. “Even when the human genome was first sequenced, it was unclear how many protein-coding genes were in it,” says Kerstin Lindblad-Toh, Ph.D., scientific director of vertebrate genome biology at the Broad Institute. “With the sequencing of the second mammalian genome, the mouse genome, it was apparent that 1.3% of genome coded for protein, but that there were many more regulatory elements to find.”

    Dr. Lindblad-Toh and colleagues have already reported the sequencing and comparative analysis of 29 eutherian genomes. The high-resolution map of human evolutionary constraint using these mammalian genomes suggests that “at least 6.5% of the human genome has undergone purifying selection.”

    “Overlap with disease-associated variants indicates that our findings will be relevant for studies of human biology, health, and disease,” according to Dr. Lindblad-Toh.Researchers have located constrained elements covering almost 4.2% of the genome. Constrained elements are regions of high sequence similarity in the genome across many species.

    As Dr. Lindblad-Toh explains, the genomewide studies involving single nucleotide polymorphisms (SNPs) and their association with disease suggest that the 29 mammalian vgenomes will help find disease mutations in the human genome. “More than 85% of signals from genome-wide association studies of human disease fall outside protein-coding genes.”

    Dr. Lindblad-Toh and colleagues have employed evolutionary signatures and comparisons with experimental datasets to assign candidate functions for nearly 60% of constrained bases, which “reveal a small number of new coding exons, candidate stop codon read-through events, and over 10,000 regions of overlapping synonymous constraint within protein-coding exons.”

    They found “220 candidate RNA structural families, and nearly a million elements overlapping potential promoter, enhancer, and insulator regions.” They have also reported specific amino acids that are positively selected for, 280,000 noncoding elements exapted from mobile elements, and over 1,000 primate- and human-accelerated elements.

    Given that 150–200 mammalian genome sequences are necessary to achieve single-base resolution for functional constraint, Dr. Lindblad-Toh’s team has undertaken sequencing of more mammals, including the squirrel monkey, the manatee, the chinchilla, the naked mole rat, the star-nosed mole, and the white rhinoceros. “The main goal is to decipher each and every single base pair in terms of its functional role and identify novel functional elements of significance to human health and disease.”

    In a different study, investigators at the Broad Institute of MIT and Harvard used full-genome sequence variation from the 1000 Genomes Project and the comprehensive multiple signals test to probe into 412 candidate selection signals. Further delving into the functional annotation, protein structure modeling, epigenetics, and association studies, they have identified and annotated candidate variants to develop a catalog for experimental follow-up.

    Among other variants that include 35 high-scoring nonsynonymous variants and 59 variants associated with expression levels of a nearby coding gene or lincRNA, the researchers identified several mutations linked with susceptibility to infectious disease and other phenotypes.

    They have experimentally characterized one candidate nonsynonymous variant in Toll-like receptor 5. Changes in NF-κB signaling are mediated by this variant in response to bacterial flagellin. As Dr. Lindblad-Toh comments, “comparing the hundreds of different mammalian genomes helps in developing models of human disease based on genetic studies and understanding of genome function.”

    Rhesus macaques (Macaca mulatta) are proven models for the study of diseases such as HIV/AIDS, cardiovascular disease, obesity and diabetes, asthma, addiction, and age-related diseases of public health significance. “Our research is focused on the development of genetically defined nonhuman primate disease models by characterizing genomic variation between and within nonhuman primate species,” says Betsy Ferguson, Ph.D., associate professor at the neuroscience division of Oregon National Primate Research Center, Oregon Health & Science University.

    Dr. Ferguson and colleagues have undertaken comparative analysis of the Indian-origin and Chinese-origin rhesus macaque genomes. The findings reveal variants that suggest currently unrecognized disease susceptibilities in the different populations.

    “While they have certain common variants, there may be unique differences that offer an advantage to studying Chinese macaques, e.g., in HIV/AIDS, in that the similarities to human disease progression are much closer,” explains Dr. Ferguson. In contrast, “the Indian rhesus might be more akin to a shorter course of the human AIDS.” On the other hand, the “Chinese species show traits related to a range of anxiety and stress behaviors that mirror those seen in humans.”

    For genome variant detection, the investigators used Illumina’s HiSeq platform to achieve deep sequence coverage (30–45x) in an initial six Indian-origin and six Chinese-origin rhesus macaque genomes. BWA aligner methodologies were used to align sequence reads to the rhesus macaque reference genome.

    The SAMtools software suite was used to check mapped reads for duplicates and SNVs. “We identified in excess of 19 million SNPs in the rhesus macaque populations,” Dr. Ferguson says. The SNP allele-frequency, potential population-specificity, and the predicted functional effects of the variants are used to identify population differences in risks for disease.

    The researchers have successfully evaluated a human exome capture design for the selective enrichment of exonic regions of nonhuman primates including nine chimpanzees, two cynomolgus macaques, and eight Japanese macaques. Their findings indicate that the human exon-capture methods offered an attractive, cost-effective approach for the comparative analysis of nonhuman primate genomes, including gene-based DNA variant discovery. It facilitated efficient enrichment of nonhuman primate gene regions.

    The investigators captured over 91% of the target regions in the nonhuman primate samples, although with decreasing specificity as evolutionary divergence from humans increased. They have identified both intraspecific and interspecific DNA variants, validating 85.4% of 41 randomly selected SNPs using Sanger-based sequencing techniques. The findings indicate that a majority (54.6–77.3%) of the variants resulted in a change of three base pairs.

    The comparative analysis of functional and nonfunctional variation in rhesus macaque serves as a model for human biology. It sheds light on how variation in population history and size altered patterns and levels of sequence variation in primates. Comparative analyses suggest that rhesus macaque has nearly three times higher SNP density and average nucleotide diversity than humans.

    The highest SNP density and average nucleotide diversity were seen in intergenic regions, while the lowest density was observed in the CDS (coding sequences) in both humans and macaques. Indeed, rhesus macaques are almost three times as diverse as the human but more closely equivalent in damaging variation.

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