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McDonald,
John F.
McDonald,
John F.
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ItemEvolutionary rates and patterns for human transcription factor binding sites derived from repetitive DNA(Georgia Institute of Technology, 2008-05-17) Polavarapu, Nalini ; Mariño-Ramírez, Leonardo ; Landsman, David ; McDonald, John F. ; Jordan, I. KingBackground The majority of human non-protein-coding DNA is made up of repetitive sequences, mainly transposable elements (TEs). It is becoming increasingly apparent that many of these repetitive DNA sequence elements encode gene regulatory functions. This fact has important evolutionary implications, since repetitive DNA is the most dynamic part of the genome. We set out to assess the evolutionary rate and pattern of experimentally characterized human transcription factor binding sites (TFBS) that are derived from repetitive versus non-repetitive DNA to test whether repeat-derived TFBS are in fact rapidly evolving. We also evaluated the position-specific patterns of variation among TFBS to look for signs of functional constraint on TFBS derived from repetitive and non-repetitive DNA. Results We found numerous experimentally characterized TFBS in the human genome, 7–10% of all mapped sites, which are derived from repetitive DNA sequences including simple sequence repeats (SSRs) and TEs. TE-derived TFBS sequences are far less conserved between species than TFBS derived from SSRs and non-repetitive DNA. Despite their rapid evolution, several lines of evidence indicate that TE-derived TFBS are functionally constrained. First of all, ancient TE families, such as MIR and L2, are enriched for TFBS relative to younger families like Alu and L1. Secondly, functionally important positions in TE-derived TFBS, specifically those residues thought to physically interact with their cognate protein binding factors (TF), are more evolutionarily conserved than adjacent TFBS positions. Finally, TE-derived TFBS show position-specific patterns of sequence variation that are highly distinct from random patterns and similar to the variation seen for non-repeat derived sequences of the same TFBS. Conclusion The abundance of experimentally characterized human TFBS that are derived from repetitive DNA speaks to the substantial regulatory effects that this class of sequence has on the human genome. The unique evolutionary properties of repeat-derived TFBS are perhaps even more intriguing. TE-derived TFBS in particular, while clearly functionally constrained, evolve extremely rapidly relative to non-repeat derived sites. Such rapidly evolving TFBS are likely to confer species-specific regulatory phenotypes, i.e. divergent expression patterns, on the human evolutionary lineage. This result has practical implications with respect to the widespread use of evolutionary conservation as a surrogate for functionally relevant non-coding DNA. Most TE-derived TFBS would be missed using the kinds of sequence conservation-based screens, such as phylogenetic footprinting, that are used to help characterize non-coding DNA. Thus, the very TFBS that are most likely to yield human-specific characteristics will be neglected by the comparative genomic techniques that are currently de rigeur for the identification of novel regulatory sites.
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ItemEndogenous retroviruses of the chicken genome(Georgia Institute of Technology, 2008-03-24) Huda, Ahsan ; Polavarapu, Nalini ; Jordan, I. King ; McDonald, John F.We analyzed the chicken (Gallus gallus) genome sequence to search for previously uncharacterized endogenous retrovirus (ERV) sequences using ab initio and combined evidence approaches. We discovered 11 novel families of ERVs that occupy more than 21 million base pairs, approximately 2%, of the chicken genome. These novel families include a number of recently active full-length elements possessing identical long terminal repeats (LTRs) as well as intact gag and pol open reading frames. The abundance and diversity of chicken ERVs we discovered underscore the utility of an approach that combines multiple methods for the identification of interspersed repeats in vertebrate genomes.
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ItemExonization of the LTR transposable elements in human genome(Georgia Institute of Technology, 2007-08-28) Piriyapongsa, Jittima ; Polavarapu, Nalini ; Borodovsky, Mark ; McDonald, John F.Background: Retrotransposons have been shown to contribute to evolution of both structure and regulation of protein coding genes. It has been postulated that the primary mechanism by which retrotransposons contribute to structural gene evolution is through insertion into an intron or a gene flanking region, and subsequent incorporation into an exon. Results: We found that Long Terminal Repeat (LTR) retrotransposons are associated with 1,057 human genes (5.8%). In 256 cases LTR retrotransposons were observed in protein-coding regions, while 50 distinct protein coding exons in 45 genes were comprised exclusively of LTR RetroTransposon Sequence (LRTS). We go on to reconstruct the evolutionary history of an alternatively spliced exon of the Interleukin 22 receptor, alpha 2 gene (IL22RA2) derived from a sequence of retrotransposon of the Mammalian apparent LTR retrotransposons (MaLR) family. Sequencing and analysis of the homologous regions of genomes of several primates indicate that the LTR retrotransposon was inserted into the IL22RA2 gene at least prior to the divergence of Apes and Old World monkeys from a common ancestor (~25 MYA). We hypothesize that the recruitment of the part of LTR as a novel exon in great ape species occurred prior to the divergence of orangutans and humans from a common ancestor (~14 MYA) as a result of a single mutation in the proto-splice site. Conclusion: Our analysis of LRTS exonization events has shown that the patterns of LRTS distribution in human exons support the hypothesis that LRTS played a significant role in human gene evolution by providing cis-regulatory sequences; direct incorporation of LTR sequences into protein coding regions was observed less frequently. Combination of computational and experimental approaches used for tracing the history of the LTR exonization process of IL22RA2 gene presents a promising strategy that could facilitate further studies of transposon initiated gene evolution.
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ItemIdentification, characterization and comparative genomics of chimpanzee endogenous retroviruses(Georgia Institute of Technology, 2006-06-26) Polavarapu, Nalini ; Bowen, Nathan J. ; McDonald, John F.Background: Retrotransposons, the most abundant and widespread class of eukaryotic transposable elements, are believed to play a significant role in mutation and disease and to have contributed significantly to the evolution of genome structure and function. The recent sequencing of the chimpanzee genome is providing an unprecedented opportunity to study the functional significance of these elements in two closely related primate species and to better evaluate their role in primate evolution. Results: We report here that the chimpanzee genome contains at least 42 separate families of endogenous retroviruses, nine of which were not previously identified. All but two (CERV 1/ PTERV1 and CERV 2) of the 42 families of chimpanzee endogenous retroviruses were found to have orthologs in humans. Molecular analysis (PCR and Southern hybridization) of CERV 2 elements demonstrates that this family is present in chimpanzee, bonobo, gorilla and old-world monkeys but absent in human, orangutan and new-world monkeys. A survey of endogenous retroviral positional variation between chimpanzees and humans determined that approximately 7% of all chimpanzee-human INDEL variation is associated with endogenous retroviral sequences. Conclusion: Nine families of chimpanzee endogenous retroviruses have been transpositionally active since chimpanzees and humans diverged from a common ancestor. Seven of these transpositionally active families have orthologs in humans, one of which has also been transpositionally active in humans since the human-chimpanzee divergence about six million years ago. Comparative analyses of orthologous regions of the human and chimpanzee genomes have revealed that a significant portion of INDEL variation between chimpanzees and humans is attributable to endogenous retroviruses and may be of evolutionary significance.