Pervasive multinucleotide mutational events in eukaryotes

Interlocus gene conversion events introduce deleterious mutations into at least 1% of human genes associated with inherited disease

Establishing the molecular basis of DNA mutations that cause inherited disease is of fundamental importance to understanding the origin, nature, and clinical sequelae of genetic disorders in humans. The majority of disease-associated mutations constitute single-base substitutions and short deletions and/or insertions resulting from DNA replication errors and the repair of damaged bases. However, pathological mutations can also be introduced by nonreciprocal recombination events between paralogous sequences, a phenomenon known as interlocus gene conversion (IGC). IGC events have thus far been linked to pathology in more than 20 human genes. However, the large number of duplicated gene sequences in the human genome implies that many more disease-associated mutations could originate via IGC. Here, we have used a genome-wide computational approach to identify disease-associated mutations derived from IGC events. Our approach revealed hundreds of known pathological mutations that could have been caused by IGC. Further, we identified several dozen high-confidence cases of inherited disease mutations resulting from IGC in ∼1% of all genes analyzed. About half of the donor sequences associated with such mutations are functional paralogous genes, suggesting that epistatic interactions or differential expression patterns will determine the impact upon fitness of specific substitutions between duplicated genes. In addition, we identified thousands of hitherto undescribed and potentially deleterious mutations that could arise via IGC. Our findings reveal the extent of the impact of interlocus gene conversion upon the spectrum of human inherited disease.

Heterogeneity of the transition/transversion ratio in Drosophila and Hominidae genomes

Mutation rate varies between sites in the genome. Part of this variation can be explained by well-recognized short nucleotide contexts, but a large component of this variation remains cryptic. We used data on interspecies divergence and intraspecies polymorphism in Drosophila and Hominidae to analyze variation of the average rate of the 12 possible kinds of single-nucleotide mutations and in the transition/transversion ratio κ at single-nucleotide resolution. Both the average mutation rate and κ vary by a factor of ~3 between nucleotide sites. The characteristic scale of variation in κ is up to at least ~30 nucleotides in Drosophila and ~5 nucleotides in Hominidae. Genome segments with locally elevated mutation rates possess lower values of κ; however, a substantial fraction of variation in κ cannot be directly explained by the local mutation rates.

The evolution of multimeric protein assemblages

Although the mechanisms by which complex cellular features evolve constitute one of the great unsolved problems of evolutionary biology, it is clear that the emergence of new protein-protein interactions, often accompanied by the diversification of duplicate genes, is involved. Using information on the levels of protein multimerization in major phylogenetic groups as a guide to the patterns that must be explained and relying on results from population-genetic theory to define the relative plausibility of alternative evolutionary pathways, a framework for understanding the evolution of dimers is developed. The resultant theory demonstrates that the likelihoods of alternative pathways for the emergence of protein complexes depend strongly on the effective population size. Nonetheless, it is equally clear that further advancements in this area will require comparative studies on the fitness consequences of alternative monomeric and dimeric proteins.

Variation in the mutation rate across mammalian genomes

It has been known for many years that the mutation rate varies across the genome. However, only with the advent of large genomic data sets is the full extent of this variation becoming apparent. The mutation rate varies over many different scales, from adjacent sites to whole chromosomes, with the strongest variation seen at the smallest scales. Some of these patterns have clear mechanistic bases, but much of the rate variation remains unexplained, and some of it is deeply perplexing. Variation in the mutation rate has important implications in evolutionary biology and underexplored implications for our understanding of hereditary disease and cancer.