[Show abstract][Hide abstract] ABSTRACT: Genome-wide association studies suggest that common genetic variants explain only a modest fraction of heritable risk for common diseases, raising the question of whether rare variants account for a significant fraction of unexplained heritability. Although DNA sequencing costs have fallen markedly, they remain far from what is necessary for rare and novel variants to be routinely identified at a genome-wide scale in large cohorts. We have therefore sought to develop second-generation methods for targeted sequencing of all protein-coding regions ('exomes'), to reduce costs while enriching for discovery of highly penetrant variants. Here we report on the targeted capture and massively parallel sequencing of the exomes of 12 humans. These include eight HapMap individuals representing three populations, and four unrelated individuals with a rare dominantly inherited disorder, Freeman-Sheldon syndrome (FSS). We demonstrate the sensitive and specific identification of rare and common variants in over 300 megabases of coding sequence. Using FSS as a proof-of-concept, we show that candidate genes for Mendelian disorders can be identified by exome sequencing of a small number of unrelated, affected individuals. This strategy may be extendable to diseases with more complex genetics through larger sample sizes and appropriate weighting of non-synonymous variants by predicted functional impact.
[Show abstract][Hide abstract] ABSTRACT: Lp(a) levels have long been recognized as a potential risk factor for coronary heart disease that is almost completely under genetic control. Much of the genetics impacting Lp(a) levels has been attributed to the highly polymorphic LPA kringle IV-2 copy number variant, and most of the variance in Lp(a) levels in populations of European-descent is inversely correlated with kringle IV copy number. However, less of the variance is explained in African-descent populations for the same structural variation. African-descent populations have, on average, higher levels of Lp(a), suggesting other genetic factors contribute to Lp(a) level variability across populations.
To identify potential cis-acting factors, we re-sequenced the gene LPA for single nucleotide polymorphism (SNP) discovery in 23 European-Americans and 24 African-Americans. We also re- sequenced the neighboring gene plasminogen (PLG) and genotyped the kringle IV copy number variant in the same reference samples.
These data are the most comprehensive description of sequence variation in LPA and its relationship with the kringle IV copy number variant. With these data, we demonstrate that only a fraction of LPA sequence diversity has been previously documented. Also, we identify several high frequency SNPs present in the African-American sample but absent in the European-American sample. Finally, we show that SNPs within PLG are not in linkage disequilibrium with SNPs in LPA, and we show that kringle IV copy number variation is not in linkage disequilibrium with either LPA or PLG SNPs.
Together, these data suggest that LPA SNPs could independently contribute to Lp(a) levels in the general population.