Exonic Variation and Its Clinical Impact in 7221 Old Order Amish

The Amish of Lancaster County, PA has been the focus of genetic studies for many years due to its demographic history and unique genetic makeup that includes a historical bottleneck event and subsequent genetic drift, resulting in a marked decrease in genetic diversity and increased frequency of some variants that have substantially shaped the health of the community. To characterize the coding variation in the Amish genome, we sequenced the exomes of 7221 adult community members, and in this report, we contrast genetic diversity between the Amish and Europeans from the UK Biobank. Exome sequences of 7221 Amish contained only 14% as many variants as the same number of UKB participants. This reduced genetic diversity has substantial clinical implications. We identified pathogenic (P) and likely pathogenic (LP) variants from ClinVar and a population-specific genetic screening panel and found that most of the variants present in the Amish were highly enriched, resulting in 5.2% of Amish individuals being homozygous for a recessive P/LP variant and 25.6% being heterozygous for at least one dominant P/LP variant. In 43.6% of the 2141 Amish spouse-pairs in our sample, at least one spouse was heterozygous for a P/LP dominant variant, and 24.3% of couples were autosomal recessive disease carrier couples, meaning that each of their children was at ~25% risk of inheriting two copies of that variant. Gene discovery efforts in other founder communities will likely uncover distinct P (and beneficial) variants impacting the health of these communities, with implications for all of human health.