Clade Distillation for Genome-wide Association Studies

Testing inferred haplotype genealogies for association with phenotypes has been a longstanding goal in human genetics with several underlying challenges. A key advantage of these methods is the potential to detect association signals caused by allelic heterogeneity — when multiple causal variants modulate a phenotype — in both coding and noncoding regions. Recent scalable methods for inferring locus-specific genealogical trees along the genome, or representations thereof, have made substantial progress towards this goal; however, the problem of testing these trees for association with phenotypes has remained unsolved due to the growth in the number of clades with increasing sample size. To address this issue, we introduce several practical improvements to the kalis ancestry inference engine, including a general optimal checkpointing algorithm for decoding hidden Markov models, thereby enabling efficient genome-wide analyses. We then propose ‘LOCATER’, a powerful new procedure based on the recently proposed Stable Distillation framework, to test local tree representations for trait association. Although LOCATER is demonstrated here in conjunction with kalis, it may be used for testing output from any ancestry inference engine, regardless of whether such engines return discrete tree structures, relatedness matrices, or some combination of the two at each locus. Using simulated quantitative phenotypes, our results indicate that LOCATER achieves substantial power gains over traditional single marker testing and window-based testing in cases of allelic heterogeneity, while also improving causal region localization relative to single marker tests. These findings suggest that genealogy based association testing will be a fruitful approach for gene discovery, especially for signals driven by multiple ultra-rare variants.