Explanations for the discrepancy between variant frequency and homozygous disease occurrence: Lessons from Ashkenazi Jewish data.

Ample data on recessive disorders among Ashkenazi Jews has been gathered and published through the years. The opportunity to integrate molecular records analyzed in actual affected individuals with data derived from population-documented frequencies enables to compare these figures. We reviewed assumed pathogenic variants reported among patients in the Israeli medical genetic database (IMGD) with a carrier frequency of 1% or more among Ashkenazi Jews in gnomAD. Among the 60 assumed pathogenic variants recorded in IMGD, 15 (25%) had either a disease incidence considerably lower than expected by the calculated carrier frequency (12 variants), or the variant was not characterized in Ashkenazi Jewish patients (three variants). Possible explanations for the rarity or absence of affected individuals despite high carrier frequency include embryonic lethality, clinical variability, and incomplete and age-related penetrance, in addition to the existence of additional assumed pathogenic variants on the founder haplotype, hypomorphic variants or digenic inheritance. The discrepancy in actual versus expected number of patients calls for caution upon designing and choosing targeted genes and recessive mutations for carrier screening.