dbSNP reports that this variant has 5% allele frequency in a 120 chromosome low-coverage CEU panel (1000 genomes) and 2.6% in a mixed panel of Caucasian & African American samples. Lorenz-Depiereux et al. point to this as suggesting that the variant is a not causal.
The predicted incidence of homozygotes (1 in 400) vs. the incidence of the disease (quite rare) implies that this variant is a highly penetrant causal variant. It may have a modifier or susceptibility effect though (note that the son of the reported family in Lorenz-Depiereux et al. & Rutsch et al. had a more severe effect compared to his father).
Edited in this revision:
The footnote to Table 3 states: “The mutation reported in this paper (c.2320C>T [p.Arg774Cys]) has since been shown to be a polymorphism (rs28933977) with a minor allele frequency of 5%.”
These authors follow up on the homozygous case from Rutsch et al, a Turkish family, and find the patient is also homozygous for E266V. The father is homozygous for the E266V variant as well but has a later onset phenotype, developing an aortic root dissection at the age of 28 yrs.