An Offhand Hypothesis

This seems to be a source of speculation among narcolepsy researchers:

African-Americans with the DQB1*0602 gene have cataplexy at a lower rate than caucasians.
Japanese people with the DQB1*0602 gene have cataplexy at a higher rate than caucasians.

I think this is explained by the interactive nature of the pathology.
African-Americans probably have better viral immunity. Viral immunity is known to correlate with APOE subtype, and the APOE genes also correlate with skin pigmentation.
Japanese people have higher rates of alcoholism than caucasians. Impaired alcohol dehydrogenase function is especially common.