Twin studies in narcolepsy

The genetic basis of narcolepsy is reflected by the strong association to human leukocyte antigen DR2 (most specifically to DQB1-0602) and the occasional familial occurrence, with several modes of transmission. At present, 12 monozygotic pairs with at least one affected twin have been reported. Of the three pairs considered concordant, the only well-documented pair is DR2 negative. Of the nine discordant pairs five are well documented, and all of these are DR2 positive. We performed a questionnaire study using a validated measure of narcoleptic symptoms (the Ullanlinna narcolepsy scale or UNS). The questionnaire was sent to 2,191 monozygotic twin pairs included in the Finnish Twin Cohort. In 225 pairs neither of the twins responded. In 1,550 pairs both twins responded, but in the answers of 228 pairs there were some missing data concerning the UNS items. Not a single case suggestive of narcolepsy was found. Narcolepsy in monozygotic twins is very rare. These facts support the hypothesis of a multifactorial etiology with a strong influence of nongenetic environmental factors.