Frequency of the Prader-Willi syndrome in the San-in district, Japan.

Nineteen cases of the Prader-Willi syndrome were collected in the San-in district, in the western part of Japan, and an epidemiological study was carried out. The male to female ratio was 1:1.11. Deletion of chromosome 15q11-13 was found in seven of 16 patients examined by means of high-resolution chromosome banding. The incidence among live-births was estimated to be 6.64 x 10(-5) (1 in 15,060 live-births) in 1980-1989, and the mutation rate was 3.32 x 10(-5) per gene per generation, as determined by a direct method. The prevalence was estimated to be 5.72 x 10(-5) (1 in 17,483) in patients under 15 years old, which was higher than that in other autosomal dominant diseases (tuberous sclerosis and congenital myotonic dystrophy) also studied in the San-in district. A birth-order effect was revealed and a slightly significant difference was found as to the fathers' age between the patients and their normal siblings. The results of these two tests are in accord with the recent discovery that this syndrome is caused by paternal chromosomal aberrations.