Facioscapulohumeral dystrophy (FSHD) is an autosomal-dominant disorder
with a characteristic and distinctive distribution of weakness but a
high degree of variation in the age of onset and rate of progression.
Monozygotic twins provide the opportunity to assess the relative impor
tance of genetic as opposed to nongenetic influences on the course of
disease. We have studied three sets of monozygotic twins with FSHD and
compared the similarity of their deg ree of involvement using quantit
ative studies of individual muscle function. Similar quantitative stud
ies of 59 other subjects with FSHD served as a reference population fo
r contrast with the twin studies. One set of twins was discordant for
FSHD, presumably as a reflection of a postzygotic mutation in the affe
cted twin. The other two sets were concordant and both had evidence of
autosomal dominantly inherited gene rearrangements. Both sets were si
milarly affected in terms of age of onset, overall degree of disabilit
y, and quantitative tests of muscle, but there were major differences
in the symmetry of involvement of specific muscles. Cerebral dominance
was not related to asymmetries of involvement. These data suggest age
of onset and severity are determined by the gene lesion in FSHD. Othe
r factors may influence the frequently encountered asymmetries in FSHD
. (C) 1995 John Wiley and Sons, Inc.