The pattern of X inactivation in lymphocyte DNA was investigated in 107 Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) carriers (102 asymptomatic and 5 manifesting carriers) and 117 normal female controls of different ages, with the aim: a) to analyze the pattern of X inactivation in blood DNA of a large number of DMD/BMD carriers as compared to normal female controls; b) to determine if there is a decrease in serum creatine kinase (CK) levels with age in obligate DMD/BMD carriers; c) to determine if there is a correlation between X-chromosome inactivation and serum CK among asymptomatic DMD/BMD carriers of different ages or with different clinical manifestations in symptomatic carriers.
A high proportion of females showed extremely skewed X inactivation (>90% of one X preferentially inactivated), which was almost the same among carriers and normal controls (19 and 24%, respectively). The mean serum CK was significantly greater among young (<20 years old) than adult (>20 years old) DMD/BMD carriers and it decreased significantly until age 20 with an apparent stabilization afterwards. No statistically significant correlation was found between the proportion of active X DMD in blood and serum CK activity in DMD/BMD carriers although it was higher among those less than 20 years old. Our observations suggest that highly skewed X-chromosome pattern in blood (with preferential inactivation of the X N chromosome) is not enough to predict that a young DMD carrier will develop muscular weakness. Am.