Dystrophin mRNA expressed in peripheral lymphocytes of individuals with X-linked
Duchenne muscular dystrophy (DMD) has been used as a source material for mutation analysis. Here we present the first report of failure of isolation of nonsense
dystrophin mRNA in lymphocytes but success in skeletal muscle in a female carrier of DMD. The mutation responsible for
dystrophin-negative muscle fibers of the carrier was analysed by direct sequencing of the reverse transcription PCR product of
dystrophin mRNA. In her peripheral lymphocytes, no
nucleotide change was detected in the 14 kb long
mRNA. Remarkably, a novel
nucleotide change of C1682T in exon 12, changing
glutamine codon to stop
codon (Q492X) was found to be present in her skeletal muscle. This change was heterozygous. Analysis of her genomic
DNA disclosed heterozygous C and T
nucleotides at nt 1682, confirming the genomic origin of the
nonsense mutation. Although
dystrophin cDNA prepared from lymphocytes was sequenced again after subcloning, mutation-retaining clone could not be isolated. This lymphocyte-specific disappearance of nonsense
mRNA strongly suggested tissue-specific skewing of X-inactivation. However, both paternal and maternal
dystrophin alleles were shown to be equally expressed in lymphocytes as well as in muscle, indicating no skewing of X-inactivation in lymphocytes. We concluded that the
dystrophin mRNA of the DMD carrier was destabilized in lymphocytes. Our results indicated that analysis of
mRNA in lymphocytes is not enough for exact carrier diagnosis of
Duchenne muscular dystrophy.