Genetic characterization in symptomatic female DMD carriers: lack of relationship between X-inactivation, transcriptional DMD allele balancing and phenotype
1 Section of Medical Genetics, Department of Experimental and Diagnostic Medicine, University of Ferrara, Ferrara, Italy
2 Istituto Di Genetica Molecolare-CNR, Istituto Ortopedico Rizzoli, Bologna, Italy
3 Department of Laboratory Medicine, Unit of Molecular Medicine, Bambino Gesù Hospital, Rome, Italy
4 Department of Child Neurology and Psychiatry, Catholic University, Rome, Italy
5 Department of Neuroscience, Neurological Clinic, University of Pisa, Pisa, Italy
6 Laboratory of Medical Genetics, I.R.C.C.S. Foundation Ca’ Granda, Maggiore Hospital, Policlinico, Milan, Italy
7 Child Neurology and Psychiatry Unit, Maggiore Hospital, Bologna, Italy
8 Neuromuscular Center, S.G. Battista Hospital, University of Turin, Turin, Italy
9 Division of Neurology, Department of Neuroscience, Civile Hospital Santa Maria della Misericordia, Rovigo, Italy
BMC Medical Genetics 2012, 13:73 doi:10.1186/1471-2350-13-73Published: 16 August 2012
Although Duchenne and Becker muscular dystrophies, X-linked recessive myopathies, predominantly affect males, a clinically significant proportion of females manifesting symptoms have also been reported. They represent an heterogeneous group characterized by variable degrees of muscle weakness and/or cardiac involvement. Though preferential inactivation of the normal X chromosome has long been considered the principal mechanism behind disease manifestation in these females, supporting evidence is controversial.
Eighteen females showing a mosaic pattern of dystrophin expression on muscle biopsy were recruited and classified as symptomatic (7) or asymptomatic (11), based on the presence or absence of muscle weakness. The causative DMD gene mutations were identified in all cases, and the X-inactivation pattern was assessed in muscle DNA. Transcriptional analysis in muscles was performed in all females, and relative quantification of wild-type and mutated transcripts was also performed in 9 carriers. Dystrophin protein was quantified by immunoblotting in 2 females.
The study highlighted a lack of relationship between dystrophic phenotype and X-inactivation pattern in females; skewed X-inactivation was found in 2 out of 6 symptomatic carriers and in 5 out of 11 asymptomatic carriers. All females were characterized by biallelic transcription, but no association was found between X-inactivation pattern and allele transcriptional balancing. Either a prevalence of wild-type transcript or equal proportions of wild-type and mutated RNAs was observed in both symptomatic and asymptomatic females. Moreover, very similar levels of total and wild-type transcripts were identified in the two groups of carriers.
This is the first study deeply exploring the DMD transcriptional behaviour in a cohort of female carriers. Notably, no relationship between X-inactivation pattern and transcriptional behaviour of DMD gene was observed, suggesting that the two mechanisms are regulated independently. Moreover, neither the total DMD transcript level, nor the relative proportion of the wild-type transcript do correlate with the symptomatic phenotype.