Molecular analysis of 4q35 rearrangements in fascioscapulohumeral muscular dystrophy (FSHD): application to family studies for a correct genetic advice and a reliable prenatal diagnosis of the disease.

In the majority of facioscapulohumeral muscular dystrophy (FSHD) families (about 95%) the genetic defect has been identified as a deletion of a variable number of KpnI repeats in the 4q35 region, although no specific transcripts from this locus have been isolated so far. Molecular diagnosis is based on the detection by probe p13E-11 of EcoRI small fragments, in the range 10-28 kb, that are resistant to BlnI digestion. In family studies this probe is used with other 4q35 polymorphic markers to assign the haplotype associated with the disease. So far, we performed DNA analysis in 145 FSHD families and identified the 4q35 DNA rearrangement not only in affected individuals, but also in healthy subjects at risk of transmitting the disease, such as non-penetrant gene carriers and somatic mosaics. In addition we applied prenatal tests to 19 fetuses, using DNA extracted from chorionic villi samples (CVS) at 10-11 weeks of gestation. The FSHD status, as determined by the presence of BlnI-resistant small fragments associated with the at risk haplotype, was assessed in nine fetuses; in the remaining 10 cases the disease was excluded. Our results show that molecular analysis of 4q35 rearrangements is a reliable indirect method to perform diagnostic, predictive and prenatal tests in FSHD.

Sequence homology between 4qter and 10qter loci facilitates the instability of subtelomeric KpnI repeat units implicated in facioscapulohumeral muscular dystrophy.

Physical mapping and in situ hybridization experiments have shown that a duplicated locus with a structural organization similar to that of the 4q35 locus implicated in facioscapulohumeral muscular dystrophy is present in the subtelomeric portion of 10q. We performed sequence analysis of the p13E-11 probe and of the adjacent KpnI tandem-repeat unit derived from a 10qter cosmid clone and compared our results with those published, by other laboratories, for the 4q35 region. We found that the sequence homology range is 98%-100% and confirmed that the only difference that can be exploited for differentiation of the 10qter from the 4q35 alleles is the presence of an additional BlnI site within the 10qter KpnI repeat unit. In addition, we observed that the high degree of sequence homology does facilitate interchromosomal exchanges resulting in displacement of the whole set of BlnI-resistant or BlnI-sensitive KpnI repeats from one chromosome to the other. However, partial translocations escape detection if the latter simply relies on the hybridization pattern from double digestion with EcoRI/BlnI and with p13E-11 as a probe. We discovered that the restriction enzyme Tru9I cuts at both ends of the array of KpnI repeats of different chromosomal origins and allows the use of cloned KpnI sequences as a probe by eliminating other spurious fragments. This approach coupled with BlnI digestion permitted us to investigate the structural organization of BlnI-resistant and BlnI-sensitive units within translocated chromosomes of 4q35 and 10q26 origin. A priori, the possibility that partial translocations could play a role in the molecular mechanism of the disease cannot be excluded.

Progress in the molecular diagnosis of facioscapulohumeral muscular dystrophy and correlation between the number of KpnI repeats at the 4q35 locus and clinical phenotype.

Genotype analysis by using the p13E-11 probe and other 4q35 polymorphic markers was performed in 122 Italian facioscapulohumeral muscular dystrophy families and 230 normal controls. EcoRI-BlnI double digestion was routinely used to avoid the interference of small EcoRI fragments of 10qter origin that were found in 15% of the controls. An EcoRI fragment ranging between 10 and 28 kb that was resistant to BlnI digestion was detected in 114 of 122 families (93%) comprising 76 familial and 38 isolated cases. Among the unaffected individuals, 3 were somatic mosaics and 7, carrying an EcoRI fragment larger than 20 kb, could be rated as nonpenetrant gene carriers. In a cohort of 165 patients with facioscapulohumeral muscular dystrophy we found an inverse correlation between fragment size and clinical severity. A severe lower limb involvement was observed in 100% of patients with an EcoRI fragment size of 10 to 13 kb (1-2 KpnI repeats left), in 53% of patients with a fragment size of 16 to 20 kb (3-4 KpnI repeats left), and in 19% of patients with a fragment size larger than 21 kb (>4 KpnI repeats left). Our results confirm that the size of the fragment is a major factor in determining the facioscapulohumeral muscular dystrophy phenotype and that it has an impact on clinical prognosis and genetic counseling of the disease.