• genotype
• facioscapulohumeral muscular dystrophy
• phenotype

Facioscapulohumeral muscular dystrophy (FSHD) has been classified as an autosomal dominant myopathy, with an almost full penetrance (>95%) by age of 20 years, associated to rearrangements occurring in a 3.3 kb repeated sequence (D4Z4) located at the 4q subtelomeric region. For last 20 years, the diagnosis of FSHD in clinical practice has been performed by DNA testing, which has been considered highly sensitive and specific. It was established that normal subjects carry on alleles greater than 45 kb (≥ 11 D4Z4 repeats), whereas alleles of 35 kb, corresponding to 8 D4Z4 units, or shorter, are present in the majority of either de novo or familial FSHD patients. An inverse correlation between the number of D4Z4 repeats and the severity of FSHD has been also traditionally reported. Nevertheless, since the advent of genetic diagnosis, several reports have described FSHD families with subjects carrying D4Z4 reduce alleles and no signs of disease, these defined as non-penetrant carriers. Moreover, a growing number of evidences has emerged to complicate the evaluation of patients, reporting a wide and unexpected variability of FSHD clinical features and outcomes, also among subjects carrying the same D4Z4 allele, even within the same family. Then, it has been proposed that the reduction of D4Z4 repeats on chromosome 4q35 could be pathogenic only in a certain chromosomal backgrounds, defined as “permissive” specific haplotypes. However, the recent studies performed by the Italian Network for FHSD show that the current DNA signature of FSHD is a common polymorphism and suggest that the genetic basis of the disease should be revisited in consideration of the important implications for diagnosis and genetic counseling of at-risk families.
The present thesis is an observational cross-sectional analysis performed on the largest cohort of FSHD families described to date, accrued through the Italian National Registry for FSHD. The aim of the study has been to evaluate the clinical expression of FSHD in relation to D4Z4 reduced allele, 4q haplotype, age, gender and degree of kinship, in order to re-establish the prognostic value of D4Z4 reduced allele and identify other possible predictors of disease outcome in clinical practice. The analysis, performed on 367 relatives from 176 unrelated FSHD families, reveals an almost complete penetrance only in carriers on reduced allele with 1-3 D4Z4 repeats. Instead, the risk of developing FSHD is around 36% in both groups of subjects carrying 4-6 and 7-8 D4Z4 repeats by age of 30 and progressively increases with age, until 71,5% and 62,9% respectively by age of 60. The study fails to identify difference in term of penetrance and age-related risk to develop FSHD between carriers of “permissive” haplotype in comparison with the “non-permissive” haplotype, thus indicating that 4q35 haplotypes cannot have a clinical predictive value. Interestingly, a higher percentage of asymptomatic subjects between relatives with lower degree of relationship with proband is observed, suggesting a possible role of the “genetic background” and a more complex mode of inheritance in FSHD. The analysis of clinical expression on affected carriers shows that subjects carrying 1-3 D4Z4 repeats are characterized by a more precocious age at onset and by a more severe phenotype, while a wide clinical variability is observed among carriers of 4-8 D4Z4 repeats, ranging to asymptomatic-paucisymptomatic to severely affected. Finally, the discover of gender differences in term of mean age at onset and severity of disease expression proves that the female gender plays a protective effect for disease expression, suggesting variables related to gender, including hormonal and/or lifestyle factors, as possible contributing factors to be further investigated.
In conclusion, this analysis definitively confirms the previous observations about the inter- and intra-familial great variability in clinical expression, strengthening the notion that FSHD is a complex disease, whose pathogenic mechanisms, at present, are not clear. The work of the present thesis indicates that, starting from a systematic analysis of FSHD families with defined mode of inheritance and phenotypic features, it turns relevant indicators of prognostic value, these useful tools in the clinical practice, but also as incitement to further investigate the pathogenesis of FSHD.