Large scale genotype-phenotype analyses indicate that novel prognostic tools are required for families with facioscapulohumeral muscular dystrophy

Abstract

Facioscapulohumeral muscular dystrophy has been genetically linked to reduced numbers (≤ 8) of D4Z4 repeats at 4q35 combined with 4A(159/161/168) DUX4 polyadenylation signal haplotype. However, we have recently reported that 1.3% of healthy individuals carry this molecular signature and 19% of subjects affected by facioscapulohumeral muscular dystrophy do not carry alleles with eight or fewer D4Z4 repeats. Therefore, prognosis for subjects carrying or at risk of carrying D4Z4 reduced alleles has become more complicated. To test for additional prognostic factors, we measured the degree of motor impairment in a large group of patients affected by facioscapulohumeral muscular dystrophy and their relatives who are carrying D4Z4 reduced alleles. The clinical expression of motor impairment was assessed in 530 subjects, 163 probands and 367 relatives, from 176 unrelated families according to a standardized clinical score. The associations between clinical severity and size of D4Z4 allele, degree of kinship, gender, age and 4q haplotype were evaluated. Overall, 32.2% of relatives did not display any muscle functional impairment. This phenotype was influenced by the degree of relation with proband, because 47.1% of second- through fifth-degree relatives were unaffected, whereas only 27.5% of first-degree family members did not show motor impairment. The estimated risk of developing motor impairment by age 50 for relatives carrying a D4Z4 reduced allele with 1-3 repeats or 4-8 repeats was 88.7% and 55%, respectively. Male relatives had a mean score significantly higher than females (5.4 versus 4.0, P = 0.003). No 4q haplotype was exclusively associated with the presence of disease. In 13% of families in which D4Z4 alleles with 4-8 repeats segregate, the diagnosis of facioscapulohumeral muscular dystrophy was reported only in one generation. In conclusion, this large-scale analysis provides further information that should be taken into account when counselling families in which a reduced allele with 4-8 D4Z4 repeats segregates. In addition, the reduced expression of disease observed in distant relatives suggests that a family's genetic background plays a role in the occurrence of facioscapulohumeral muscular dystrophy. These results indicate that the identification of new susceptibility factors for this disease will require an accurate classification of families.

Keywords: D4Z4 reduced allele; disease expression; facioscapulohumeral muscular dystrophy; genotype–phenotype correlations; penetrance.

Figure 1

(A) Preliminary selection of probands/families from the Italian National Registry for FSHD (INRF). (B) Selection of the cohort of probands and their relatives for genotype–phenotype correlation analysis.

Figure 2

Age-specific cumulative risk of reported muscle impairment according to D4Z4 allele size. Estimates obtained on 361 relatives using the Kaplan-Meier analysis. Blue line refers to carriers of 1–3 D4Z4 units; red line refers to carriers of 4–6 D4Z4 units; green line refers to carriers of 7–8 D4Z4 units. Carriers of 7–8 versus 4–6 units Log rank test P value = 0.002.

Figure 3

Distribution of clinical severity among relatives carrying D4Z4 reduced allele according to D4Z4 allele size and age. Subjects were subdivided by age: (A) 18–30 years, (B) 31–55 years, (C) 56–70 years and by D4Z4 allele size: 1–3, 4–6 and 7–8 units. In each subgroup, percentages of subjects who received FSHD score equal to 0, 1–2, 3–6 and ≥ 7 are reported.

Figure 4

Age-specific cumulative risk of reported muscle impairment according to sex. (A) Estimates obtained on 361 relatives using the Kaplan-Meier analysis. Log-rank test P-value = 0.113. (B) Estimates obtained on 160 probands using the Kaplan-Meier analysis. Blue line refers to male; red line refers to female. Log-rank test P-value = 0.028.