FSHD1 and FSHD2 form a disease continuum

Abstract

Objective: To compare the clinical features of patients showing a classical phenotype of facioscapulohumeral muscular dystrophy (FSHD) with genetic and epigenetic characteristics of the FSHD1 and FSHD2 loci D4Z4 and SMCHD1.

Methods: This is a national multicenter cohort study. We measured motor strength, motor function, and disease severity by manual muscle testing sumscore, Brooke and Vignos scores, clinical severity score (CSS), and age-corrected CSS, respectively. We correlated these scores with genetic (D4Z4 repeat size and haplotype; SMCHD1 variant status) and epigenetic (D4Z4 methylation) parameters.

Results: We included 103 patients: 54 men and 49 women. Among them, we identified 64 patients with FSHD1 and 20 patients with FSHD2. Seven patients had genetic and epigenetic characteristics of FSHD1 and FSHD2, all carrying repeats of 9-10 D4Z4 repeat units (RU) and a pathogenic SMCHD1 variant. In the remaining patients, FSHD was genetically excluded or remained unconfirmed. All clinically affected SMCHD1 mutation carriers had a D4Z4 repeat of 9-16 RU on a disease permissive 4qA haplotype. These patients are significantly more severely affected by all clinical scales when compared to patients with FSHD1 with upper-sized FSHD1 alleles (8-10 RU).

Conclusion: The overlap between FSHD1 and FSHD2 patients in the 9-10 D4Z4 RU range suggests that FSHD1 and FSHD2 form a disease continuum. The previously established repeat size threshold for FSHD1 (1-10 RU) and FSHD2 (11-20 RU) needs to be reconsidered.

Clinicaltrialsgov identifier: NCT01970735.

Figure 1. Breakdown of patients

The major groups 1 (57 patients with facioscapulohumeral dystrophy [FSHD] type 1, 2 [20 patients with FSHD type 2], and 3 [7 patients with FSHD1 + FSHD2]) are indicated. RU = repeat units.

Figure 2. Clinical assessments of the facioscapulohumeral dystrophy (FSHD) subpopulations

(A) Significant differences in the age-corrected clinical severity score (CSS) between FSHD type 1 (FSHD1) (8–10) and FSHD1 (4–7) (p = 0.0009***), FSHD1+ FSHD type 2 (FSHD2) (p = 0.0002***), or FSHD2 (p < 0.0001****) subgroups are observed. A significant difference between FSHD1 (4–7) and FSHD2 is also present (p = 0.048*). (B) The CSS of the FSHD1 (8–10) subpopulation is significantly different from those of FSHD1+2 (p = 0.036*), FSHD2 (p = 0.002**), and FSHD1 (4–7) (p = 0.003**) subgroups. (C) Significant differences in the manual muscle testing (MMT) score between FSHD1 (8–10) and FSHD1 (4–7) (p < 0.0001****) or FSHD2 (p = 0.047*) subgroups are observed. (D) The Brooke score of the FSHD1 (8–10) subpopulation is significantly different from those of FSHD2 (p = 0.0002***) and FSHD1 (4–7) (p = 0.012*) subgroups. (E) Significant differences observed between Vignos scores of FSHD1 (8–10) and FSHD 1+2 (p = 0.034) or FSHD2 (p = 0.044*) subgroups.

Figure 3. Analysis of the DR1 region in the D4Z4 repeats methylation level in the facioscapulohumeral dystrophy (FSHD) subgroups

(A) The level of methylation of the DR1 region in the D4Z4 repeats is significantly different between FSHD type 1 (FSHD1) (8–10) patients and FSHD1+ FSHD type 2 (FSHD2) (p = 0.005**) or FSHD2 subgroups (p < 0.0001****). We also observe a significant difference in methylation between FSHD1 (4–7) and FSHD1+2 (p = 0.0002***) or FSHD2 subgroups (p < 0.0001****). FSHD1+2 and FSHD2 patients show a hypomethylation status at the DR1 region (mean level < 30%). (B) A significant correlation between the methylation level of the DR1 region and the age-corrected clinical severity score (CSS) is obtained (Spearman r = −0.37, p = 0.0005***). The disease severity is increased in patients with low methylation levels (significant p values only are indicated [*]).

Figure 4. Facioscapulohumeral dystrophy type 1 (FSHD1) and facioscapulohumeral dystrophy type 2 (FSHD2), a disease continuum

Patients with FSHD1 have a contraction of the D4Z4 repeat to a size of 1–10 repeat units (RU) (X-axis). Patients with FSHD2 show hypomethylation of the DR1 region in the presence of a normal sized D4Z4 repeat of 11–20 RU (Y axis). FSHD1+2 can be considered part of a FSHD disease continuum characterized by a contraction of the D4Z4 repeat (9–10 RU) and a hypomethylation status of the DR1 region because of mutations in D4Z4 chromatin modifiers.