CLIA Laboratory Testing for Facioscapulohumeral Dystrophy: A Retrospective Analysis

OBJECTIVE: To summarize facioscapulohumeral muscular dystrophy (FSHD) diagnostic testing results from the University of Iowa Molecular Pathology Laboratory. METHODS: All FSHD tests performed in the diagnostic laboratory from January 2015 to July 2019 were retrospectively reviewed. Testing was by restriction enzyme digestion and Southern blot analysis with sequencing of SMCHD1, if indicated. Cases were classified as FSHD1 (4q35 EcoRI size ≤40 kb; 1–10 D4Z4 repeats), FSHD2 (permissive 4q35A allele, D4Z4 hypomethylation, and pathogenic SMCHD1 variant), or non-FSHD1,2. We also noted cases with borderline EcoRI fragment size (41–43 kb; 11 D4Z4 repeats), cases that meet criteria for both FSHD1 and FSHD2, somatic mosaicism, and cases with hybrid alleles that add complexity to test interpretation. RESULTS: Of the 1,594 patients with FSHD tests included in the analysis, 703 (44.1%) were diagnosed with FSHD. Among these positive tests, 664 (94.5%) met criteria for FSHD1 and 39 (5.5%) met criteria for FSHD2. Of all 1,594 cases, 20 (1.3%) had a 4q35 allele of borderline size, 23 (1.5%) were somatic mosaics, and 328 (20.9%) had undergone translocation events. Considering only cases with at least 1 4q35A allele, D4Z4 repeat number differed significantly among groups: FSHD1 cases median 6.0 (interquartile range [IQR] 4–7) repeats, FSHD2 cases 15.0 (IQR 12–22) repeats, and non-FSHD1,2 cases 28.0 (IQR 19–40) repeats. CONCLUSION: FSHD1 accounts for 94.5% of genetically confirmed cases of FSHD. The data show a continuum of D4Z4 repeat numbers with FSHD1 samples having the fewest, FSHD2 an intermediate number, and non-FSHD1,2 the most.