Integrated D4Z4 structural, epigenetic and exome-based evaluation of facioscapulohumeral muscular dystrophy in a tertiary referral cohort from Türkiye.

Contraction of the D4Z4 macrosatellite on a permissive haplotype (FSHD1) or hypomethylation linked to SMCHD1 variants (FSHD2), assessed by single-molecule repeat sizing, DR1 methylation profiling and exome sequencing.

This study evaluated 135 unrelated referrals with suspected facioscapulohumeral muscular dystrophy at a Turkish tertiary referral centre between 2019 and 2026. First-line testing combined single-molecule D4Z4 repeat sizing, haplotyping and structural analysis by molecular combing or optical genome mapping; DR1 methylation profiling and WES served as second line in unresolved, borderline, non-contracted or clinically atypical cases. Diagnosis was confirmed in 121 of 135 referrals (89.6%): FSHD1 in 109, FSHD1+2 in 4 and FSHD2 in 8. Single-molecule analysis revealed mosaicism, allelic imbalance, homozygous contracted genotypes and complex 4q configurations, while WES identified six SMCHD1 variants, four of them new, along with alternative or dual diagnoses.

Synthesis written by Geno'X. For the full original abstract, please refer to the source publication.

A real-world demonstration of what a tiered diagnostic algorithm brings in FSHD: 89.6% confirmation and, above all, resolution of borderline cases that D4Z4 sizing alone would leave unsettled. The practical value lies in explicitly positioning DR1 methylation and WES as second line, indication by indication, rather than as ad hoc fallback. The high confirmation rate reflects expert-centre referral bias and should not be read as the expected performance on unselected requests.

Analysis by Dr Thibaut Benquey