CLIA Laboratory Testing for Facioscapulohumeral Dystrophy: A Retrospective Analysis

Current diagnostic approaches for muscular dystrophies rely on molecular techniques to distinguish subtypes accurately. While facioscapulohumeral muscular dystrophy (FSHD) is a well-recognized condition, its genetic heterogeneity complicates test interpretation. Prior research has shown that FSHD involves two primary subtypes, FSHD1 and FSHD2, each with distinct genetic markers. However, the frequency and characteristics of these subtypes in diagnostic settings remain unclear. No prior work had resolved the distribution of FSHD subtypes in a large, real-world laboratory cohort. This gap motivated a retrospective analysis of FSHD diagnostic testing data to better understand subtype prevalence and test outcomes. The study aimed to clarify how often borderline or complex cases arise in routine testing. It also sought to quantify the proportion of FSHD1 versus FSHD2 diagnoses. By examining a large dataset, the research provided insights into the practical challenges of FSHD classification in a clinical setting.

The objective of this study was to analyze FSHD diagnostic testing outcomes from a single molecular pathology laboratory over a four-year period. Researchers aimed to determine the frequency of FSHD1 and FSHD2 diagnoses among tested individuals. They also wanted to identify how often borderline or complex test results occurred. The study focused on the practical implications of diagnostic criteria in a clinical setting. It sought to quantify how often cases met both FSHD1 and FSHD2 criteria simultaneously. The researchers also aimed to assess the impact of somatic mosaicism and translocation events on test interpretation. By summarizing these findings, the study aimed to inform diagnostic practices and test interpretation guidelines. The results could help improve the accuracy of FSHD diagnosis in clinical laboratories.

The study involved a retrospective analysis of FSHD diagnostic tests conducted at the University of Iowa Molecular Pathology Laboratory. Testing was performed using restriction enzyme digestion and Southern blot analysis. When indicated, sequencing of the SMCHD1 gene was also performed. Cases were categorized as FSHD1, FSHD2, or non-FSHD1,2 based on specific genetic criteria. The researchers also identified cases with borderline EcoRI fragment sizes and somatic mosaicism. They examined hybrid alleles and translocation events that complicated test interpretation. The dataset included all FSHD tests performed from January 2015 to July 2019. A total of 1,594 patient cases were analyzed for this study.

Out of 1,594 patients tested, 703 (44.1%) received a diagnosis of FSHD. Among these, 664 (94.5%) met criteria for FSHD1 and 39 (5.5%) for FSHD2. Twenty cases (1.3%) had borderline EcoRI fragment sizes. Twenty-three (1.5%) were somatic mosaics, and 328 (20.9%) had translocation events. When considering only cases with at least one 4q35A allele, FSHD1 cases had a median of 6.0 D4Z4 repeats (IQR 4-7). FSHD2 cases had a median of 15.0 repeats (IQR 12-22). Non-FSHD1,2 cases had a median of 28.0 repeats (IQR 19-40). These findings suggest a continuum of D4Z4 repeat numbers across FSHD subtypes.

The study found that FSHD1 accounts for the majority of genetically confirmed FSHD cases. The data show a clear distinction in D4Z4 repeat numbers between FSHD subtypes. Borderline cases and somatic mosaicism were relatively rare but still occurred in clinical testing. Translocation events were more common and added complexity to test interpretation. The findings suggest that FSHD diagnostic testing requires careful evaluation of multiple genetic factors. The study highlights the importance of standardized diagnostic criteria for accurate classification. It also emphasizes the need for laboratories to account for hybrid alleles and translocations. These results may help improve diagnostic accuracy in FSHD testing.