What is the recommended testing for diagnosing facioscapulohumeral muscular dystrophy (FSHD)?

Diagnostic Testing for Facioscapulohumeral Muscular Dystrophy (FSHD)

Genetic testing for D4Z4 repeat contraction on chromosome 4q35 is the recommended first-line diagnostic test for facioscapulohumeral muscular dystrophy (FSHD), with additional methylation analysis recommended for cases with normal D4Z4 repeat numbers but clinical symptoms consistent with FSHD. 1, 2, 3

Clinical Presentation Warranting Testing

• FSHD should be suspected in patients presenting with asymmetric muscle weakness, particularly affecting facial, scapular, and humeral muscles 4
• Testing should be considered in patients with:
  • Progressive muscle weakness with characteristic distribution (face, shoulders, upper arms)
  • Family history of similar symptoms (autosomal dominant inheritance pattern) 4
  • Unexplained elevated creatine kinase levels 5

Recommended Diagnostic Algorithm

First-Line Testing

1. Genetic testing for D4Z4 repeat contraction:
   • Southern blot analysis or optical genome mapping (OGM) to determine the number of D4Z4 repeats on chromosome 4q35 1, 6
   • FSHD1 is diagnosed when there is contraction to 1-10 D4Z4 repeat units on a permissive 4qA haplotype 1, 2

Second-Line Testing (if First-Line is Negative or Inconclusive)

1. Methylation analysis of D4Z4 repeats:

   • FSHD-MPA (methylation profile analysis) to determine methylation levels of D4Z4 repeats 3
   • Significant hypomethylation of D4Z4 repeats supports FSHD diagnosis 2, 3

2. Genetic sequencing for FSHD2:

   • Exome sequencing to identify pathogenic variants in epigenetic regulator genes (SMCHD1, DNMT3B, or LRIF1) 1, 2
   • FSHD2 is diagnosed when there is hypomethylation of D4Z4 repeats with normal or mildly contracted repeat numbers 2

Advanced Testing Options

• Optical Genome Mapping (OGM) with EnFocus™ FSHD analysis:

  • Particularly valuable for cases with inconclusive Southern blot results 1
  • Can simultaneously detect D4Z4 repeat size and distinguish between chromosomes 4q and 10q 1

• Nanopore CRISPR/Cas9-targeted resequencing:

  • Allows simultaneous detection of D4Z4 repeat length and methylation status at nucleotide level 2
  • Helpful for complex cases requiring detailed molecular characterization 2

Diagnostic Challenges and Considerations

• Borderline D4Z4 repeat numbers (8 or more repeats):

  • No definitive diagnostic cut-off point separating FSHD from controls 4
  • Clinical correlation is essential as patients with borderline repeat numbers may present with:
    • Typical FSHD
    • Facial-sparing FSHD
    • FSHD with atypical features
    • Non-FSHD muscle disease 4

• Technical considerations:

  • Cross-hybridization between 4q and 10q D4Z4 arrays can complicate interpretation 6
  • Deletions including the genomic p13E-11 sequence and part of D4Z4 may require specialized testing approaches 6
  • Translocations between 4q and 10q D4Z4 arrays can occur 6

Clinical Correlation

• Methylation level of D4Z4 repeats correlates strongly with clinical severity and may be a better predictor of disease status than genetic parameters alone 3
• Epigenetic parameters (D4Z4 methylation) should be considered alongside genetic findings for accurate diagnosis and prognosis 3

Post-Diagnostic Management

• Refer patients to specialized neuromuscular centers for multidisciplinary care 5
• Provide genetic counseling for patients and family members 5
• Connect patients with support organizations and resources 5

Remember that while D4Z4 repeat contraction testing is the gold standard for diagnosis, the complex genetics and epigenetics of FSHD may require a comprehensive testing approach, particularly in cases with atypical presentations or borderline D4Z4 repeat numbers.