Objective To identify the genetic and epigenetic defects in patients presenting with a facioscapulohumeral (FSHD) clinical phenotype without D4Z4 contractions on chromosome 4q35 tested by linear gel electrophoresis and Southern blot analysis.

Design and patients The authors studied 16 patients displaying an FSHD-like phenotype, with normal cardiovascular and respiratory function, a myopathic pattern on electromyography, and a muscle biopsy being normal or displaying only mild and aspecific dystrophic changes. They sequenced the genes calpain 3 (CAPN3), valosin containing protein (VCP) and four-and-a-half LIM domains protein 1 (FHL1), and they analysed the D4Z4 repeat arrays by extensive genotyping and DNA methylation analysis.

Results The authors identified one patient carrying a complex rearrangement in the FSHD locus that masked the D4Z4 contraction associated with FSHD1 in standard genetic testing, one patient with somatic mosaicism for the D4Z4 4q35 contraction, six patients that were diagnosed as having FSHD2, four patients with CAPN3 mutations and two patients with a VCP mutation, No mutations were detected in FHL1, and in two patients, the authors could not identify the genetic defect.

Conclusions In patients presenting with an FSHD-like clinical phenotype with a negative molecular testing for FSHD, consider (1) detailed genetic testing including D4Z4 contraction of permissive hybrid D4Z4 repeat arrays, p13E-11 probe deletions, and D4Z4 hypomethylation in the absence of repeat contraction as observed in FSHD2; (2) mutations in CAPN3 even in the absence of protein deficiency on western blot analysis; and (3) VCP mutations even in the absence of cognitive impairment, Paget disease and typical inclusion in muscle biopsy.