De novo facioscapulohumeral muscular dystrophy: frequent somatic mosaicism, sex-dependent phenotype, and the role of mitotic transchromosomal repeat interaction between chromosomes 4 and 10

Abstract

Autosomal dominant facioscapulohumeral muscular dystrophy (FSHD) is caused by deletion of most copies of the 3.3-kb subtelomeric D4Z4 repeat array on chromosome 4q. The molecular mechanisms behind the deletion and the high proportion of new mutations have remained elusive. We surveyed 35 de novo FSHD families and found somatic mosaicism in 40% of cases, in either the patient or an asymptomatic parent. Mosaic males were typically affected; mosaic females were more often the unaffected parent of a nonmosaic de novo patient. A genotypic-severity score, composed of the residual repeat size and the degree of somatic mosaicism, yields a consistent relationship with severity and age at onset of disease. Mosaic females had a higher proportion of somatic mosaicism than did mosaic males. The repeat deletion is significantly enhanced by supernumerary homologous repeat arrays. In 10% of normal chromosomes, 4-type repeat arrays are present on chromosome 10. In mosaic individuals, 4-type repeats on chromosome 10 are almost five times more frequent. The reverse configuration, also 10% in normal chromosomes, was not found, indicating that mutations may arise from transchromosomal interaction, to which the increase in 4-type repeat clusters is a predisposing factor. The somatic mosaicism suggests a mainly mitotic origin; mitotic interchromosomal gene conversion or translocation between fully homologous 4-type repeat arrays may be a major mechanism for FSHD mutations.

Figure 1

Subtelomeric repeat-array constitutions on chromosomes 4 and 10 in the Dutch control and FSHD population. Chromosomes 4 are shaded, whereas chromosomes 10 are blackened. Top, in the control population, 80% of the individuals carry a standard configuration, with 4-type repeats on chromosome 4 and 10-type repeats on chromosome 10. In 10%, a 4-type repeat is also present on one of the chromosomes 10 (“4 on 10”), and 10% carry a 10-type repeat on chromosome 4 (“10 on 4”). Bottom, the repeat-array constitutions of mosaic individuals from de novo FSHD families. The deletion is indicated by an open bar. These individuals carry two cell populations indicated within a box. In the original population, no FSHD-associated rearrangement is present, whereas, in the other population, a deletion has occurred on chromosome 4. In mosaic individuals, 54% carry a standard allele configuration, and 46% carry a “4 on 10” allele configuration.

Figure 2

Relationship between clinical and genotypic severity in mosaic individuals. The clinical severity is on the Y axis, whereas the genotypic severity, calculated by [(5 − number of remaining repeat units) × fraction of cells with that deletion] is on the X axis. Males are represented by blackened squares, whereas females are represented by blackened circles.

Figure 3

A, Somatic mosaicism in a de novo FSHD patient (FSHD1). DNA, digested by EcoRI/HindIII (H) or EcoRI/BlnI (B) and separated by PFGE, was visualized with probe p13E-11. The patient inherited a 65-kb 4-type repeat array from his father, which is reduced in 50% of his PBLs to the FSHD range of 10 kb (both alleles have a reduced intensity [arrows]). Note that this patient inherited from his mother two 4-type repeat arrays and, therefore, has a 4 on 10 repeat-array configuration. Y alleles are marked with an asterisk (*). The son of this patient inherited both grandmaternal alleles and is therefore not affected. B, Somatic mosaicism in a female carrier of a de novo FSHD kindred (FSHD9). DNA, digested by EcoRI/HindIII (H) or EcoRI/BlnI (B) and separated by PFGE, was visualized with probe p13E-11. The patient carries a short BlnI-insensitive repeat array of 10 kb (arrow), indicating FSHD. This allele (arrow) is also weakly present in the mother, who carries on chromosomes 4 and 10 four-type repeat arrays, identified on the basis of their BlnI insensitivity. The Y chromosomal cohybridizing fragment is marked with an asterisk (*).