Assignment of a form of congenital muscular dystrophy with secondary merosin deficiency to chromosome 1q42

Abstract

We have previously reported an autosomal recessive form of congenital muscular dystrophy, characterized by proximal girdle weakness, generalized muscle hypertrophy, rigidity of the spine, and contractures of the tendo Achilles, in a consanguineous family from the United Arab Emirates. Early respiratory failure resulting from severe diaphragmatic involvement was present. Intellect and the results of brain imaging were normal. Serum creatine kinase levels were grossly elevated, and muscle-biopsy samples showed dystrophic changes. The expression of the laminin-alpha2 chain of merosin was reduced on several fibers, but linkage analysis excluded the LAMA2 locus on chromosome 6q22-23. Here, we report the results of genomewide linkage analysis of this family, by use of homozygosity mapping. In all four affected children, an identical homozygous region was identified on chromosome 1q42, spanning 6-15 cM between flanking markers D1S2860 and D1S2800. We have identified a second German family with two affected children having similar clinical and histopathological features; they are consistent with linkage to the same locus. The cumulative LOD score was 3.57 (straight theta=.00) at marker D1S213. This represents a novel locus for congenital muscular dystrophy. We suggest calling this disorder "CMD1B." The expression of three functional candidate genes in the CMD1B critical region was investigated, and no detectable changes in their level of expression were observed. The secondary reduction in laminin-alpha2 chain in these families suggests that the primary genetic defect resides in a gene coding for a protein involved in basal lamina assembly.

Figure 1

Pedigrees of families 1 (A) and 2 (B), showing the haplotypes for chromosome 1q42 markers. The disease haplotype is denoted by the gray-shaded bars. The diagonally hatched section of a bar denotes an allele that could not be unequivocally assigned. The CMD1B critical region is defined, in family 1, by recombination events in patients II.2 and II.4.

Figure 2

Frozen sections of the muscle-biopsy samples from patients II.4 (a–c, e, and f) and II.3 (d) from family 1; sections were stained with hematoxylin and eosin (a) and were immunolabeled for laminin-α2 (b), heparan sulfate proteoglycan (c), nidogen (d), α-actinin 2 (e), and actin (f). Note the dystrophic morphology; the reduced laminin-α2 on some fibers but normal expression of heparan sulfate proteoglycan; and the presence of nidogen, α-actinin 2, and actin. Bars = 50 μm (bar in a also applies to panels b–d, and e).

Figure 3

Ideogram of chromosome 1 and markers within the CMD1B critical region. The unbroken-and-dashed line represents the minimal and maximal critical regions. These regions are defined by the recombination events in family 1 (in patient II.2, between D1S459 and D1S2800; in patient II.4, between D1S2860 and D1S2871). Distances between adjacent markers are in centimorgans, on the basis of the data from the Généthon database. The map positions of four functional candidate genes, on the basis of the GeneMap'98 (International Radiation Hybrid Mapping Consortium) GB4 map, are indicated as follows: LBR, ACTA1, NID, and ACTN2.