The limb girdle and congenital muscular dystrophies (LGMD and CMD) are characterized by skeletal muscle weakness and dystrophic muscle changes. The onset of symptoms in CMD is within the first few months of life, whereas in LGMD they can occur in late childhood, adolescence or adult life. We have recently demonstrated that the fukutin-related protein gene (FKRP) is mutated in a severe form of CMD (MDC1C), characterized by the inability to walk, leg muscle hypertrophy and a secondary deficiency of laminin alpha2 and alpha-dystroglycan. Both MDC1C and LGMD2I map to an identical region on chromosome 19q13.3. To investigate whether these are allelic disorders, we undertook mutation analysis of FKRP in 25 potential LGMD2I families, including some with a severe and early onset phenotype. Mutations were identified in individuals from 17 families. A variable reduction of alpha-dystroglycan expression was observed in the skeletal muscle biopsy of all individuals studied. In addition, several cases showed a deficiency of laminin alpha2 either by immunocytochemistry or western blotting. Unexpectedly, affected individuals from 15 families had an identical C826A (Leu276Ileu) mutation, including five that were homozygous for this change. Linkage analysis identified at least two possible haplotypes in linkage disequilibrium with this mutation. Patients with the C826A change had the clinically less severe LGMD2I phenotype, suggesting that this is a less disruptive FKRP mutation than those found in MDC1C. The spectrum of LGMD2I phenotypes ranged from infants with an early presentation and a Duchenne-like disease course including cardiomyopathy, to milder phenotypes compatible with a favourable long-term outcome.

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Inherited limb malformations provide a valuable resource for the identification of genes involved in limb development. Brachydactyly type B (BDB), an autosomal dominant disorder, is the most severe of the brachydactylies and characterized by terminal deficiency of the fingers and toes. In the typical form of BDB, the thumbs and big toes are spared, sometimes with broadening or partial duplication. The BDB1 locus was previously mapped to chromosome 9q22 within an interval of 7.5 cM (refs 9,10). Here we describe mutations in ROR2, which encodes the orphan receptor tyrosine kinase ROR2 (ref. 11), in three unrelated families with BDB1. We identified distinct heterozygous mutations (2 nonsense, 1 frameshift) within a 7-amino-acid segment of the 943-amino-acid protein, all of which predict truncation of the intracellular portion of the protein immediately after the tyrosine kinase domain. The localized nature of these mutations suggests that they confer a specific gain of function. We obtained further evidence for this by demonstrating that two patients heterozygous for 9q22 deletions including ROR2 do not exhibit BDB. Expression of the mouse mouse orthologue, Ror2, early in limb development indicates that BDB arises as a primary defect of skeletal patterning.

Limb-girdle muscular dystrophy type 2A (LGMD2A) is an autosomal recessive disorder characterized mainly by symmetrical and selective atrophy of the proximal limb muscles. It derives from defects in the human CAPN3 gene, which encodes the skeletal muscle-specific member of the calpain family. This report represents a compilation of the mutations and variants identified so far in this gene. To date, 97 distinct pathogenic calpain 3 mutations have been identified (4 nonsense mutations, 32 deletions/insertions, 8 splice-site mutations, and 53 missense mutations), 56 of which have not been described previously, together with 12 polymorphisms and 5 nonclassified variants. The mutations are distributed along the entire length of the CAPN3 gene. Thus far, most mutations identified represent private variants, although particular mutations have been found more frequently. Knowledge of the mutation spectrum occurring in the CAPN3 gene may contribute significantly to structure/function and pathogenesis studies. It may also help in the design of efficient mutation-screening strategies for calpainopathies.

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by early contractures of the elbows and Achilles tendons, slowly progressive muscle wasting and weakness, and life-threatening cardiomyopathy with conduction blocks. We recently identified LMNA encoding two nuclear envelope proteins, lamins A and C, to be implicated in the autosomal dominant form of EDMD. Here, we report on the variability of the phenotype and spectrum of LMNA mutations in 53 autosomal dominant EDMD patients (36 members of 6 families and 17 sporadic cases). Twelve of the 53 patients showed cardiac involvement exclusively, although the remaining 41 all showed muscle weakness and contractures. We were able to identify a common phenotype among the patients with skeletal muscle involvement, consisting of humeroperoneal wasting and weakness, scapular winging, rigidity of the spine, and elbow and Achilles tendon contractures. The disease course was generally slow, but we observed either a milder phenotype characterized by late onset and a mild degree of weakness and contractures or a more severe phenotype with early presentation and a rapidly progressive course in a few cases. Mutation analysis identified 18 mutations in LMNA (i.e., 1 nonsense mutation, 2 deletions of a codon, and 15 missense mutations). All the mutations were distributed between exons 1 and 9 in the region of LMNA that is common to lamins A and C. LMNA mutations arose de novo in 76% of the cases; 2 of these de novo mutations were typical hot spots, and 2 others were identified in 2 unrelated cases. There was no clear correlation between the phenotype and type or localization of the mutations within the gene. Moreover, a marked inter- and intra-familial variability in the clinical expression of LMNA mutations exists, ranging from patients expressing the full clinical picture of EDMD to those characterized only by cardiac involvement, which points toward a significant role of possible modifier genes in the course of this disease. In conclusion, the high proportion of de novo mutations together with the large spectrum of both LMNA mutations and the expression of the disease should now prompt screening for LMNA in familial and sporadic cases of both EDMD and dilated cardiomyopathy associated with conduction system disease.

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