The critical region for the congenital eye movement disorder, CFEOM1, has been reduced to a 3 cM region at the centromere of human chromosome 12 [2
]. In an effort to identify the gene which when mutated gives rise to CFEOM1, a combination of positional and candidate gene approaches has been undertaken. The sarcospan gene is composed of three exons that are located on human chromosome 12p11.2 and the sarcospan protein has been shown to be an integral component of the sarcoglycan complex [5
]. Mutations in any one of the other four members of this complex have been shown to cause muscular dystrophy. As the extraocular muscle is one of the few muscles spared in Duchenne Muscular Dystrophy, the dystrophin associated protein complex present in extraocular muscle may respond differently to mutation of the complex than other striated muscle [17
]. By virtue of its genomic localization and expression in extraocular muscle, sarcospan is a candidate gene for autosomal dominant CFEOM1. We therefore chose to study sarcospan more closely within the context of this disease.
Immunofluorescence data showed comparable patterns of sarcospan staining in CFEOM1 and control patient samples implying that there was no haplo-insufficiency, altered accumulation or increased degradation of the sarcospan protein in these patients. Direct sequencing of the three coding exons of sarcospan in CFEOM1 patients confirmed that they were normal, rendering alteration of the sarcospan protein as the primary genetic defect in CFEOM1 unlikely. This conclusion is supported by the electronic localization of sarcospan to BACs outside of the CFEOM1 critical region.
The normal staining pattern of sarcospan in the autosomal dominant CFEOM1 patient muscle indicates that the primary genetic mutation is likely to be found in another, unrelated gene. Patients with autosomal dominant limb girdle muscular dystrophy type 1C, caused by mutations in caveolin-3 [19
], or autosomal recessive LGMD2B, caused by mutations in dysferlin, show normal dystrophin and sarcoglycan-sarcospan complex staining patterns [21
]. Sarcospan immunoreactivity is altered in both Duchenne Muscular Dystrophy and the sarcoglycanopathies (LGMD2C-F) and appears to be very sensitive to disturbances in the dystrophin associated protein complex [16
]. Thus the finding that sarcospan is normal in CFEOM1 patient muscle suggests that sarcospan itself and the rest of the dystrophin associated protein complex are not involved in the pathogenesis of the disease.
A sarcospan null mouse has recently been generated and appears to display a normal phenotype [22
]. This does not, however, rule out the possibility of sarcospan playing a primary role in a muscle disorder. There may, for example, be a homologous tetraspanin protein that compensates for the absence of sarcospan in mice, but not in humans. There is, of course, also a difference between complete absence of a protein and a protein with an altered sequence. This may hold especially true, as sarcospan appears to be member of the tetraspanin family of proteins, which have been described as molecular facilitators; alterations of such a protein's sequence could affect the proteins with which it interacts. We are therefore expanding our patient analysis to include patients with other forms of muscular abnormalities including unlinked muscular dystrophies.
The effort to identify the CFEOM1 disease gene is continuing with analysis of other genes and expressed sequence tags from the critical region at the centromere of chromosome 12.