CFEOM1, the Classic Familial Form of Congenital Fibrosis of the Extraocular Muscles: Refinement of the Disease Locus and Analysis of Candidate Genes

Abstract

Abstract: : Purpose: The most common inherited form of 'congenital fibrosis of the extraocular muscles', CFEOM1, is characterized by congenital bilateral static hypotropic ophthalmoplegia with ptosis. Our previous genetic studies of 18 families with CFEOM1 localized the disease locus (FEOM1) to a 3.0-cM region on chromosome 12, flanked by D12S1584 and D12S1668. Pathologic study of an affected member of a FEOM1-linked CFEOM1 family revealed absence of the superior division of the oculomotor nerve and absence of the corresponding alpha-motoneurons in the midbrain. To delineate the FEOM1 locus and to evaluate candidate genes within the critical region, we performed linkage analysis of new CFEOM1 families and mutation analysis of candidate genes within the FEOM1 critical region. Methods: Linkage and haplotype analysis of a large Japanese family with CFEOM1 was performed using markers spanning the FEOM1 region. Affected individuals from 21 FEOM1-lined CFEOM1 families were screened for mutations in candidate genes. The genomic structure of each gene was determined and each exon and corresponding intron-exon boundaries was PCR amplified and subjected to analysis by WAVE (denaturing high-performance liquid chromatography) and/or direct DNA sequencing. Results: A new critical recombination event was detected in an affected individual in the Japanese family between D12S345 and D12S1692, refining the FEOM1 locus to a 2.1-cM region between markers D12S345 and D12S1668. Among the known genes in the FEOM1 locus, we chose to analyze BICD1 first. BICD1 is homologous to bicaudal-D, a Drosophila protein that is involved in establishing the asymmetric cytoplasm in the developing oocyte. In addition, it has been suggested that BICD1 may be involved in human neuronal migration. No disease causing mutations were identified, likely excluding BICD1 as the FEOM1 gene. Similar analysis also excluded two additional candidate genes from the FEOM1 locus. Conclusions: The critical recombination event identified in this study refined the localization of the FEOM1 gene to a 2.1-cM region. The results of mutation analysis provide considerable evidence for exclusion of three genes as candidates for the FEOM1 gene. Identifying the FEOM1 gene should provide insight into specific features of developing ocular motoneurons and extraocular muscles.

Keywords: positional cloning • strabismus • linkage analysis