Study Of Candidate Genes In Congenital Inherited Nuclear Cataract

Autosomal dominant congenital cataract (ADCC) is a clinically and genetically heterogeneous lens disorder that is usually considered to be a sight-threatening trait in childhood. Cataract results from a loss in transparency of the crystalline lens. Despite effective surgical treatment, demand outstrips supply in both Western and developing countries, for many reasons, and other disease modifying strategies must be considered. A strong genetic predisposition to the development of congenital cataract and age related cataract has been well documented. Inherited cataract accounts for at least 30% of all congenital cataracts, and shows marked inter-and intrafamilial variation.Cataract characterized by opacities of the lens remains the leading cause of human blindness worldwide with one third of cases are familial. Cataract may be inherited as autosomal dominant, autosomal recessive, or X-linked recessive traits, and 17 specific genes associated with inherited isolated cataract have been identified to date; including seven genes coding for crystallins (CRYAA, CRYAB, CRYBA1/A3, CRYBB1, CRYBB2, CRYGC, CRYGD), two for gap junctional channel protein (GJA3 and GJA8), two for lens membrane protein (LIM2 and MIP), one for beaded filament structural protein 2 (BFSP2), four for ranscription factor 4 (PITX3, PAX6. MAF, HSF4), one for glucosaminyl (N-acetyl) transferase 2 gene (GCNT2). It is likely that more genes remain to be discovered.Here we tried to identify the genetic defect causing ADCC in a five-generation family in the northeast of China. The molecular genetics characterisation of a Chinese family with an autosomal dominant congenital nuclear cataract is described. With informed consent, genomic DNA was extracted from peripheral blood leucocytes using standard protocols. The affected status was determined before genetic analysis. Three panels of cataract candidate loci were selected forpreliminary haplotype analysis. Polymerase chain reaction based genotyping of microsatellite markers was done on a Pharmacia automated sequencer. The marked order and intermarker distances (cM) were obtained from NCBI. Linkage analysis was carried out with ploymorphic microsatellites on the Human MapPairs marker set, special known loci. Mutation analysis of the candidate gene in the critical region was performed to detect the potential mutation. From this research we concluded that:1. Sixteen family members seven affected and nine unffected participated and had a full ocular assessment to document the phenotype. The phenotype was characterised by autosomal dominant, bilateral, congenital nuclear cataracts. Other ocular defects were present, including amblyopia strabismus and nystagmus.2. The five generation Chinese family composed of 16 individuals, including 7 affected members (two male and five female), with autosomal dominant pattern of inheritance and aprroxmitaly 100% penetrance.3. There was no evidence of defects in karyotype analysis.4. Candidate gene mapping. The maximum lod score (2.44 at recombination fraction theta=0 ) was obtained for markers D1S498> D1S3O5^ DlS2844.The cataract locus in this family constellation was mapped to Iq21.1 and 21.44cM interval between D1S2344 and D1S2844.5. We were known to flank the gene coding Connexin 50 (Cx50) or gap junction protein alpha-8 (GAJ8). Sequencing of the coding region of GJA8 gene showed a heterozygous transversion T —G in exon2, which resulted in the substitution of glycine for valine at amino acid 64, and this position is in the first connexin signature region that characterize this protein. This sequence change segregated with the disease phenotype in the family and was not found in 100 control individuals.6. The V64G mutation was located in the first extracellular domain, learning from EBI proteinbank.7. Using PROSITE software we predicted that Connexin50 -64V locate in the signature 1 function motif.