Molecular genetics of cataract.

Studies on hereditary congenital cataracts have led to the identification of genes involved in formation of these cataracts. Knowledge of the structure and function of a particular gene and the effect of disease-associated mutations on its function are providing insights into the mechanisms of cataract. Identification of the disease gene requires both the relevant clinical data as well as genetic data on the entire pedigree in which the disease is found to occur. Genes for hereditary cataract have been mapped by genetic linkage analysis, in which one examines the inheritance pattern of DNA markers throughout the genome in all individuals of the pedigree, and compares those with the inheritance of the disease. Cosegregation of a set of markers with disease implies that the disease gene is present at the same chromosomal location as those markers. The genes so far identified for hereditary cataracts in both humans and animal models encode structural lens proteins, gap junction proteins, membrane proteins and regulatory proteins involved in lens development. Understanding of the mechanisms of hereditary cataract may also help us understand the manner in which environmental and nutritional factors act on the lens to promote opacification.

Investigations into size heterogeneity of the alpha B-crystallin mRNA.

alpha B-crystallin is expressed in a variety of developmental, physiological and pathological conditions in a number of tissues. Based on northern blots of total RNA, the existence of at least two size classes of alpha B-crystallin mRNAs has been reported, one smaller predominant species, 0.8-0.9 kb, and the other 1.2 to 1.4 kb. This heterogeneity has been attributed to alternative upstream transcriptional initiation. We have investigated the origin of the size heterogeneity of alpha B-crystallin mRNA by using 5'-upstream-, coding- and 3'-untranslated-region probes in RNAse protection and northern blot assays. RNAse protection assays indicate that there is only one predominant initiation site as previously reported and that the second polyadenylation signal is not used in the rat gene. Importantly, northern blot data obtained with coding region-only probe shows that the size of alpha B mRNA detected in the heart and the lens is similar (0.78 kb) in poly A+ as well as in total RNA. On the other hand, in the brain and in the lung, the larger hybridizing species (1.05 kb and 1.22 kb respectively) seen in total RNA are not detected in poly A+RNA which shows a 0.95 kb species in both tissues. The 5' upstream probe (-1 to -499) produces weak hybridization patterns in the brain and the lung, similar to those obtained with coding region-only probe. The 5' probe did not show hybridization in the heart and the lens RNAs. These data suggest that upstream initiations represent a minor population of transcripts and that higher size transcripts (about 500 bp larger) actually represent non-polyadenylated RNAs that may not contribute to the generation of the actual gene product.