Screening a Family with Hereditary Congenital Cataract for Mutations in Q9UKB5

Cataracts occur when the proteins of the lens, mainly crystallins, clump together, making the normally transparent lens cloudy. This causes the light hitting by the lens to be scattered rather than finely focused on the retina. As a result, a person with cataracts suffers from blurred vision. Cataracts can be treated with surgery and corrective lenses, greatly improving vision in the majority of patients. However, cataracts still account for approximately 50% of blindness in the world.
The majority of cataracts are age-related (>40 years). Although congenital cataracts, those present at birth, are not as common as age-related cataracts, inherited congenital cataracts may play a crucial role in understanding the age-related form because there is increasing evidence that age-related cataract has a strong genetic component. Cataracts are also a very common symptom of genetic diseases, such as Down Syndrome and Myotonic Dystrophy.

In this study, our lab collected DNA from a family with an autosomal dominant inherited posterior polar cataract and used linkage analysis to map the family’s disease to chromosome 1p36 using micro-satellite markers. Within this region, there are 6 known genes, 5 hypothetical genes (mRNA is supported by experimental evidence but the protein has not been identified), and 4 novel genes (predicted by computer program). None of the six known genes were expressed in the lens or likely to cause cataract. As a result, I chose to begin screening for mutations in the gene Q9UKB5 because it codes for hypothetical protein MOT8, which may be a member of the aldehyde dehydrogenase (ALDH) family of enzymes. ALDH is similar to omega-crystallin, the primary protein found in the scallop lens. Q9UKB5’s similarity to omega-crystallin was the best connection with cataracts of the 15 genes in the region, as crystallin account for 99% of water-soluble proteins in the lens.

In order to search for mutations in Q9UKB5, I sequenced its 5 exons using wild type DNA and DNA from two affected individuals from our family. To do this, I used the Polymerase-Chain Reaction (PCR) to amplify the exons, and then subjected the exons to direct sequencing. To determine whether or not mutations were present in a particular exon, I visually analyzed the sequence data, looking for instances of two different alleles, a mutant disease-causing allele and a normal allele.

I was able to exclude 3 of the gene’s 5 exons as having disease-causing mutations. However, the remaining exons will need to be examined further before Q9UKB5 can be eliminated as the disease gene for congenital cataracts in the family studied.