Lanosterol Synthase Mutations and a Modifier Gene Identified in the Hereditary Cataractous SCR Rat

Abstract

The Shumiya cataract rat (SCR) is a hereditary cataractous strain. Previous crossing experiments revealed that one genetically recessive locus and one copy of a recessive gene associated with embryonic lethality were a prerequisite for cataract onset in the SCR. In this study, we revealed unique molecular genetic features of cataract onset in the SCR by identifying a gene responsible for cataract and embryonic lethality in the SCR. Genetic linkage analysis and positional cloning revealed mutations of the gene for lanosterol synthase (Lss) on chromosome 20 in SCR, which functions in the cholesterol biosynthesis pathway. Accordingly, the cataract in SCR could be classified under cholesterol deficiency-associated cataracts. Cataract onset in SCR was uniquely regulated by a specific combination of different mutant (Lss^l) and polymorphic alleles (Lss^s) on the Lss locus. Lss^s was a hypomorphic allele with a G to A nucleotide substitution in exon 4 that reduced LSS activity to about 40% of normal by substituting an amino acid (D139N) . The G to A nucleotide substitution also caused aberrant splicing, which led to transcripts with a deletion of a sequence corresponding to the last 47 nucleotides of the exon 4. Thus, the Lss^s allele was unique in that one nucleotide substitution acted synergistically to reduce enzyme activity. Lss^l was a null allele with a deletion of thirteen consecutive nucleotides (1405-1407del) plus a single nucleotide insertion (1419-1420insT), leading to a deletion of four consecutive amino acids (H469-C472del) . Thus, Lss^s and Lss^l could be assumed to be cataract and embryonic lethal alleles, respectively. These observations indicated that the cataract onset in the SCR complied with a threshold model. The prerequisite for cataract onset was the Lss^s/Lss^l genotype, which reduced LSS activity below the threshold, that is about 12-24% of normal. Furthermore, the linkage analysis revealed that a locus on chromosome 15 of normal ACT rat strain (Cats2^A) suppressed cataract onset with incomplete penetrance, indicating that cataract in SCR is an oligogenic trait.The through elucidation of the mechanisms for cataract in SCR might afford an excellent insight into the studies of threshold diseases as well as of other oligogenic traits.