| Mice deficient in Klotho gene expression exhibit a syndrome resembling human aging, including atherosclerosis, osteoporosis, emphysema, and infertility—phenotypes not associated with normal mouse aging. In this thesis, we studied polymorphisms at the human KLOTHO locus to determine if this gene product contributes to human aging. We applied two newly characterized polymorphic microsatellite markers flanking the gene in a population-based association study. In a cohort chosen for its homogeneity, Bohemian Czechs, we demonstrated significant differences in selected marker allele frequencies between newborn and elderly individuals ( P < 0.05). These results precipitated a search for functional variants of klotho. We identified an allele, termed KL-VS, containing six sequence variants in complete linkage disequilibrium, two of which result in amino acid substitutions F352V and C370S. Homozygous elderly individuals were underrepresented in three distinct populations: Bohemian Czechs, Baltimore Caucasians and Baltimore African Americans (combined OR = 2.59, P < 0.0023). Since major changes were manifest in the cardiovascular system of klotho mice, we examined KLOTHO allele sharing between sibs concordant and discordant for early-onset coronary artery disease (CAD). While the KL-VS allele was not found to influence early-onset CAD, increased KLOTHO allele sharing may be associated with increased likelihood of concordance for CAD status (P < 0.066). Based, in part, on the identification of a secreted form, it has been proposed that klotho functions as a humoral factor. In a transient transfection assay, extracellular levels of klotho harboring V352 are reduced 6-fold, without decrease in protein synthesis or stability, suggesting reduced secretion. Surprisingly, extracellular levels of klotho harboring S370 are increased 2.9-fold. The V352/S370 double-mutant exhibits an intermediate phenotype (1.6-fold increase), providing a rare example of intragenic complementation in cis by human SNPs. The remarkable conservation of F352 among homologous proteins suggests that it is functionally important. The corresponding substitution, F289V, in the closest human klotho paralog with a known substrate, cBGL1, completely eliminates its ability to cleave p-nitrophenyl-β-D-glucoside. These results suggest that the KL-VS allele influences the trafficking and catalytic activity of klotho, and that variation in klotho function contributes to heterogeneity in the onset and severity of human age-related phenotypes. |
