Lens transparency, a requirement of unobstructed vision, is achieved by ordered events of lens cell differentiation. Disturbances during the developmental process of lens differentiation or impact of environmental factors can lead to a change in transparency and to the development of childhood or age-related cataract. The majority of these defects are inherited dominantly but recessive patterns are also known. Using a large family segregating congenital cataract we mapped the disease locus to a region on chromosome 10 and identified a premature termination codon mutation in a coding exon of SLC16A12, which codes for the monocarboxylate transporter, MCT12. Mutation screen in DNA from cataract patients revealed an SNP in the 5’UTR as likely risk factor for age-related cataract only. Furthermore we found a substantial number of missense mutations but also splice site mutations.
We were able to identify creatine as one substrate of MCT12 by combining a metabolomics approach with the heterologous expression system of Xenopus laevis. Another creatine transporter (CRT1) was known which is encoded by SLC6A8 and mutations lead to mental retardation. We are investigating the specificity of MCT12 in distinction to CRT1 with respect to gene expression and transport characteristics.
To understand the mechanism of the transport activity of MCT12 we are testing the cataract associated mutations for their ability to transport creatine, kinetic differences and membrane localization. Initially we are using protein modeling combined with substrate docking predictions, which will be tested in Xenopus oocytes and mammalian cell culture.
- Simone Camargo and Francois Verrey, Institute of Physiology, University Zurich
- Endre Laczko, Functional Genomics Center, University Zurich, ETHZ
- Nancy Philp, Thomas Jefferson University, Philadelphia, PA, USA
- Alex Levine, Wills Eye Institute and Thomas Jefferson University, Philadelphia, PA, USA
- Tien Wong, Singapore Eye Research Institute (SERI), Singapore