Purpose: SLC16A12 encodes an orphan member of the monocarboxylate transporter family, MCT12. A nonsense mutation in SLC16A12 (c.643C>T; p.Q215X) causes juvenile cataract with a dominant inheritance pattern. In the present study, in vitro and in vivo experimental models were used to gain insight into how the SLC16A12 (c.643C>T) mutation leads to cataract formation.
Methods: MCT12 peptide antibodies were generated and used to examine the expression of MCT12 in the lens using immuno-confocal microscopy. To determine whether loss of Slc16a12 resulted in cataract formation, a Slc16a12 hypomorphic rat generated by transposon insertional mutagenesis was characterized using RT-PCR, slit lamp microscopy and histologic methods. Exogenous expression of MCT12 and MCT12:214Δ, a mimic of the mutant allele, were used to assess protein expression and trafficking.
Results: MCT12 protein was detected in the lens epithelium and secondary fiber cells at postnatal day 1. In the Slc16a12(TKO) rat, complete loss of MCT12 did not result in any detectable ocular phenotype. Exogenous expression of MCT12-GFP and MCT12:214Δ-GFP revealed that the full-length protein was trafficked to the plasma membrane (PM), whereas the truncated protein was retained in the endoplasmic reticulum (ER). When both MCT12 and MCT12:214Δ were coexpressed, to mimic the heterozygous patient genotype, the truncated protein was retained in the ER whereas full-length MCT12 was trafficked to the PM. Furthermore, MCT12 was identified as another MCT isoform that requires CD147 for trafficking to the cell surface.
Conclusions: These data support a model whereby the SLC16A12 (c.643C>T) mutation causes juvenile cataract by a defect in protein trafficking rather than by haploinsufficiency.