Batten disease: evaluation of CLN3 mutations on protein localization and function

Juvenile neuronal ceroid lipofuscinosis (JNCL), Batten disease, is an autos omal recessive lysosomal storage disease associated with mutations in CLN3. CLN3 has no known homology to other proteins and a function has not yet be en described. The predominant mutation in CLN3 is a 1.02 kb genomic deletio n that accounts for nearly 85% of the disease alleles, In this mutation, tr uncation of the protein by a premature stop codon results in the classical phenotype. Additional missense and nonsense mutations have been described, Some missense substitutions result in a protracted phenotype, with delays i n the onset of classical clinical features, whereas others lead to classica l JNCL. In this study, we examined the effect of naturally occurring point mutations on the intracellular localization of CLN3 and their ability to co mplement the CLN3-deficient yeast, btn1-Delta. We also examined a putative farnesylation motif thought to be involved in CLN3 trafficking. All of the point mutations, like wild-type CLN3, were highly associated with lysosome- associated membrane protein II in non-neuronal cells and with synaptophysin in neuronal cell lines. In the yeast functional assay, point mutations cor relating with a mild phenotype also demonstrated CLN3 activity, whereas the mutations associated with severe disease failed to restore CLN3 function c ompletely. CLN3 with a mutation in the farnesylation motif trafficked norma lly but was functionally impaired. These data suggest that these clinically relevant point mutations, causative of Batten disease, do not affect prote in trafficking but rather exert their effects by impairing protein function .