Cross–regulation of CLN5 and CLN6 gene expression in ovine Batten disease models

Abstract

Sheep with naturally occurring CLN5 and CLN6 forms of Batten disease (neuronal ceroid lipofuscinoses, NCLs) are studied as models of the human diseases. Despite having mutations in different genes, the development of clinical and neuropathological disease in CLN5 and CLN6 affected sheep is very similar. Common features, including the near-ubiquitous accumulation of lysosome-derived storage bodies, regional brain atrophy from progressive neuronal loss, retinal degeneration, seizures and psychomotor decline culminating in premature death follow a very similar time-course in both forms. This is unexpected as the CLN5 and CLN6 genes and mutations are quite different and apparently unrelated. CLN5 disease is caused by an intronic splice-site mutation resulting in excision of an exon from the mRNA for a soluble lysosomal protein, while CLN6 results from a large 5’UTR-exon 1 deletion from the gene for a membrane bound protein of unknown function.

Antibodies specific for the ovine CLN5 protein revealed a band of concentrated endogenous staining in the hippocampus of unaffected control sheep, that was not apparent in CLN5 affected sheep. Surprisingly there was very little CLN5 expression in CLN6 affected sheep either, and only intermediate staining in heterozygous CLN6+/-sheep. The implied interaction of CLN5 and CLN6 expression was explored further in a quantitative PCR study of brain samples. Full length CLN5 mRNA expression was up-regulated 3-4 fold in CLN6 affected brains, thus there is plenty of CLN5 mRNA but this does not result in successful CLN5 protein expression. Heterozygotes gave intermediate values. Similar results were obtained from other non-neuronal tissues, showing that they did not arise from neurodegeneration. Control CLN6 expression was 10 fold less than CLN5 expression and was up-regulated in some brain regions in CLN5 affected sheep.

These results suggest that the CLN5 and CLN6 protein and gene expressions are cross- regulated. CLN6 is an endoplasmic reticulum resident protein. If it is required for the correct processing of the CLN5 protein, then it may be that no CLN5 protein reaches the lysosome in CLN6 affected sheep, resulting in an induced lysosomal CLN5 deficiency disease. CLN5 transcription may be up-regulated in a vain attempt to rectify this. The CLN6 protein expression may become up-regulated likewise if it receives no substrate.