👤 Claudia Kitzmüller

The neuronal ceroid lipofuscinoses (NCLs) form a group of autosomal recessively inherited neurodegenerative disorders that mainly affect children. Ten NCL forms can be distinguished by age at onset, clinicopathologic features, and genetics. In eight of these forms, the underlying genes have been identified. At present, approximately 10% of all patients do not fall into one of the eight known genetic forms of NCL. We have identified two Asian families with two novel homozygous mutations in the CLN5 gene. In the first Pakistani family, two children developed symptoms of an early juvenile NCL. After exclusion of mutations in genes known to be associated with this age of onset in families from many different countries (CLN1, CLN2, CLN3, CLN6, CLN8 and CLN10) SNP array-based homozygosity mapping led to the identification of a novel homozygous mutation c.1072₁₀₇₃delTT (p.Leu358AlafsX4) in CLN5. In the second Afghan family, two children developed symptoms of a late infantile NCL. The mutation c.1137G>T (p.Trp379Cys) in CLN5 was identified. The affected children in these families represent the first reported CLN5 patients originating in Asian sibships. Expression analysis showed that mutant p.Leu358AlafsX4 CLN5 is truncated and lacks a used N-glycosylation site at Asn401. The missense mutation p.Trp379Cys affected neither the size nor glycosylation of the CLN5 protein. Double immunofluorescence microscopy showed that while the wild-type CLN5 protein is localized in lysosomes, both mutant CLN5 proteins are retained in the endoplasmic reticulum rather than reaching the lysosome. Show less

The neuronal ceroid lipofuscinoses (NCLs) are common neurodegenerative disorders of childhood and are classified as lysosomal storage diseases since affected cells exhibit lysosomes containing ceroid and lipofuscin-like material. CLN3 is the most widely conserved NCL gene, suggesting that it has a basic eukaryotic cell function; its loss might be expected to cause the earliest onset and/or most severe disease. However, mutations in CLN3 are linked to juvenile NCL (JNCL), the latest onset and mildest form of NCL in children. We sought to explain this paradox. Almost all patients with JNCL are homozygous or heterozygous for an intragenic 1 kb deletion within CLN3, hitherto presumed to be a null mutation. We hypothesized that the 1 kb mutation may allow CLN3 residual function. We confirmed the presence of CLN3 transcripts in JNCL patient cells. When RNA silencing was used to deplete these transcripts in cells from JNCL patients, the lysosomes significantly increased in size, confirming the presence of functional protein in these cells. Consistently, overexpression of mutant CLN3 transcript caused lysosomes to decrease in size. We modelled the JNCL mutant transcripts and those corresponding to mouse models for Cln3 in Schizosaccharomyces pombe and confirmed that most transcripts retained significant function as we predicted. Therefore, we concluded that the common mutant CLN3 protein does indeed retain significant function and that JNCL is a mutation-specific disease phenotype. This finding has important consequences for recognition and diagnosis of disease caused by mutations in CLN3 and for the development of therapy for JNCL. Show less