Retention of lysosomal protein CLN5 in the endoplasmic reticulum causes neuronal ceroid lipofuscinosis in Asian Sibship

Abstract

ABSTRACT: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.1072_1073delTT (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.

Proteolytic processing of the neuronal ceroid lipofuscinosis related lysosomal protein CLN5

“…For some NCL proteins it's mainly due to the low expression in tissues. For CLN5, several groups have attempted to generate antibodies without much success in detecting endogenous protein expression [27,25,29,30,19]. Here we identified an antibody that is able to recognize endogenous CLN5 in various tissues and cell lines from human and mice.…”

NCL diseases — clinical perspectives

“…Mutations in the same gene may also lead to very different disease courses [3,4]. Other designations such as “Finnish” or “Turkish” NCL variant are outdated, as mutations in the respective genes in fact occur worldwide [5]. Therefore, the hitherto existing nomenclature is obsolete.…”

“…This form of late-infantile NCL [20] has been called the “Finnish variant,” but it occurs world-wide [5]. Age at onset is more variable than in classic CLN2 disease, a mean age at onset of 5.6 years (range 4–17 years).…”

Interactions of the proteins of neuronal ceroid lipofuscinosis: clues to function

“…CLN5 is most likely a soluble protein based on the presence of CLN5 in culture medium due to secretion from transiently transfected BHK-21 cells [98], the ability to isolate CLN5 by mannose-6-phosphate affinity purification [101], and the likelihood that a N-terminal signal peptide is cleaved from CLN5 in its maturation to the lysosome [98, 102]. Further, both mouse and human CLN5 were reported to be soluble by Triton X-114 fractionation with the appropriate controls, using PDI (a soluble protein) and transferrin receptor (a membrane protein) for confirmation [100].…”

“…CLN5 predominantly colocalizes with lysosomal associated membrane protein-1 (LAMP1) in several studies [95, 98–102, 104–106]. Mutations in CLN5 result in Finnish variant late infantile NCL (Fin-vLINCL) [97, 107, 108]; recently, mutations in CLN5 have been found outside Finnish populations [97, 102, 106, 109, 110].…”