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Cln3 – A novel key to juvenile neuronal lipofuscinosis?

Juvenile neuronal lipofuscinosis (JNCL) is an autosomal recessive disease in children involving seizures, visual, motor and cognitive decline, and premature death. It is caused by mutations in the CLN3 gene. Although gene function is not fully clear yet it is believed that CLN3, a primarily endosomal-lysosomal protein in mammalian cells, plays a major role in post-Golgi, endocytic, autophagic and lysosomal trafficking.

Four mouse models of JNCL exist and have been characterised to various degrees. All display recessive features of JNCL including accumulation of ceroid lipofuscin, brain gliosis, neurological dysfunction and neurodegeneration. The knock-in mouse model used by Starapoli et al. however represents the only genetically accurate JNCL mouse model and therefore may be most predictive of the earliest molecular and cellular consequences of CLN3 mutation in JNCL. This mouse model, the Cln3∆ex7/8 mouse, has been systemically phenotyped in the German Mouse Clinic to obtain thorough knowledge of the early stages of the disease process.

Early defects in sensory and motor function
Young (10 weeks) old male mutant mice displayed reduced pre pulse inhibition (PPI) when compared to heterozygous littermates. Female homozygous Cln3∆ex7/8 mice did not show the same reduction in PPI, but displayed reduced startle reactivity compared to wild-type littermates. Homozygous Cln3∆ex7/8 mice might thus harbour early, subtle defects in sensory and motor functions.

Abnormalities in perhipheral blood
Further analyses in clinical chemistry and haematology as well as immunological screenings revealed additional alterations. By flow cytometry, a significantly lower frequency of T cells in male heterozygous and homozygous Cln3∆ex7/8 mice was observed compared to controls. The relative proportions of the CD4+ and CD8+ T cell populations were also altered in male heterozygous and homozygous Cln3∆ex7/8 mice; the ratio of CD4+/CD8+ T cells was significantly reduced in male homozygous mutant mice and tended to be lower in male heterozygous Cln3∆ex7/8 mice. In female homozygous Cln3∆ex7/8 mice, the CD4+/CD8+ ratio was also significantly reduced, though no other genotypic differences among the other leukocyte populations were observed among the female mice. Elevated serum ferritin and increased reticulocyte counts were also reproducibly detected in homozygous male and female Cln3∆ex7/8 mice. Taken together, these data suggest that CLN3 function plays a role in hematopoiesis.

Strong vacuolation in male reproductive tract
Evaluation of peripheral blood smears for vacuolated lymphocytes is a useful diagnostic tool in the workup of JNCL patients. Importantly, this feature of JNCL was replicated in homozygous Cln3∆ex7/8 mice. Surprisingly, we also discovered a profound vacuolation in cells of the male reproductive tract that was absent in all wild-type and heterozygous Cln3∆ex7/8 mice examined. Transmission electron microscopic (TEM) analysis confirmed the presence of massive, mostly translucent vacuoles in the clear cells, and further revealed an accumulation of smaller vacuoles in many of the cells as well. Despite the presence of these giant vacuoles, male homozygous Cln3∆ex7/8 mice were able to successfully breed.

This study demonstrates that very early processes could be identified that were detectable even before the first degenerative problems become obvious. These data have initiated new approaches to identifying CLN3 function and the development of new biomarker tools for pre-clinical and clinical use .

Large-Scale Phenotyping of an Accurate Genetic Mouse Model of JNCL Identifies Novel Early Pathology Outside the Central Nervous System”, Staropoli et al., PLoS One, Vol 7(6):e38310