Novel likely disease-causing CLN5 variants identified in Pakistani patients with neuronal ceroid lipofuscinosis

Pediatric onset neuronal ceroid lipofuscinoses: Unraveling clinical and genetic specifications

Objective

To unravel the clinical and genetic specifications of Neuronal ceroid lipofuscinosis (NCL).

Methods

This is a retrospective cross-sectional study conducted in the Department of Pediatric Neurology Children Hospital and University of Child Health Sciences, Lahore, Pakistan from March 2017 to March 2022. The primary outcome was to measure genotype-phenotype correlation by segregation of phenotypes according to genotype. The secondary outcomes included a correlation between genotype and distribution of age(s) of onset.

Results

One hundred fifty three patients clinically diagnosed with NCL underwent genetic testing and pathologic mutation was identified in 32.7% of patients. About 59.6% were male and 37.2% had an affected sibling. The median age was 5.46±1.95 years at the onset of the first symptom i.e., myoclonic seizures in 68%, and motor difficulty in 24%. Other features found were global developmental delay (56%), hypotonia (23%), visual impairment (80%), ataxia (22%), and disc pallor (56%). The most common type was CLN6 (Ceroid lipofuscinosis neuronal) (42%), CLN2 (16%) followed by CLN7 (12%). When 50 patients with recognized mutations were compared with 103 patients with no mutation, family history (p=0.049), early visual loss (p=0.016), hypotonia (p=0.001), white matter signals (p=0.026) and pan-atrophy(p=0.047) was statistically significant in the genetically confirmed NCL. Multiple pairwise comparisons indicated that the estimated age of onset for the CLN1 and CLN2 mutation group was significantly lower than other genotypes including CLN6 (p 0.012), CLN10 (p 0.007) and CLN12 (p 0.007).

Conclusion

Following a detailed review of NCL symptomatology, a clinically-oriented approach should be used for a rapid diagnosis with confirmation by targeted molecular testing for future genetic counseling.

Characterization of neuropathology in ovine CLN5 and CLN6 neuronal ceroid lipofuscinoses (Batten disease)

Sheep with naturally occurring CLN5 and CLN6 forms of neuronal ceroid lipofuscinoses (Batten disease) share the key clinical features of the human disease and represent an ideal model system in which the clinical efficacy of gene therapies is developed and test. However, it was first important to characterize the neuropathological changes that occur with disease progression in affected sheep. This study compared neurodegeneration, neuroinflammation, and lysosomal storage accumulation in CLN5 affected Borderdale, CLN6 affected South Hampshire, and Merino sheep brains from birth to end-stage disease at ≤24 months of age. Despite very different gene products, mutations, and subcellular localizations, the pathogenic cascade was remarkably similar for all three disease models. Glial activation was present at birth in affected sheep and preceded neuronal loss, with both spreading from the visual and parieto-occipital cortices most prominently associated with clinical symptoms to the entire cortical mantle by end-stage disease. In contrast, the subcortical regions were less involved, yet lysosomal storage followed a near-linear increase across the diseased sheep brain with age. Correlation of these neuropathological changes with published clinical data identified three potential therapeutic windows in affected sheep-presymptomatic (3 months), early symptomatic (6 months), and a later symptomatic disease stage (9 months of age)-beyond which the extensive depletion of neurons was likely to diminish any chance of therapeutic benefit. This comprehensive natural history of the neuropathological changes in ovine CLN5 and CLN6 disease will be integral in determining what impact treatment has at each of these disease stages.

A novel CLN5 mutation in Turkish patient with variant late-onset neuronal ceroid lipofuscinosis and recurrent fractures that causes severe morbidity

Neuronal ceroid lipofuscinosis (NCL) is characterized by ataxia, epilepsy, mental and motor deterioration, and visual loss. The phenotype of patients is highly heterogeneous. We report a patient with late-infantile-onset psychomotor retardation, visual loss, seizure, movement disorder, and recurrent bone fractures. Clinical exome sequencing revealed a novel homozygous c.1113_1116del, p.Y371fs mutation in CLN5. No variant was detected associated with simple bone cyst. While NCL disease is difficult disease in itself, recurrent fractures significantly increased morbidity. This case report contributes to genotypic spectrum of CLN5 and emphasizes clinical importance of Turkish patients with CLN5 mutations, and non-NCL factors/diseases can adversely affect morbidity.

A lysosomal enigma CLN5 and its significance in understanding neuronal ceroid lipofuscinosis

Neuronal Ceroid Lipofuscinosis (NCL), also known as Batten disease, is an incurable childhood brain disease. The thirteen forms of NCL are caused by mutations in thirteen CLN genes. Mutations in one CLN gene, CLN5, cause variant late-infantile NCL, with an age of onset between 4 and 7 years. The CLN5 protein is ubiquitously expressed in the majority of tissues studied and in the brain, CLN5 shows both neuronal and glial cell expression. Mutations in CLN5 are associated with the accumulation of autofluorescent storage material in lysosomes, the recycling units of the cell, in the brain and peripheral tissues. CLN5 resides in the lysosome and its function is still elusive. Initial studies suggested CLN5 was a transmembrane protein, which was later revealed to be processed into a soluble form. Multiple glycosylation sites have been reported, which may dictate its localisation and function. CLN5 interacts with several CLN proteins, and other lysosomal proteins, making it an important candidate to understand lysosomal biology. The existing knowledge on CLN5 biology stems from studies using several model organisms, including mice, sheep, cattle, dogs, social amoeba and cell cultures. Each model organism has its advantages and limitations, making it crucial to adopt a combinatorial approach, using both human cells and model organisms, to understand CLN5 pathologies and design drug therapies. In this comprehensive review, we have summarised and critiqued existing literature on CLN5 and have discussed the missing pieces of the puzzle that need to be addressed to develop an efficient therapy for CLN5 Batten disease.