Neuronal ceroid lipofuscinoses (NCLs) are a group of autosomal recessive neurogenetic disorders caused by mutations in 14 different genes. CLN6 disease manifests as variant late-infantile NCL (vLINCL) or as an adult variant. In childhood, symptoms include speech delay, vision loss, cognitive and motor decline, seizures, and early death. An in-depth characterization of a naturally occurring Cln6 mutant mouse (Cln6nclf) is presented, with implications for translational research. The expanded phenotype provides data showing early death, vision loss, and motor deficits in male and female Cln6nclf mice. Diminished visual acuity in Cln6nclf mice was noted at 28 weeks of age, but the pathological loss of retinal layers began as early as 2 weeks or postnatal day 14 (P14). Apoptosis was confirmed by TUNEL staining in the Cln6nclf mouse brain at P8 and in the retina at P12. A peak in glial fibrillary acidic protein (GFAP) expression was established as a normal developmental phenomenon in the wild-type and Cln6nclf mouse brain cerebellum and the CA2-CA3 regions of the hippocampus at P8. In Cln6nclf mice, GFAP levels were elevated at P12 in the cerebellum and hippocampus. In the retina, a developmental peak in gliosis was absent, with increased astrogliosis noted at P6 and P8 in female and male Cln6nclf mice, respectively. This highlights the lack of a sex-dependent response in wild-type mice. These novel data position the Cln6nclf mouse model as a useful tool for screening potential therapeutics for human CLN6 disease.

Neuronal ceroid lipofuscinoses (NCLs) are genetically heterogeneous neurodegenerative disorders, characterized by progressive cognitive and motor decline, epilepsy, visual impairment, and shortened life-expectancy. CLN6-related NCLs include both late-infantile and adult myoclonic form. We report a 21-year-old patient, with mild developmental delay, who developed occipital seizures at 14 years, and subsequently cognitive decline, cortical myoclonus, and photosensitivity at low and higher frequencies. Overall, the picture suited progressive myoclonus epilepsy. Electroretinogram was normal. A skin biopsy revealed a mixed storage of curvilinear and fingerprint profiles. A brain MRI showed severe cortical atrophy. Performing genetic analyses, two biallelic variants were identified in the CLN6 gene, each inherited from one of the healthy parents, one c.722T>C, p.(Met241Thr) already described in the late-infantile form and the other one c.486+28T>C, intronic and novel, causing aberrant splicing. In the patient, the expression of the allele containing c.722T>C variant was increased, in comparison with the carrier parent. The peculiar genetic pattern observed in the patient could explain a milder clinical picture when compared with late-infantile form, since CLN6 expression was partially preserved. However, the presence of a delay, and the early cognitive decline suggested a continuum phenotype connecting late-infantile and adult CLN6-related forms. PLAIN LANGUAGE SUMMARY: We report a patient with CLN6 disease who developed symptoms at an intermediate age: 9 years for mild intellectual disability and 14 years for occipital seizures and progressive myoclonus epilepsy, without visual impairment. The patient is compound heterozygous for a CLN6 missense variant c.722T>C, p.(Met241Thr) already described in the late-infantile form and for a novel intronic variant c.486+28T>C, causing aberrant splicing. In the patient, the expression of the allele containing c.722T>C variant was increased, compared with the carrier parent. The splice site variant had a milder effect. The peculiar genetic pattern may explain the continuum phenotype between late-infantile and adult forms.

Ceroid lipofuscinoses neuronal 6 (CLN6) disease belongs to the neuronal ceroid lipofuscinoses (NCLs), complex and genetically heterogeneous disorders with wide geographical and phenotypic variation. The first clinical signs usually appear between 18 months and 8 years, but examples of later-onset have also been reported. Common manifestations include ataxia, seizures, vision impairment, and developmental regression. Because these are shared by other neurological diseases, identification of CLN6 genetic variants is imperative for early diagnosis. We present one of the largest cohorts to date of genetically diagnosed CLN6 patients screened at a single center. In total 97 subjects, originating from 20 countries were screened between 2010 and 2020. They comprised 86 late-infantile, eight juvenile, and three adult-onset cases (two patients with Kufs disease type A, and one with teenage progressive myoclonic epilepsy). The male to female ratio was 1.06: 1.00. The age at referral was between six months and 33 years. The time from disease onset to referral ranged from less than 1 month to 8.3 years. The clinical phenotype consisted of a combination of symptoms, as reported before. We characterized a total of 45 distinct variants defining 45 distinct genotypes. Twenty-four were novel variants, some with distinct geographic associations. Remarkably, c.257A > G (p.H86R) was present in five out of 23 unrelated Egyptian individuals but in no patients from other countries. The most common genotype was homozygosity for the c.794_796del in-frame deletion. It was present in about one-third of CLN6 patients (28 unrelated cases, and 2 familial cases), all with late-infantile onset. Variants with a high likelihood of causing loss of CLN6 function were found in 21% of cases and made up 33% of all distinct variants. Forty-four percent of variants were classified as pathogenic or likely pathogenic. Our study significantly expands the number of published clinical cases and the mutational spectrum of disease-associated CLN6 variants, especially for the Middle Eastern and North African regions. We confirm previous observations regarding the most prevalent symptoms and recommend including CLN6 in the genetic diagnosis of patients presenting with early-onset abnormalities of the nervous system, musculoskeletal system, and eye.

CLN6, spanning the endoplasmic reticulum membrane, is a protein of unknown function. Mutations in the CLN6 gene are linked to an autosomal recessively inherited disorder termed CLN6 disease, classified as a form of the neuronal ceroid lipofuscinoses (NCL). The pathogenesis of CLN6 disease remains poorly understood due to a lack of information about physiological roles CLN6 plays. We previously demonstrated that CLN6 has the ability to prevent protein aggregate formation, and thus hypothesized that the abrogation of CLN6's anti-aggregate activity underlies the development of CLN6 disease. To test this hypothesis, we narrowed down the region vital for CLN6's anti-aggregate activity, and subsequently investigated if pathogenic mutations within the region attenuate CLN6's anti-aggregate activity toward four aggregation-prone αB-crystallin (αBC) mutants. None of the four αBC mutants was prevented from aggregating by the Arg106ProfsX truncated CLN6 mutant, the human counterpart of the nclf mutant identified in a naturally occurring mouse model of late infantile-onset CLN6 disease. In contrast, the Arg149Cys and the Arg149His CLN6 mutants, both associated with adult-onset CLN6 disease, blocked aggregation of two out of and all of the four αBC mutants, respectively, indicating that CLN6's anti-aggregate activity is differentially modulated according to the substitution pattern at the same amino acid position. Collectively, we here propose that the graded reduction in CLN6's anti-aggregate activity governs the clinical course of late infantile- and adult-onset NCL.