Complete loss of CASK causes severe ataxia through cerebellar degeneration in human and mouse

Abstract

Heterozygous loss of X-linked genes like CASK and MeCP2 (Rett syndrome) causes neurodevelopmental disorders (NDD) in girls, while in boys such loss leads to profound encephalopathy. The cellular basis for these disorders remains unknown. CASK is presumed to work through the Tbr1-reelin pathway in neuronal migration during brain development. Here we report our clinical and histopathological analysis of a deceased 2-month-old boy with a CASK-null mutation. We demonstrate that although smaller in size, the CASK-null human brain exhibits normal lamination without defective neuronal differentiation, migration, or axonal guidance, excluding the role of reelin in CASK-linked pathology. The disproportionately hypoplastic cerebellum in humans without CASK expression is associated with cerebellar astrogliosis, a marker for neuronal loss. Cerebellum-specific deletion in mouse confirms a post-developmental degeneration of cerebellar granular neurons that results in a small cerebellum. Mechanistically, cerebellar hypoplasia in CASK mutation thus results from neurodegeneration rather that developmental defects. Zygosity-pathology correlation suggests that NDDs like CASK mutation and Rett syndrome are pathologically neurodegenerative; however, random X-chromosome inactivation in the typical heterozygous mutant girls results in 50% of cells expressing the functional gene, resulting in a non-progressive pathology, whereas complete loss of the only allele in boys leads to unconstrained degeneration and encephalopathy.

The Non-Linear Path from Gene Dysfunction to Genetic Disease: Lessons from the MICPCH Mouse Model

“…In contrast to girls, who have two alleles of the CASK gene, hemizygous boys with CASK loss-of-function variants present with a more severe phenotype. In addition to MICPCH, they are also diagnosed with epileptic encephalopathies such as Ohtahara syndrome and West syndrome (infantile spasms) due to the presence of early-onset epilepsy and diagnostic EEG (electroencephalogram) patterns such as burst-suppression and hypsarrhythmia [ 35 , 36 , 37 , 38 ]. Boys with mosaic variants in CASK that arise during embryogenesis present with MICPCH, just like girls [ 33 , 35 ].…”

“…Overall the phenotype indicates that the role of the CASK gene itself does not have any sex-specificity, but rather that differences in phenotype between girls and boys arise simply due to the fact that in boys there is only one X-chromosome and hence one CASK gene. Analysis of boys with CASK -null variants suggests that the lethality observed in boys is due to thinning of the brain stem which leads to aberrant respiratory and deglutition reflexes manifesting systemically as central neurogenic respiratory failure, airway infection, and pneumonia [ 35 , 36 , 37 ]. A previously published histopathology of a posthumous CASK -variant case suggested that there could be defects in cortical and cerebellar layering, indicating a defect in neuronal migration.…”

“…No specific defects were found in the vasculature or meninges. The hindbrain, including the cerebellum and brain stem, was found to be extremely hypoplastic, and histologically, a decrease in granule cells in the cerebellum and gliosis was noted; no hallmarks of defective neuronal migration were evident [ 37 ].…”

“…CASK variants can also be associated with nonspecific phenotypes in organs other than the brain [ 33 , 35 , 37 ]. In some cases, CASK variants have been associated with microcephaly without cerebellar hypoplasia [ 39 ].…”

“…To better address the question of degeneration in the setting of early development, we next synchronized the timing of Cask deletion with the completion of neuronal migration in the cerebella of mice. We observed that this manipulation leads to rapid atrophy of the cerebellum, specifically of the granule cell layer, beginning at 2 months [ 37 ]. The mice remain otherwise healthy but lose their balance completely.…”