Classic "PCH" Genes are a Rare Cause of Radiologic Pontocerebellar Hypoplasia

IMPDH2 filaments protect from neurodegeneration in AMPD2 deficiency

Metabolic dysregulation is one of the most common causes of pediatric neurodegenerative disorders. However, how the disruption of ubiquitous and essential metabolic pathways predominantly affect neural tissue remains unclear. Here we use mouse models of a childhood neurodegenerative disorder caused by AMPD2 deficiency to study cellular and molecular mechanisms that lead to selective neuronal vulnerability to purine metabolism imbalance. We show that mouse models of AMPD2 deficiency exhibit predominant degeneration of the hippocampal dentate gyrus, despite a general reduction of brain GTP levels. Neurodegeneration-resistant regions accumulate micron-sized filaments of IMPDH2, the rate limiting enzyme in GTP synthesis, while these filaments are barely detectable in the hippocampal dentate gyrus. Furthermore, we show that IMPDH2 filament disassembly reduces GTP levels and impairs growth of neural progenitor cells derived from individuals with human AMPD2 deficiency. Together, our findings suggest that IMPDH2 polymerization prevents detrimental GTP deprivation, opening the possibility of exploring the induction of IMPDH2 assembly as a therapy for neurodegeneration.

Case report: A severe clinical phenotype of pontocerebellar hypoplasia type 7 with compound heterozygous variants of TOE1

Pontocerebellar Hypoplasia (PCH) is a rare autosomal recessive hereditary neurological degenerative disease. To elaborate upon the clinical phenotypes of PCH and explore the correlation between TOE1 gene mutations and clinical phenotype, we analyze the clinical and genetic features of a Chinese infant afflicted with pontocerebellar dysplasia accompanied by gender reversal with bioinformatics methods. The main clinical features of this infant with TOE1 gene mutation included progressive lateral ventricle widening, hydrocephalus, severe postnatal growth retardation, and hypotonia, and simultaneously being accompanied by 46, XY female sex reversal. Whole exome sequencing revealed a compound heterozygous mutation in the TOE1 gene (c.299T > G, c.1414T > G), with the protein homology modeling-generated structure predicting a pathogenic variation, which is closely related to the clinical manifestations in the patient. The new mutation sites, c.299T > G and c.1414T > G, in the TOE1 gene are pathogenic variants of pontocerebellar hypoplasia type 7.

IMPDH2 filaments protect from neurodegeneration in AMPD2 deficiency

Metabolic dysregulation is one of the most common causes of pediatric neurodegenerative disorders. However, how the disruption of ubiquitous and essential metabolic pathways predominantly affect neural tissue remains unclear. Here we use mouse models of AMPD2 deficiency to study cellular and molecular mechanisms that lead to selective neuronal vulnerability to purine metabolism imbalance. We show that AMPD deficiency in mice primarily leads to hippocampal dentate gyrus degeneration despite causing a generalized reduction of brain GTP levels. Remarkably, we found that neurodegeneration resistant regions accumulate micron sized filaments of IMPDH2, the rate limiting enzyme in GTP synthesis. In contrast, IMPDH2 filaments are barely detectable in the hippocampal dentate gyrus, which shows a progressive neuroinflammation and neurodegeneration. Furthermore, using a human AMPD2 deficient neural cell culture model, we show that blocking IMPDH2 polymerization with a dominant negative IMPDH2 variant, impairs AMPD2 deficient neural progenitor growth. Together, our findings suggest that IMPDH2 polymerization prevents detrimental GTP deprivation in neurons with available GTP precursor molecules, providing resistance to neurodegeneration. Our findings open the possibility of exploring the involvement of IMPDH2 assembly as a therapeutic intervention for neurodegeneration.