Pontocerebellar Hypoplasia Type 9: A New Case with a Novel Mutation and Review of Literature

Pontocerebellar hypoplasia type 9 (PCH-9) is a very rare autosomal recessive neurodegenerative disorder. Affected infants present early with severe developmental delay, spasticity, with the unique magnetic resonance imaging picture of thin corpus callosum, atrophied pons, and cerebellum. It is caused by loss of function mutations in the AMPD2 gene, encoding for the adenosine monophosphate deaminase enzyme-paralog 2. This gene is expressed in different somatic tissues with high level of expression in cerebellum and its encoded enzyme catalyzes a critical step in de novo biosynthesis of purines and its deficiency in the developing neurons severely affects neuronal differentiation and cell viability. We clinically evaluated an Emirati patient presented with severe developmental and growth delay, as well as corpus callosum agenesis and atrophy of brainstem and cerebellum. We performed exome sequencing, Sanger sequencing, and segregation analysis to identify the genetic cause of the phenotype, followed by in silico and in vitro analysis. We identified the novel variant (NM_004037.9:c.1471G > A) in AMPD2 gene leading to a single amino acid substitution (p.Gly491Arg) in adenosine monophosphate deaminase-2 enzyme. This variant is predicted to be pathogenic using several in silico tools, and resulted in a decrease in the enzyme function in the patient's polymorphonuclear cells by 82% (95% confidence interval: 73.3-91.7%, p  = 0.029) compared with the control. This data establishes that the affected child is affected by PCH-9. Furthermore, we review all reported cases in literature to summarize the main clinical features of this rare disease.