Elucidating the clinical and genetic spectrum of inositol polyphosphate phosphatase INPP4A-related neurodevelopmental disorder.

Purpose: Biallelic INPP4A variants have recently been associated with severe neurodevelopmental disease in single-case reports. Here, we expand and elucidate the clinical-genetic spectrum and provide a pathomechanistic explanation for genotype-phenotype correlations.

Methods: Clinical and genomic investigations of 30 individuals were undertaken alongside molecular and in silico modelling and translation reinitiation studies.

Results: We characterize a clinically variable disorder with cardinal features, including global developmental delay, severe-profound intellectual disability, microcephaly, limb weakness, cerebellar signs, and short stature. A more severe presentation associated with biallelic INPP4A variants downstream of exon 4 has additional features of (ponto)cerebellar hypoplasia, reduced cerebral volume, peripheral spasticity, contractures, intractable seizures, and cortical visual impairment. Our studies identify the likely pathomechanism of this genotype-phenotype correlation entailing translational reinitiation in exon 4 resulting in an N-terminal truncated INPP4A protein retaining partial functionality, associated with less severe disease. We also identified identical reinitiation site conservation in Inpp4a^-/- mouse models displaying similar genotype-phenotype correlation. Additionally, we show fibroblasts from a single affected individual exhibit disrupted endocytic trafficking pathways, indicating the potential biological basis of the condition.

Conclusion: Our studies comprehensively characterize INPP4A-related neurodevelopmental disorder and suggest genotype-specific clinical assessment guidelines. We propose that the potential mechanistic basis of observed genotype-phenotype correlations entails exon 4 translation reinitiation.