Attenuated cerebellar phenotypes in Inpp4a truncation mutants with preserved phosphatase activity

Phosphoinositides (PIPs) act as intracellular signaling molecules that regulate various cellular processes. Abnormalities in PIP metabolism cause various pathological conditions, including neurodegenerative diseases, cancer, and immune disorders. Several neurological diseases with diverse phenotypes, such as ataxia with cerebellar atrophy or intellectual disability without brain malformation, are caused by mutations in INPP4A, which encodes a phosphoinositide phosphatase. This study examined two strains of Inpp4a mutant mice with distinct cerebellar phenotypes: the first Inpp4aΔEx1,2 mutant exhibited striatal degeneration without cerebellar atrophy, and the other Inpp4aΔEx23 mutant exhibited a severe striatal phenotype with cerebellar atrophy. Both strains exhibited reduced expressions of Inpp4a mutant proteins in the cerebellum. N-terminal truncated Inpp4a proteins were expressed from Inpp4aΔEx1,2 allele by alternative translation initiation and had phosphatase activity for PI(3,4)P2, whereas the Inpp4a mutant protein encoded by Inpp4aΔEx23 completely lacked phosphatase activity. The diverse phenotypes observed in Inpp4a-related neurological diseases could be due to the varying protein expression levels and retained phosphatase activity in different Inpp4a variants. These findings provide insights into the role of Inpp4a mutations in disease pathogenesis and may help to develop personalized therapy.