Date of Award
Medical Doctor (MD)
Pietro De Camilli
Next-generation sequencing has recently allowed for the identification of mutations in phosphatidylinositol 4-kinase type IIIα (PI4KIIIα) and its adaptor proteins as the causative factor in several genetic diseases. PI4KIIIα catalyzes production of phosphatidylinositol 4-phosphate (PI4P) at the plasma membrane, where it exists in complex with two subunits, the membrane protein EFR3 and the scaffold TTC7. Meanwhile, mutations in the FAM126A gene have been reported to be associated with hypomyelination and congenital cataract (HCC), a disorder characterized by abnormal development of central and peripheral myelin, bilateral cataracts, and neurological impairment. Prior studies have suggested that the encoded protein, FAM126A/hyccin, may interact with the PI4KIIIα complex. However, the cellular function of FAM126A and its precise relation to PI4KIIIα remain unknown.
To address these questions, we investigated the possible role of FAM126A in regulating PI4KIIIα activity. We demonstrate here that FAM126A directly interacts with and stabilizes the PI4KIIIα complex. Imaging of the PI4KIIIα complex and structural studies reveal that FAM126A specifically binds to TTC7, and in vitro assays show that binding of FAM126A and TTC7 to PI4KIIIα improves the enzymatic catalysis of PI4P production. In concordance with these findings, primary skin fibroblasts from patients with HCC have reduced levels of PI4KIIIα complex proteins and decreased amounts of plasma membrane PI4P as compared to controls. We show that this phenotype results from a destabilization of the affected proteins, which can be rescued by expression of GFP- tagged FAM126A. In addition, we studied a mouse model with FAM126A knockout to observe its effects specifically in neural tissue. We show that these knockout mice have lower levels of PI4KIIIα complex components, and that this effect is most pronounced in cells of oligodendroglial origin. These results imply that deficient PI4P generation due to the absence of FAM126A plays a critical role in causing the impaired myelin formation observed in HCC.
Supporting the critical role of proper PI4P metabolism in early development is the existence of other genetic diseases and disorders that arise due to mutations in PI4KA and TTC7A. Mutations in the former lead to a variety of severe neurological defects, while TTC7A deficiency causes a syndrome of combined immunodeficiency and multiple intestinal atresias (CID-MIA). We examined primary skin fibroblasts from patients with these mutations, and demonstrate that these cells exhibit PI4KIIIα complex dysfunction similar to that found in HCC. Differing expression patterns of these component proteins may partly explain the diversity of clinical phenotypes that can be linked to a common underlying mechanism of abnormal PI4P metabolism.
Oh, Michael, "Molecular Mechanisms Of Genetic Disorders Associated With The Pi4kiiiα Complex" (2016). Yale Medicine Thesis Digital Library. 2068.