Investigation of Neural Pathogenic Mechanisms Associated with Congenital Disorders of Glycosylation

Icahn School Of Medicine At Mount Sinai, New York NY

Investigators

Abstract

PROJECT SUMMARY Mutations in the Phosphatidylinositol glycan class A (PIGA) gene cause PIGA deficiency, an X-linked epilepsy and intellectual developmental disorder. PIGA deficiency is a rare Congenital Disorder of Glycosylation (CDG) with only 20 patients reported. PIGA deficiency is characterized by neonatal hypotonia, myoclonic seizures, epilepsy, dysmorphic features, and a number of other congenital anomalies. PIGA is involved in the first step of glycosylphosphatidylinositol (GPI) anchor biosynthesis. The GPI attaches a protein to the cell surface. GPI- anchored proteins play a number of roles in cell-cell signaling, cell migration, and immunity. With such wide reaching effects, loss PIGA is likely to affect many different cell and tissue types and in very different ways. It remains unclear how mutations in PIGA contribute to the large spectrum of phenotypes observed in patients. Forty two different disease causing variants have been reported among the 79 reported patients. Most of these variants are missense or in-frame deletions in evolutionarily invariant amino acids, spanning the entire protein, with only one protein truncating mutation. Epilepsy and movement disorder are the two most common phenotypes among these patients. There are also a number of phenotypes, including metabolic dysfunction and dysmorphic features that are only observed in some patients. It is unknown, however, how mutations in PIGA result in epilepsy and a movement disorder. Further, it is unknown whether the phenotypic variation observed among patients is due to differences in the PIGA pathogenic mutations or differences in genetic background. Similar to PIGA, mutations in PMM2 also cause epilepsy and movement disorder; however mutations in PMM2 disrupt an early step in N-linked glycosylation. PMM2 CDG also shows extensive phenotypic variation and it is unknown whether patient mutations or genetic background contribute to this variability. We hypothesize that the spectrum of phenotypes observed among patients arises from a complex interaction between tissue-specific effects, the specific pathogenic mutation, and genetic background modifiers. We will use powerful Drosophila genetic tools to address this. Drosophila is particularly well suited for this because 1) it is simple to perform tissue specific analysis; 2) it is possible to generate many different mutant lines in a short amount of time; and 3) there are unique, efficient methods for identifying modifier genes. In Aim 1, we propose to 1) determine the differential requirement for PIGA function in different cell types and 2) determine how patient mutations affect these cell- type-specific functions. In Aim 2, we propose to study genetic modifiers of a model of PIGA deficiency. In Aim 3, we will apply the same techniques to begin studies on PMM2. Understanding the sources of disease differences among patients will begin to shed light on potential personalized therapies.