PNH. PIGA codes for a protein involved in the early steps of GPI anchor synthesis, hydrophobic anchors that are attached to a range of proteins, which allows them to be attached to the membrane. This mechanism is important for protein sorting in the endoplasmatic reticulum and the Golgi apparatus. Acquired mutations in PIGA are known to cause paroxysmal nocturnal hemoglobinuria (PNH), an anemia due to destruction of red blood cells. In a recent paper in Neurology, de novo mutations in PIGA are now identified in a complex genetic syndrome, which has early-onset intractable epilepsy as the most prominent feature.
GPI anchor. In parallel to our previous post on FIG4 phenotypes, PIGA is probably best understood by explaining the physiological function. In eurokaryotic cells, proteins have various possibilities to be attached to membranes. For transmembrane proteins, this is straightforward, but there are various other possibilities. One of the frequently used mechanisms is a process called glypiation. Basically, proteins that would otherwise not attach to the membrane are linked an anchor. This anchor, glycosylphosphatidylinositol (GPI), prevents the protein from floating away into the cytoplasm and is the prerequisite for sorting in the ER and Golgi. The process of glypiation is crucial in cells – a complete disruption is not compatible with life.
The sugar code. Glycosylation, i.e. attaching carbohydrate residues to proteins is a crucial prerequisite for many cellular functions. It is assumed that up to 2% of the human genome codes for proteins implicated in generating or recognizing the “sugar code“, the carbohydrate modification of proteins. Up to 100 genetic disorders are due to malfunction in these pathways and in 2013, a new congenital disorder of glycosylation was identified every 17 days. The generation of GPI anchors also involves linking up mannose and glucosamine building blocks. Therefore, genetic disorders of GPI synthesis or remodeling also belong to the group of glycosylation disorders. Mutations in PIGO, PIGV, PGAP2 and PGAP3 affect enzymes that are involved in proper assembly of GPI anchors. Mutations in these genes often lead to intellectual disability with hyperphosphatasia. Many of these patients also have seizures. In fact, in some patients, epileptic encephalopathies are a very prominent feature. In their recent publication, Kato and collaborators find mutations in PIGA – the encoded protein catalyzes the very first step in GPI anchor synthesis.
PIGA. In the field of human disease, PIGA is no stranger. Acquired mutations in PIGA are known to cause paroxysmal nocturnal hemoglobinuria (PNH). The pathomechanism in PNH directly results from the defect in GPI anchor synthesis: as some proteins can no longer be anchored to the cell membrane, the red blood cells become a target of the complement system, which leads to a destruction of the red blood cells. As the erythrocytes are no longer equipped with the proper protein coat, they are mistaken as pathogens and eliminated. In contrast to the acquired PIGA mutation in PNH that are limited to hemopoetic cells, the epileptic encephalopathy is due to somatic mutations. Interestingly, severe epileptic encephalopathy is the most prominent feature.
PIGA encephalopathy.Kato and collaborators performed exome sequencing in 172 patients with epileptic encephalopathies and identified 4 patients with de novo mutations in PIGA. The epilepsy phenotype of the patients was classified as Early Myoclonic Encephalopathy (EME), West syndrome, or unclassified early onset epileptic encephalopathy. The authors also demonstrated that more severe epilepsies are due to mutations with a more dramatic functional effect on PIGA activity. In addition to the very prominent intractable epileptic encephalopathy, the patients showed various minor dysmorphic features, delay in myelination, and callosal hypoplasia. The EEG in most patients showed suppression-burst activity, indicating a very severe impairment of cerebral function. One patient described by Kato and collaborators became seizure-free on topiramate, but this medication was not effective in other patients.
Lessons learned. PIGA mutations are a recurrent cause of early onset epileptic encephalopathies encompassing catastrophic epilepsies including Early Myoclonic Epilepsy and Ohtahara Syndrome. This gene is an example for known disease-related genes that are rediscovered in the context of epileptic encephalopathies. The mechanism of disease, however, remains intriguing. It will be interesting to know which GPI-anchored proteins are involved in the epilepsy phenotype, as most of the GPI-anchored proteins have not been implicated in epilepsy before.