Early infantile epileptic encephalopathy due to biallelic pathogenic variants in PIGQ: Report of seven new subjects and review of the literature

Abstract

We investigated seven children from six families to expand the phenotypic spectrum associated with an early infantile epileptic encephalopathy caused by biallelic pathogenic variants in the phosphatidylinositol glycan anchor biosynthesis class Q (PIGQ) gene. The affected children were all identified by clinical or research exome sequencing. Clinical data, including EEGs and MRIs, was comprehensively reviewed and flow cytometry and transfection experiments were performed to investigate PIGQ function. Pathogenic biallelic PIGQ variants were associated with increased mortality. Epileptic seizures, axial hypotonia, developmental delay and multiple congenital anomalies were consistently observed. Seizure onset occurred between 2.5 months and 7 months of age and varied from treatable seizures to recurrent episodes of status epilepticus. Gastrointestinal issues were common and severe, two affected individuals had midgut volvulus requiring surgical correction. Cardiac anomalies including arrythmias were observed. Flow cytometry using granulocytes and fibroblasts from affected individuals showed reduced expression of glycosylphosphatidylinositol (GPI)-anchored proteins. Transfection of wildtype PIGQ cDNA into patient fibroblasts rescued this phenotype. We expand the phenotypic spectrum of PIGQ-related disease and provide the first functional evidence in human cells of defective GPI-anchoring due to pathogenic variants in PIGQ.

Keywords: GPI; IGD; PIGQ; epileptic encephalopathy; exome sequencing; rare diseases.

FIGURE 1

A, Variants in PIGQ identified in this study localized on isoform 1 (NM_148920.2) of the gene. Variants reported in previous studies are in red with corresponding reference numbers in superscript. Brain MRI of subject 4 was performed at age 9 days, B, and 7 months, C. At each age, T2‐weighted image sequences were performed with axial (top) and coronal (bottom) images shown. Brain MRI at 7 months showed progressive cortical volume loss with increased prominence of the lateral ventricles, consistent with loss of subcortical white matter volume compared to the prior study. Mild delay in myelination was apparent. D, Diffusion weighted axial images identified restricted diffusion in the medial lemniscus tracts, bilaterally

FIGURE 2

Impact of the PIGQ variants on the expression of GPI‐APs. Flow cytometry analysis of granulocytes from fresh blood from subject 4 (top row) and subject 5 (bottom row) and compared to healthy controls and parents, respectively. Fluorescently labeled proaerolysin “FLAER” directly binds the GPI‐anchor, whereas CD16, CD55, and CD59 all stain for GPI‐APs

FIGURE 3

Impact of the PIGQ variants on the expression of GPI‐APs. Flow cytometry analysis of fibroblasts derived from subject 3b (top row) and subject 5 (bottom row) and compared to healthy controls and parental derived cell‐lines, respectively. Subject cell lines were further transfected with empty lentivirus or lentivirus expressing WT PIGQ cDNA. Fluorescently labeled proaerolysin “FLAER” directly binds the GPI‐anchor, whereas CD73 and CD109 stain for GPI‐APs