Deletion 1p36 (del1p36) syndrome is the most common human disorder resulting from a terminal autosomal deletion. This condition is molecularly and clinically heterogeneous. Deletions involving two non-overlapping regions, known as the distal (telomeric) and proximal (centromeric) critical regions, are sufficient to cause the majority of the recurrent clinical features, although with different facial features and dysmorphisms. SPEN encodes a transcriptional repressor commonly deleted in proximal del1p36 syndrome and is located centromeric to the proximal 1p36 critical region. Here, we used clinical data from 34 individuals with truncating variants in SPEN to define a neurodevelopmental disorder presenting with features that overlap considerably with those of proximal del1p36 syndrome. The clinical profile of this disease includes developmental delay/intellectual disability, autism spectrum disorder, anxiety, aggressive behavior, attention deficit disorder, hypotonia, brain and spine anomalies, congenital heart defects, high/narrow palate, facial dysmorphisms, and obesity/increased BMI, especially in females. SPEN also emerges as a relevant gene for del1p36 syndrome by co-expression analyses. Finally, we show that haploinsufficiency of SPEN is associated with a distinctive DNA methylation episignature of the X chromosome in affected females, providing further evidence of a specific contribution of the protein to the epigenetic control of this chromosome, and a paradigm of an X chromosome-specific episignature that classifies syndromic traits. We conclude that SPEN is required for multiple developmental processes and SPEN haploinsufficiency is a major contributor to a disorder associated with deletions centromeric to the previously established 1p36 critical regions.Neurodevelopmental disorders (NDDs) and intellectual disability (ID) affect approximately 1%-3% of the general population. 1-3 NDDs/ID are largely genetically determined, but identification of the underlying molecular causes has been hampered by clinical and genetic heterogeneity. During the last decades, the use of high-resolution array-based copy number variant (CNV) analysis and second-generation sequencing techniques has improved our knowledge of the genetic basis of both syndromic and non-syndromic NDDs/ID. [2][3][4] Notwithstanding these achievements, the genetic basis of NDDs/ID is still unsolved in a large proportion of affected individuals.Deletion 1p36 (del1p36) syndrome, first described by Shapira and colleagues in 1997, 5 is the most common autosomal terminal deletion syndrome in humans, occurring in about 1 in 5,000 births. [6][7][8] This disorder is characterized by developmental delay (DD)/ID, behavioral abnormalities, hypotonia, seizures, brain anomalies, vision problems, hearing loss, orofacial clefting, congenital heart defects (CHDs), cardiomyopathy, renal anomalies, short

Background and ObjectivesTo expand the clinical knowledge of GPAA1-related glycosylphosphatidylinositol (GPI) deficiency.MethodsAn international case series of 7 patients with biallelic GPAA1 variants were identified. Clinical, biochemical, and neuroimaging data were collected for comparison. Where possible, GPI-anchored proteins were assessed using flow cytometry.ResultsTen novel variants were identified in 7 patients. Flow cytometry samples of 3 available patients confirmed deficiency of several GPI-anchored proteins on leukocytes. Extensive phenotypic information was available for each patient. The majority experienced developmental delay, seizures, and hypotonia. Neuroimaging revealed cerebellar anomalies in the majority of the patients. Alkaline phosphatase was within the normal range in 5 individuals and low in 1 individual, as has been noted in other transamidase defects. We notably describe individuals either less affected or older than the ones published previously.DiscussionClinical features of the cases reported broaden the spectrum of the known phenotype of GPAA1-related GPI deficiency, while outlining the importance of using functional studies such as flow cytometry to aid in variant classification.