Highlights d Three groups of highly genetically-related disorders among 8 psychiatric disorders d Identified 109 pleiotropic loci affecting more than one disorder d Pleiotropic genes show heightened expression beginning in 2 nd prenatal trimester d Pleiotropic genes play prominent roles in neurodevelopmental processes Authors Cross-Disorder Group of the Psychiatric Genomics Consortium

Infantile spasms (IS) is an early-onset epileptic encephalopathy of unknown etiology in ∼40% of patients. We hypothesized that unexplained IS cases represent a large collection of rare single-gene disorders. We investigated 44 children with unexplained IS using comparative genomic hybridisation arrays (aCGH) (n = 44) followed by targeted sequencing of 35 known epilepsy genes (n = 8) or whole-exome sequencing (WES) of familial trios (n = 18) to search for rare inherited or de novo mutations. aCGH analysis revealed de novo variants in 7% of patients (n = 3/44), including a distal 16p11.2 duplication, a 15q11.1q13.1 tetrasomy and a 2q21.3-q22.2 deletion. Furthermore, it identified a pathogenic maternally inherited Xp11.2 duplication. Targeted sequencing was informative for ARX (n = 1/14) and STXBP1 (n = 1/8). In contrast, sequencing of a panel of 35 known epileptic encephalopathy genes (n = 8) did not identify further mutations. Finally, WES (n = 18) was very informative, with an excess of de novo mutations identified in genes predicted to be involved in neurodevelopmental processes and/or known to be intolerant to functional variations. Several pathogenic mutations were identified, including de novo mutations in STXBP1, CASK and ALG13, as well as recessive mutations in PNPO and ADSL, together explaining 28% of cases (5/18). In addition, WES identified 1-3 de novo variants in 64% of remaining probands, pointing to several interesting candidate genes. Our results indicate that IS are genetically heterogeneous with a major contribution of de novo mutations and that WES is significantly superior to targeted re-sequencing in identifying detrimental genetic variants involved in IS.

Hereditary spastic paraplegia (HSP) is a progressive upper-motor neurodegenerative disease. The eighth HSP locus, SPG8, is on chromosome 8p24.13. The three families previously linked to the SPG8 locus present with relatively severe, pure spastic paraplegia. We have identified three mutations in the KIAA0196 gene in six families that map to the SPG8 locus. One mutation, V626F, segregated in three large North American families with European ancestry and in one British family. An L619F mutation was found in a Brazilian family. The third mutation, N471D, was identified in a smaller family of European origin and lies in a spectrin domain. None of these mutations were identified in 500 control individuals. Both the L619 and V626 residues are strictly conserved across species and likely have a notable effect on the structure of the protein product strumpellin. Rescue studies with human mRNA injected in zebrafish treated with morpholino oligonucleotides to knock down the endogenous protein showed that mutations at these two residues impaired the normal function of the KIAA0196 gene. However, the function of the 1,159-aa strumpellin protein is relatively unknown. The identification and characterization of the KIAA0196 gene will enable further insight into the pathogenesis of HSP.