Zollo et al. report that mutations in PRUNE1, a phosphoesterase superfamily molecule, underlie primary microcephaly and profound global developmental delay in four unrelated families from Oman, India, Iran and Italy. The study highlights a potential role for prune during microtubule polymerization, suggesting that prune syndrome may be a tubulinopathy.

We investigated the genetic, phenotypic, and interferon status of 46 patients from 37 families with neurological disease due to mutations in ADAR1. The clinicoradiological phenotype encompassed a spectrum of Aicardi–Goutières syndrome, isolated bilateral striatal necrosis, spastic paraparesis with normal neuroimaging, a progressive spastic dystonic motor disorder, and adult-onset psychological difficulties with intracranial calcification. Homozygous missense mutations were recorded in five families. We observed a p.Pro193Ala variant in the heterozygous state in 22 of 23 families with compound heterozygous mutations. We also ascertained 11 cases from nine families with a p.Gly1007Arg dominant-negative mutation, which occurred de novo in four patients, and was inherited in three families in association with marked phenotypic variability. In 50 of 52 samples from 34 patients, we identified a marked upregulation of type I interferon-stimulated gene transcripts in peripheral blood, with a median interferon score of 16.99 (interquartile range [IQR]: 10.64–25.71) compared with controls (median: 0.93, IQR: 0.57–1.30). Thus, mutations in ADAR1 are associated with a variety of clinically distinct neurological phenotypes presenting from early infancy to adulthood, inherited either as an autosomal recessive or dominant trait. Testing for an interferon signature in blood represents a useful biomarker in this context.

Gamma-aminobutyric acid (GABA) and glutamate are the most abundant amino acid neurotransmitters in the brain. GABA, an inhibitory neurotransmitter, is synthesized by glutamic acid decarboxylase (GAD). Its predominant isoform GAD67, contributes up to ∼90% of base-level GABA in the CNS, and is encoded by the GAD1 gene. Disruption of GAD1 results in an imbalance of inhibitory and excitatory neurotransmitters, and as Gad1−/− mice die neonatally of severe cleft palate, it has not been possible to determine any potential neurological dysfunction. Furthermore, little is known about the consequence of GAD1 disruption in humans. Here we present six affected individuals from six unrelated families, carrying bi-allelic GAD1 variants, presenting with developmental and epileptic encephalopathy, characterized by early-infantile onset epilepsy and hypotonia with additional variable non-CNS manifestations such as skeletal abnormalities, dysmorphic features and cleft palate. Our findings highlight an important role for GAD1 in seizure induction, neuronal and extraneuronal development, and introduce GAD1 as a new gene associated with developmental and epileptic encephalopathy.

Developmental and/or epileptic encephalopathies (DEEs) are a group of devastating genetic disorders, resulting in early-onset, therapy-resistant seizures and developmental delay. Here we report on 22 individuals from 15 families presenting with a severe form of intractable epilepsy, severe developmental delay, progressive microcephaly, visual disturbance and similar minor dysmorphisms. Whole exome sequencing identified a recurrent, homozygous variant (chr2:64083454A > G) in the essential UDP-glucose pyrophosphorylase (UGP2) gene in all probands. This rare variant results in a tolerable Met12Val missense change of the longer UGP2 protein isoform but causes a disruption of the start codon of the shorter isoform, which is predominant in brain. We show that the absence of the shorter isoform leads to a reduction of functional UGP2 enzyme in neural stem cells, leading to altered glycogen metabolism, upregulated unfolded protein response and premature neuronal differentiation, as modeled during pluripotent stem cell differentiation in vitro. In contrast, the complete lack of all UGP2 isoforms leads to differentiation defects in multiple lineages in human cells. Reduced expression of Ugp2a/Ugp2b in vivo in zebrafish mimics visual disturbance and mutant animals show a behavioral phenotype. Our study identifies a recurrent start codon mutation in UGP2 as a cause of a novel autosomal recessive DEE syndrome. Importantly, it also shows that isoform-specific start-loss mutations causing expression loss of a tissue-relevant isoform of an essential protein can cause a genetic disease, even when an organism-wide protein absence is incompatible with life. We provide additional examples where a similar disease mechanism applies.

2 52 Running title: Loss of UGP2 causes a severe epileptic encephalopathy 53 54 55 Abstract: 60 Developmental and/or epileptic encephalopathies (DEEs) are a group of devastating genetic 61 disorders, resulting in early onset, therapy resistant seizures and developmental delay. Here we 62 report on 12 individuals from 10 families presenting with a severe form of intractable epilepsy, 63 severe developmental delay, progressive microcephaly and visual disturbance. Whole exome 64 sequencing identified a recurrent, homozygous variant (chr2:64083454A>G) in the essential UDP-65 glucose pyrophosphorylase (UGP2) gene in all probands. This rare variant results in a tolerable 66Met12Val missense change of the longer UGP2 protein isoform but causes a disruption of the start 67 codon of the shorter isoform. We show that the absence of the shorter isoform leads to a reduction 68 of functional UGP2 enzyme in brain cell types, leading to altered glycogen metabolism, upregulated 69unfolded protein response and premature neuronal differentiation, as modelled during pluripotent 70 stem cell differentiation in vitro. In contrast, the complete lack of all UGP2 isoforms leads to 71 differentiation defects in multiple lineages in human cells. Reduced expression of Ugp2a/Ugp2b in 72 vivo in zebrafish mimics visual disturbance and mutant animals show a behavioral phenotype. Our 73 study identifies a recurrent start codon mutation in UGP2 as a cause of a novel autosomal recessive 74 DEE. Importantly, it also shows that isoform specific start-loss mutations causing expression loss of a 75 tissue relevant isoform of an essential protein can cause a genetic disease, even when an organism-76 wide protein absence is incompatible with life. We provide additional examples where a similar 77 disease mechanism applies. 78 79 80 81 82 83 84 85 86 87 3 Introduction: 88 Developmental and/or epileptic encephalopathies (DEEs) are a heterogeneous group of genetic 89 disorders, characterized by severe epileptic seizures in combination with developmental delay or 90 regression 1 . Genes involved in multiple pathophysiological pathways have been implicated in DEEs, 91 including synaptic impairment, ion channel alterations, transporter defects and metabolic processes 92 such as disorders of glycosylation 2 . Mostly, dominant acting, de novo mutations have been identified 93 in children suffering from DEEs 3 , and only a limited number of genes with a recessive mode of 94 inheritance are known so far, with a higher occurrence rate in consanguineous populations 4 . A recent 95 cohort study on DEEs employing whole exome sequencing (WES) and copy-number analysis, 96however, found that up to 38% of diagnosed cases might be caused by recessive genes, indicating 97 that the importance of this mode of inheritance in DEEs has been underestimated 5 . 98