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Almaghlouth IA, Mohamed JY, Al-Amoudi M, Al-Ahaidib L, Al-Odaib A, Alkuraya FS. 5-Oxoprolinase deficiency: report of the first human OPLAH mutation. Gamma-glutamyl cycle is a six-enzyme cycle that represents the primary pathway for glutathione synthesis and degradation. 5-Oxoprolinase deficiency is an extremely rare disorder of the gamma-glutamyl cycle with only eight patients reported to date. Debate continues as to whether this is a benign biochemical defect because of the heterogeneity of the clinical presentation which ranges from normal to significant neurological involvement. Here, we report the first molecularly characterized patients with 5-oxoprolinase deficiency due to a mutation in OPLAH (which encodes 5-oxoprolinase). The largely benign clinical course of the patients described herein despite persistent 5-oxoprolinuria highlights the importance of establishing a molecular diagnosis in the few cases with abnormal neurological outcome to exclude potentially overlapping biochemical defects and to explore potential genotype/phenotype correlation. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented regarding pediatric cases involving mutation in brittle cornea syndrome.

WD repeat domain 83 opposite strand (WDR83OS) encodes the 106-aa (amino acid) protein Asterix, which heterodimerizes with CCDC47 to form the PAT (protein associated with ER translocon) complex. This complex functions as a chaperone for large proteins containing transmembrane domains to ensure proper folding. Until recently, little was known about the role of WDR83OS or CCDC47 in human disease traits. However, biallelic variants in CCDC47 were identified in four unrelated families with trichohepatoneurodevelopmental syndrome, characterized by a neurodevelopmental disorder (NDD) with liver dysfunction. Three affected siblings in an additional family share a homozygous truncating WDR83OS variant and a phenotype of NDD, dysmorphic features, and liver dysfunction. Using family-based rare variant analyses of exome sequencing (ES) data and case matching through GeneMatcher, we describe the clinical phenotypes of 11 additional individuals in eight unrelated families (nine unrelated families, 14 individuals in total) with biallelic putative truncating variants in WDR83OS. Consistent clinical features include NDD (14/14), facial dysmorphism (13/14), intractable itching (9/14), and elevated bile acids (5/6). Whereas bile acids were significantly elevated in 5/6 of individuals tested, bilirubin was normal and liver enzymes were normal to mildly elevated in all 14 individuals. In three of six individuals for whom longitudinal data were available, we observed a progressive reduction in relative head circumference. A zebrafish model lacking Wdr83os function further supports its role in the nervous system, craniofacial development, and lipid absorption. Taken together, our data support a disease-gene association between biallelic loss-of-function of WDR83OS and a neurological disease trait with hypercholanemia., Competing Interests: Declaration of interests The Department of Molecular & Human Genetics at Baylor College of Medicine receives revenue from clinical genetic testing conducted at Baylor Genetics Laboratories. J.R.L. serves on the Scientific Advisory Board of Baylor Genetics. J.R.L. has stock ownership in 23andMe, is a paid consultant for Genome International, and is a co-inventor on multiple United States and European patents related to molecular diagnostics for inherited neuropathies, eye diseases, genomic disorders, and bacterial genomic fingerprinting., (Copyright © 2024 American Society of Human Genetics. Published by Elsevier Inc. All rights reserved.)

Background: Founder variants are ancestral variants shared by individuals who are not closely related. The large effect size of some of these variants in the context of Mendelian disorders offers numerous precision medicine opportunities., Methods: Using one of the largest datasets on Mendelian disorders in the Middle East, we identified 2,908 medically relevant founder variants derived from 18,360 exomes and genomes and investigated their contribution to the clinical annotation of the human genome., Findings: Strikingly, ∼34% of Arab founder variants are absent in gnomAD. We found a strong contribution of Arab founder variants to the identification of novel gene-disease links (n = 224) and the support/dispute (n = 81 support, n = 101 dispute) of previously reported candidate gene-disease links. The powerful segregation evidence generated by Arab founder variants allowed many ClinVar and Human Gene Mutation Database variants to be reclassified. Overall, 39.5% of diagnostic reports from our clinical lab are based on founder variants, and 19.41% of tested individuals carry at least one pathogenic founder variant. The presumptive loss-of-function mechanism that typically underlies autosomal recessive diseases means that Arab founder variants also offer unique opportunities in "druggable genome" research. Arab founder variants were also informative of migration patterns in the Middle East consistent with documented historical accounts., Conclusions: We highlight the contribution of founder variants from an under-represented population group to precision medicine and inform future prevention programs. Our study also sheds light on the added value of these variants in supplementing other lines of research in tracing population history., Funding: There is no funding for this work., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Purpose: Pediatric cholestasis is the phenotypic expression of clinically and genetically heterogeneous disorders of bile acid synthesis and flow. Although a growing number of monogenic causes of pediatric cholestasis have been identified, the majority of cases remain undiagnosed molecularly., Methods: In a cohort of 299 pediatric participants (279 families) with intrahepatic cholestasis, we performed exome sequencing as a first-tier diagnostic test., Results: A likely causal variant was identified in 135 families (48.56%). These comprise 135 families that harbor variants spanning 37 genes with established or tentative links to cholestasis. In addition, we propose a novel candidate gene (PSKH1) (HGNC:9529) in 4 families. PSKH1 was particularly compelling because of strong linkage in 3 consanguineous families who shared a novel hepatorenal ciliopathy phenotype. Two of the 4 families shared a founder homozygous variant, whereas the third and fourth had different homozygous variants in PSKH1. PSKH1 encodes a putative protein serine kinase of unknown function. Patient fibroblasts displayed abnormal cilia that are long and show abnormal transport. A homozygous Pskh1 mutant mouse faithfully recapitulated the human phenotype and displayed abnormally long cilia. The phenotype could be rationalized by the loss of catalytic activity observed for each recombinant PSKH1 variant using in vitro kinase assays., Conclusion: Our results support the use of genomics in the workup of pediatric cholestasis and reveal PSKH1-related hepatorenal ciliopathy as a novel candidate monogenic form., Competing Interests: Conflict of Interest The authors declare no conflicts of interest., (Copyright © 2024 American College of Medical Genetics and Genomics. Published by Elsevier Inc. All rights reserved.)

Haploinsufficiency of FOXP1 gene is responsible for a neurodevelopmental disorder presenting with intellectual disability (ID), autism spectrum disorder (ASD), hypotonia, mild dysmorphic features, and multiple congenital anomalies. Joint contractures are not listed as a major feature of FOXP1-related disorder. We report five unrelated individuals, each harboring likely gene disruptive de novo FOXP1 variants or whole gene microdeletion, who showed multiple joint contractures affecting at least two proximal and/or distal joints. Consistent with the phenotype of FOXP1-related disorder, all five patients showed developmental delay with moderate-to-severe speech delay, ID, ASD, and facial dysmorphic features. FOXP1 is implicated in neuronal differentiation and in organizing motor axon projections, thus providing a potential developmental basis for the joint contractures. The combination of joint contractures and neurodevelopmental disorders supports the clinical suspicion of FOXP1-related phenotype., (© 2024 Wiley Periodicals LLC.)

Myotubularin-Related Protein 5 (MTMR5) is an inactive, poorly characterized D3-phosphatidylinositol phosphatase. Mutations in MTMR5 have been linked to Charcot-Marie-Tooth Disease Type 4B3 (CMT4B3), a rare, early-onset, recessive peripheral neuropathy. Here, we describe the establishment and validation of three human induced pluripotent stem cell (iPSC) lines derived from unrelated CMT4B3 patients, each harboring homozygous MTMR5/Sbf1 mutations. Current MTMR5 -/- animal models do not clearly link Sbf1 mutations to severe neuropathy, so such a resource is highly desired to further elucidate the relationship between MTMR5 dysfunction and peripheral nerve degeneration., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Mario Saporta reports financial support was provided by Hunter’s CMT4B3 Research Foundation. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

Combined oxidative phosphorylation deficiency (COXPD) is a rare multisystem disorder which is clinically and genetically heterogeneous. Genome sequencing identified biallelic MRPL49 variants in individuals from five unrelated families with presentations ranging from Perrault syndrome (primary ovarian insufficiency and sensorineural hearing loss) to severe childhood onset of leukodystrophy, learning disability, microcephaly and retinal dystrophy. Complexome profiling of fibroblasts from affected individuals revealed reduced levels of the small and, a more pronounced reduction of, the large mitochondrial ribosomal subunits. There was no evidence of altered mitoribosomal assembly. The reductions in levels of OXPHOS enzyme complexes I and IV are consistent with a form of COXPD associated with biallelic MRPL49 variants, expanding the understanding of how disruption of the mitochondrial ribosomal large subunit results in multi-system phenotypes., Competing Interests: Declaration of interest: The authors declare no competing interests.

Background: Delineating base-resolution breakpoints of complex rearrangements is crucial for an accurate clinical understanding of pathogenic variants and for carrier screening within family networks or the broader population. However, despite advances in genetic testing using short-read sequencing (SRS), this task remains costly and challenging., Methods: This study addresses the challenges of resolving missing disease-causing breakpoints in complex genomic disorders with suspected homozygous rearrangements by employing multiple long-read sequencing (LRS) strategies, including a novel and efficient strategy named nanopore-based rapid acquisition of neighboring genomic regions (NanoRanger). NanoRanger does not require large amounts of ultrahigh-molecular-weight DNA and stands out for its ease of use and rapid acquisition of large genomic regions of interest with deep coverage., Findings: We describe a cohort of 16 familial cases, each harboring homozygous rearrangements that defied breakpoint determination by SRS and optical genome mapping (OGM). NanoRanger identified the breakpoints with single-base-pair resolution, enabling accurate determination of the carrier status of unaffected family members as well as the founder nature of these genomic lesions and their frequency in the local population. The resolved breakpoints revealed that repetitive DNA, gene regulatory elements, and transcription activity contribute to genome instability in these novel recessive rearrangements., Conclusions: Our data suggest that NanoRanger greatly improves the success rate of resolving base-resolution breakpoints of complex genomic disorders and expands access to LRS for the benefit of patients with Mendelian disorders., Funding: M.L. is supported by KAUST Baseline Award no. BAS/1/1080-01-01 and KAUST Research Translation Fund Award no. REI/1/4742-01., Competing Interests: Declaration of interests A US provisional patent application (application no. 63/588,122) based on methods described in this paper has been filed by KAUST, in which M.L. and Y.Z. are listed as inventors. F.S.A. is on the advisory board of Med., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Purpose: FLVCR1 encodes a solute carrier protein implicated in heme, choline, and ethanolamine transport. Although Flvcr1 -/- mice exhibit skeletal malformations and defective erythropoiesis reminiscent of Diamond-Blackfan anemia (DBA), biallelic FLVCR1 variants in humans have previously only been linked to childhood or adult-onset ataxia, sensory neuropathy, and retinitis pigmentosa., Methods: We identified individuals with undiagnosed neurodevelopmental disorders and biallelic FLVCR1 variants through international data sharing and characterized the functional consequences of their FLVCR1 variants., Results: We ascertained 30 patients from 23 unrelated families with biallelic FLVCR1 variants and characterized a novel FLVCR1-related phenotype: severe developmental disorders with profound developmental delay, microcephaly (z-score -2.5 to -10.5), brain malformations, epilepsy, spasticity, and premature death. Brain malformations ranged from mild brain volume reduction to hydranencephaly. Severely affected patients share traits, including macrocytic anemia and skeletal malformations, with Flvcr1 -/- mice and DBA. FLVCR1 variants significantly reduce choline and ethanolamine transport and/or disrupt mRNA splicing., Conclusion: These data demonstrate a broad FLVCR1-related phenotypic spectrum ranging from severe multiorgan developmental disorders resembling DBA to adult-onset neurodegeneration. Our study expands our understanding of Mendelian choline and ethanolamine disorders and illustrates the importance of anticipating a wide phenotypic spectrum for known disease genes and incorporating model organism data into genome analysis to maximize genetic testing yield., Competing Interests: Conflict of Interest James R. Lupski has stock ownership in 23andMe, is a paid consultant for Genome International, and is a coinventor on multiple US and European patents related to molecular diagnostics for inherited neuropathies, eye diseases, genomic disorders, and bacterial genomic fingerprinting. The Department of Molecular and Human Genetics at Baylor College of Medicine receives revenue from clinical genetic testing conducted at Baylor Genetics Laboratories. Amber Begtrup and Houda Elloumi are employees of GeneDx, LLC. All other authors declare no conflicts of interest., (Copyright © 2024 American College of Medical Genetics and Genomics. Published by Elsevier Inc. All rights reserved.)

Purpose: This study aims to comprehensively delineate the phenotypic spectrum of ACTL6B-related disorders, previously associated with both autosomal recessive and autosomal dominant neurodevelopmental disorders. Molecularly, the role of the nucleolar protein ACTL6B in contributing to the disease has remained unclear., Methods: We identified 105 affected individuals, including 39 previously reported cases, and systematically analysed detailed clinical and genetic data for all individuals. Additionally, we conducted knockdown experiments in neuronal cells to investigate the role of ACTL6B in ribosome biogenesis., Results: Biallelic variants in ACTL6B are associated with severe-to-profound global developmental delay/intellectual disability (GDD/ID), infantile intractable seizures, absent speech, autistic features, dystonia, and increased lethality. De novo monoallelic variants result in moderate-to-severe GDD/ID, absent speech, and autistic features, while seizures and dystonia were less frequently observed. Dysmorphic facial features and brain abnormalities, including hypoplastic corpus callosum, parenchymal volume loss/atrophy, are common findings in both groups. We reveal that in the nucleolus, ACTL6B plays a crucial role in ribosome biogenesis, in particular in pre-rRNA processing., Conclusion: This study provides a comprehensive characterization of the clinical spectrum of both autosomal recessive and dominant forms of ACTL6B-associated disorders. It offers a comparative analysis of their respective phenotypes provides a plausible molecular explanation and suggests their inclusion within the expanding category of 'ribosomopathies'., (Copyright © 2024. Published by Elsevier Inc.)

Background: NOTCH3 encodes a transmembrane receptor critical for vascular smooth muscle cell function. NOTCH3 variants are the leading cause of hereditary cerebral small vessel disease (SVD). While monoallelic cysteine-involving missense variants in NOTCH3 are well-studied in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), patients with biallelic variants in NOTCH3 are extremely rare and not well characterised., Methods: In this study, we present clinical and genetic data from 25 patients with biallelic NOTCH3 variants and conduct a literature review of another 25 cases (50 patients in total). Brain magnetic resonance imaging (MRI) were analysed by expert neuroradiologists to better understand the phenotype associated with biallelic NOTCH3 variants., Findings: Our systematic analyses verified distinct genotype-phenotype correlations for the two types of biallelic variants in NOTCH3. Biallelic loss-of-function variants (26 patients) lead to a neurodevelopmental disorder characterised by spasticity, childhood-onset stroke, and periatrial white matter volume loss resembling periventricular leukomalacia. Conversely, patients with biallelic cysteine-involving missense variants (24 patients) fall within CADASIL spectrum phenotype with early adulthood onset stroke, dementia, and deep white matter lesions without significant volume loss. White matter lesion volume is comparable between patients with biallelic cysteine-involving missense variants and individuals with CADASIL. Notably, monoallelic carriers of loss-of-function variants are predominantly asymptomatic, with only a few cases reporting nonspecific headaches., Interpretation: We propose a NOTCH3-SVD classification depending on dosage and variant type. This study not only expands our knowledge of biallelic NOTCH3 variants but also provides valuable insight into the underlying mechanisms of the disease, contributing to a more comprehensive understanding of NOTCH3-related SVD., Funding: The Wellcome Trust, the MRC., Competing Interests: Declaration of interests Wendy Chung is on the board of directors of Prime Medicine. Stephan Zuchner has received consultancy honoraria from Neurogene, AegleaBioTherapeutics, Applied Therapeutics, and is an unpaid officer of the TGP foundation, all unrelated to the present manuscript. Elisa Fazzi has received honoraria from GW Pharma. Nicola De Stefano has received honoraria from Biogen-Idec, Genzyme, Immunic, Merck, Novartis, Roche, Celgene, and Teva for consulting services, speaking, and travel support. He serves on advisory boards for Merck, Novartis, Biogen-Idec, Immunic, Roche, and Genzyme, and he has received research grant support from the Italian MS Society. Lyn R Griffiths has received grants from the Australian National Health and Medical Research Council, Variant Bio, US Department of Defense and US Migraine Research Foundation as well as honoraria from Teva, Springer Nature, and Association of Migraine Disorders and she is Board of Censors, Diagnostic Genomics Human Genetics Assoc Australia and member of the Human Genetics Australasia Advisory Board. Hugh S Markus has received peer reviewed grants from the Medical Research Council, British Heart Foundation, National Institute of Health Research, and the Alzheimer Society, and is editor in chief of the International Journal of Stroke., (Copyright © 2024 The Author(s). Published by Elsevier B.V. All rights reserved.)

Background: Biallelic ZBTB11 variants have previously been associated with an ultrarare subtype of autosomal recessive intellectual developmental disorder (MRT69)., Objective: The aim was to provide insights into the clinical and genetic characteristics of ZBTB11-related disorders (ZBTB11-RD), with a particular emphasis on progressive complex movement abnormalities., Methods: Thirteen new and 16 previously reported affected individuals, ranging in age from 2 to 50 years, with biallelic ZBTB11 variants underwent clinical and genetic characterization., Results: All patients exhibited a range of neurodevelopmental phenotypes with varying severity, encompassing ocular and neurological features. Eleven new patients presented with complex abnormal movements, including ataxia, dystonia, myoclonus, stereotypies, and tremor, and 7 new patients exhibited cataracts. Deep brain stimulation was successful in treating 1 patient with generalized progressive dystonia. Our analysis revealed 13 novel variants., Conclusions: This study provides additional insights into the clinical features and spectrum of ZBTB11-RD, highlighting the progressive nature of movement abnormalities in the background of neurodevelopmental phenotype., (© 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.)

Meier-Gorlin syndrome (MGORS) is an autosomal recessive disorder characterized by short stature, microtia, and patellar hypoplasia, and is caused by pathogenic variants of cellular factors involved in the initiation of DNA replication. We previously reported that biallelic variants in GINS3 leading to amino acid changes at position 24 (p.Asp24) cause MGORS. Here, we describe the phenotype of a new individual homozygous for the Asp24Asn variant. We also report the clinical characteristics of an individual harboring a novel homozygous GINS3 variant (Ile25Phe) and features suggestive of MGORS. Modification of the corresponding residue in yeast Psf3 (Val9Phe) compromised S phase progression compared to a humanized Psf3 Val9Ile variant. Expression of Psf3 Val9Phe in yeast also caused sensitivity to elevated temperature and the replicative stress-inducing drug hydroxyurea, confirming partial loss of function of this variant in vivo and allowing us to upgrade the classification of this variant. Taken together, these data validate the critical importance of the GINS DNA replication complex in the molecular etiology of MGORS., (© 2024 The Authors. Clinical Genetics published by John Wiley & Sons Ltd.)

Purpose: To describe the rate, characteristics, and outcomes of rhegmatogenous retinal detachment (RD) in patients with Knobloch syndrome., Design: A single-center retrospective cohort study., Participants: Fifty patients with Knobloch syndrome diagnosed clinically, with or without molecular confirmation of recessive pathogenic COL18A1 variants., Methods: A retrospective chart review of all patients diagnosed with Knobloch syndrome from November 1, 1983 to March 31, 2023. Demographic data, ophthalmic evaluation at baseline and follow-up, interventions, and final anatomic and visual outcomes were collected., Main Outcome Measures: Rate, time of onset, characteristics, and treatment outcomes of RD., Results: Fifty patients with Knobloch syndrome were included. Males constituted 56% of cases. The diagnosis was confirmed with molecular genetic testing in 37 (74%) patients. Twenty-two patients (44%) had documented occipital bony defects or scalp lesions. Forty-eight of 100 eyes (48%) developed RD at a mean (standard deviation [SD]) age of 6.5 (6.1) years. The mean (SD) follow-up was 7.7 (5.6) years (range, 6 months to 24.3 years). Macular hole-related RD comprised 33% of RD cases. The overall single-surgery success rate was 36% and the final anatomic success rate was 70%. Macular hole-related RD carried a slightly worse prognosis with a 58% final anatomic success rate. Vitrectomy with adjunct scleral buckle and silicone oil tamponade provided the highest single-surgery success (62.2%). In eyes with measurable best-corrected visual acuity (BCVA), the mean preoperative BCVA was 1.2 logarithm of the minimum angle of resolution (Snellen equivalent, 20/320). After successful repair, mean visual acuity was 1.3 logarithm of the minimum angle of resolution (Snellen equivalent, 20/500)., Conclusions: Retinal detachment in Knobloch syndrome is frequent and occurs in young children. Macular hole-related RD comprises one third of RD cases and requires careful macular evaluation. Vitrectomy, combined with scleral buckling and silicone oil tamponade, appears to provide the best anatomic outcomes., Financial Disclosures: The authors have no proprietary or commercial interest in any materials discussed in this article., (Copyright © 2024 American Academy of Ophthalmology. Published by Elsevier Inc. All rights reserved.)

Developmental and epileptic encephalopathies (DEEs) feature altered brain development, developmental delay and seizures, with seizures exacerbating developmental delay. Here we identify a cohort with biallelic variants in DENND5A, encoding a membrane trafficking protein, and develop animal models with phenotypes like the human syndrome. We demonstrate that DENND5A interacts with Pals1/MUPP1, components of the Crumbs apical polarity complex required for symmetrical division of neural progenitor cells. Human induced pluripotent stem cells lacking DENND5A fail to undergo symmetric cell division with an inherent propensity to differentiate into neurons. These phenotypes result from misalignment of the mitotic spindle in apical neural progenitors. Cells lacking DENND5A orient away from the proliferative apical domain surrounding the ventricles, biasing daughter cells towards a more fate-committed state, ultimately shortening the period of neurogenesis. This study provides a mechanism for DENND5A-related DEE that may be generalizable to other developmental conditions and provides variant-specific clinical information for physicians and families., (© 2024. The Author(s).)

Pulmonary hypoplasia, Diaphragmatic anomalies, Anophthalmia/microphthalmia, and Cardiac defects (PDAC) syndrome is a genetically heterogeneous multiple congenital malformation syndrome. Although pathogenic variants in RARB and STRA6 are established causes of PDAC, many PDAC cases remain unsolved at the molecular level. Recently, we proposed biallelic WNT7B variants as a novel etiology based on several families with typical features of PDAC syndrome albeit with variable expressivity. Here, we report three patients from two families that share a novel founder variant in WNT7B (c.739C > T; Arg247Trp). The phenotypic expression of this variant ranges from typical PDAC features to isolated genitourinary anomalies. Similar to previously reported PDAC-associated WNT7B variants, this variant was found to significantly impair WNT7B signaling activity further corroborating its proposed pathogenicity. This report adds further evidence to WNT7B-related PDAC and expands its variable expressivity., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Germline gain of function variants in the oncogene ABL1 cause congenital heart defects and skeletal malformations (CHDSKM) syndrome. Whether a corresponding ABL1 deficiency disorder exists in humans remains unknown although developmental defects in mice deficient for Abl1 support this notion. Here, we describe two multiplex consanguineous families, each segregating a different homozygous likely loss of function variant in ABL1. The associated phenotype is multiple congenital malformations and distinctive facial dysmorphism that are opposite in many ways to CHDSKM. We suggest that a tight balance of ABL1 activity is required during embryonic development and that both germline gain of function and loss of function variants result in distinctively different allelic congenital malformation disorders., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Loss-of-function mutation of ABCC9, the gene encoding the SUR2 subunit of ATP sensitive-potassium (KATP) channels, was recently associated with autosomal recessive ABCC9-related intellectual disability and myopathy syndrome (AIMS). Here we identify nine additional subjects, from seven unrelated families, harbouring different homozygous loss-of-function variants in ABCC9 and presenting with a conserved range of clinical features. All variants are predicted to result in severe truncations or in-frame deletions within SUR2, leading to the generation of non-functional SUR2-dependent KATP channels. Affected individuals show psychomotor delay and intellectual disability of variable severity, microcephaly, corpus callosum and white matter abnormalities, seizures, spasticity, short stature, muscle fatigability and weakness. Heterozygous parents do not show any conserved clinical pathology but report multiple incidences of intra-uterine fetal death, which were also observed in an eighth family included in this study. In vivo studies of abcc9 loss-of-function in zebrafish revealed an exacerbated motor response to pentylenetetrazole, a pro-convulsive drug, consistent with impaired neurodevelopment associated with an increased seizure susceptibility. Our findings define an ABCC9 loss-of-function-related phenotype, expanding the genotypic and phenotypic spectrum of AIMS and reveal novel human pathologies arising from KATP channel dysfunction., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Guarantors of Brain.)