Detailed Analysis of ITPR1 Missense Variants Guides Diagnostics and Therapeutic Design.

BACKGROUND: The ITPR1 gene encodes the inositol 1,4,5-trisphosphate (IP3 ) receptor type 1 (IP3 R1), a critical player in cerebellar intracellular calcium signaling. Pathogenic missense variants in ITPR1 cause congenital spinocerebellar ataxia type 29 (SCA29), Gillespie syndrome (GLSP), and severe pontine/cerebellar hypoplasia. The pathophysiological basis of the different phenotypes is poorly understood. OBJECTIVES: We aimed to identify novel SCA29 and GLSP cases to define core phenotypes, describe the spectrum of missense variation across ITPR1, standardize the ITPR1 variant nomenclature, and investigate disease progression in relation to cerebellar atrophy. METHODS: Cases were identified using next-generation sequencing through the Deciphering Developmental Disorders study, the 100,000 Genomes project, and clinical collaborations. ITPR1 alternative splicing in the human cerebellum was investigated by quantitative polymerase chain reaction. RESULTS: We report the largest, multinational case series of 46 patients with 28 unique ITPR1 missense variants. Variants clustered in functional domains of the protein, especially in the N-terminal IP3 -binding domain, the carbonic anhydrase 8 (CA8)-binding region, and the C-terminal transmembrane channel domain. Variants outside these domains were of questionable clinical significance. Standardized transcript annotation, based on our ITPR1 transcript expression data, greatly facilitated analysis. Genotype-phenotype associations were highly variable. Importantly, while cerebellar atrophy was common, cerebellar volume loss did not correlate with symptom progression. CONCLUSIONS: This dataset represents the largest cohort of patients with ITPR1 missense variants, expanding the clinical spectrum of SCA29 and GLSP. Standardized transcript annotation is essential for future reporting. Our findings will aid in diagnostic interpretation in the clinic and guide selection of variants for preclinical studies. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

De Novo VPS4A Mutations Cause Multisystem Disease with Abnormal Neurodevelopment.

The endosomal sorting complexes required for transport (ESCRTs) are essential for multiple membrane modeling and membrane-independent cellular processes. Here we describe six unrelated individuals with de novo missense variants affecting the ATPase domain of VPS4A, a critical enzyme regulating ESCRT function. Probands had structural brain abnormalities, severe neurodevelopmental delay, cataracts, growth impairment, and anemia. In cultured cells, overexpression of VPS4A mutants caused enlarged endosomal vacuoles resembling those induced by expression of known dominant-negative ATPase-defective forms of VPS4A. Proband-derived fibroblasts had enlarged endosomal structures with abnormal accumulation of the ESCRT protein IST1 on the limiting membrane. VPS4A function was also required for normal endosomal morphology and IST1 localization in iPSC-derived human neurons. Mutations affected other ESCRT-dependent cellular processes, including regulation of centrosome number, primary cilium morphology, nuclear membrane morphology, chromosome segregation, mitotic spindle formation, and cell cycle progression. We thus characterize a distinct multisystem disorder caused by mutations affecting VPS4A and demonstrate that its normal function is required for multiple human developmental and cellular processes.

Further delineation of phenotypic spectrum of SCN2A-related disorder.

SCN2A-related disorders include intellectual disability, autism spectrum disorder, seizures, episodic ataxia, and schizophrenia. In this study, the phenotype-genotype association in SCN2A-related disorders was further delineated by collecting detailed clinical and molecular characteristics. Using previously proposed genotype-phenotype hypotheses based on variant function and position, the potential of phenotype prediction from the variants found was examined. Patients were identified through the Deciphering Developmental Disorders study and gene matching strategies. Phenotypic information and variant interpretation evidence were collated. Seventeen previously unreported patients and five patients who had been previously reported (but with minimal phenotypic and segregation data) were included (10 males, 12 females; median age 10.5 years). All patients had developmental delays and the majority had intellectual disabilities. Seizures were reported in 15 of 22 (68.2%), four of 22 (18.2%) had autism spectrum disorder and no patients were reported with episodic ataxia. The majority of variants were de novo. One family had presumed gonadal mosaicism. The correlation of the use of sodium channel-blocking antiepileptic drugs with phenotype or genotype was variable. These data suggest that variant type and position alone can provide some predictive information about the phenotype in a proportion of cases, but more precise assessment of variant function is needed for meaningful phenotype prediction.

A fetus with de novo 2q33.2q35 deletion including MAP2 with brain anomalies, esophageal atresia, and laryngeal stenosis.

Deletions of the long arm of chromosome 2 are rare. Few cases of interstitial deletions of the 2q33q35 region have been reported. Individuals with deletions in this region have growth retardation, psychomotor retardation, micrognathia, microcephaly, and apparently low-set ears. We describe a female fetus with a de novo deletion of 2q33.2 to q35 with delayed gyral formation with widespread neuronal heterotopia of the white matter, small cerebellum, esophageal atresia, laryngeal stenosis, micrognathia, and intrauterine growth retardation. With the use of karyotyping and high-resolution array comparative genomic hybridization the boundaries and gene content of the deletion were identified. Our aim was to determine whether a candidate gene for the brain phenotype was present in the deletion. By this means and based on literature we pinpointed the microtubule associated gene MAP2 as a candidate for the brain anomalies.

Mutations in the U4 snRNA gene RNU4-2 cause one of the most prevalent monogenic neurodevelopmental disorders.

Most people with intellectual disability (ID) do not receive a molecular diagnosis following genetic testing. To identify new etiologies of ID, we performed a genetic association analysis comparing the burden of rare variants in 41,132 noncoding genes between 5,529 unrelated cases and 46,401 unrelated controls. RNU4-2, which encodes U4 small nuclear RNA, a critical component of the spliceosome, was the most strongly associated gene. We implicated de novo variants among 47 cases in two regions of RNU4-2 in the etiology of a syndrome characterized by ID, microcephaly, short stature, hypotonia, seizures and motor delay. We replicated this finding in three collections, bringing the number of unrelated cases to 73. Analysis of national genomic diagnostic data showed RNU4-2 to be a more common etiological gene for neurodevelopmental abnormality than any previously reported autosomal gene. Our findings add to the growing evidence of spliceosome dysfunction in the etiologies of neurological disorders.

Abrogation of MAP4K4 protein function causes congenital anomalies in humans and zebrafish.

We report 21 families displaying neurodevelopmental differences and multiple congenital anomalies while bearing a series of rare variants in mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4). MAP4K4 has been implicated in many signaling pathways including c-Jun N-terminal and RAS kinases and is currently under investigation as a druggable target for multiple disorders. Using several zebrafish models, we demonstrate that these human variants are either loss-of-function or dominant-negative alleles and show that decreasing Map4k4 activity causes developmental defects. Furthermore, MAP4K4 can restrain hyperactive RAS signaling in early embryonic stages. Together, our data demonstrate that MAP4K4 negatively regulates RAS signaling in the early embryo and that variants identified in affected humans abrogate its function, establishing MAP4K4 as a causal locus for individuals with syndromic neurodevelopmental differences.

17q12 microdeletion syndrome: three patients illustrating the phenotypic spectrum.

Deletions of 17q12 are associated with renal cysts and maturity onset diabetes of the young, and have also been identified in women with reproductive tract anomalies due to Mullerian aplasia. Although initially identified in patients with normal cognitive ability, some patients with this recurrent microdeletion syndrome have learning problems. We identified a 17q12 microdeletion in three patients with renal cystic disease by array comparative genomic hybridization and the phenotypic spectrum of the 17q12 microdeletion syndrome is illustrated by the description of these patients. Of two patients who are old enough to be assessed, one has significant speech delay, autism spectrum disorder, and mild learning difficulty, while the other patient has only mild speech delay. This highlights the variability of cognitive involvement in this condition. The third patient presented with Alagille syndrome-like features in the neonatal period. All three patients had transient hypercalcemia in the neonatal period, a finding that has not previously been described in this condition. Moreover, two patients have mild or no dysmorphism, while one displays striking facial dysmorphism in addition to minor congenital anomalies. We suggest that while patients with 17q12 microdeletion syndrome can present with type 2 diabetes or renal cysts without any dysmorphic features, a subgroup may have dysmorphic features or present with neonatal cholestasis. Transient neonatal hypercalcemia may be a feature of this microdeletion syndrome.

Phenotypic spectrum associated with CASK loss-of-function mutations.

BACKGROUND: Heterozygous mutations in the CASK gene in Xp11.4 have been shown to be associated with a distinct brain malformation phenotype in females, including disproportionate pontine and cerebellar hypoplasia. METHODS: The study characterised the CASK alteration in 20 new female patients by molecular karyotyping, fluorescence in situ hybridisation, sequencing, reverse transcriptase (RT) and/or quantitative real-time PCR. Clinical and brain imaging data of a total of 25 patients were reviewed. RESULTS: 11 submicroscopic copy number alterations, including nine deletions of ~11 kb to 4.5 Mb and two duplications, all covering (part of) CASK, four splice, four nonsense, and one 1 bp deletion are reported. These heterozygous CASK mutations most likely lead to a null allele. Brain imaging consistently showed diffuse brainstem and cerebellar hypoplasia with a dilated fourth ventricle, but of remarkably varying degrees. Analysis of 20 patients in this study, and five previously reported patients, revealed a core clinical phenotype comprising severe developmental delay/intellectual disability, severe postnatal microcephaly, often associated with growth retardation, (axial) hypotonia with or without hypertonia of extremities, optic nerve hypoplasia, and/or other eye abnormalities. A recognisable facial phenotype emerged, including prominent and broad nasal bridge and tip, small or short nose, long philtrum, small chin, and/or large ears. CONCLUSIONS: These findings define the phenotypic spectrum associated with CASK loss-of-function mutations. The combination of developmental and brain imaging features together with mild facial dysmorphism is highly suggestive of this disorder and should prompt subsequent testing of the CASK gene.

Rare Genetic Variation in 135 Families With Family History Suggestive of X-Linked Intellectual Disability.

Families with multiple male children with intellectual disability (ID) are usually suspected of having disease due to a X-linked mode of inheritance and genetic studies focus on analysis of segregating variants in X-linked genes. However, the genetic cause of ID remains elusive in approximately 50% of affected individuals. Here, we report the analysis of next-generation sequencing data in 274 affected individuals from 135 families with a family history suggestive of X-linked ID. Genetic diagnoses were obtained for 19% (25/135) of the families, and 24% (33/135) had a variant of uncertain significance. In 12% of cases (16/135), the variants were not shared within the family, suggesting genetic heterogeneity and phenocopies are frequent. Of all the families with reportable variants (43%, 58/135), we observed that 55% (32/58) were in X-linked genes, but 38% (22/58) were in autosomal genes, while the remaining 7% (4/58) had multiple variants in genes with different modes on inheritance. This study highlights that in families with multiple affected males, X linkage should not be assumed, and both individuals should be considered, as different genetic etiologies are common in apparent familial cases.

Clinical, biochemical, cellular and molecular characterization of mitochondrial DNA depletion syndrome due to novel mutations in the MPV17 gene.

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are severe autosomal recessive disorders associated with decreased mtDNA copy number in clinically affected tissues. The hepatocerebral form (mtDNA depletion in liver and brain) has been associated with mutations in the POLG, PEO1 (Twinkle), DGUOK and MPV17 genes, the latter encoding a mitochondrial inner membrane protein of unknown function. The aims of this study were to clarify further the clinical, biochemical, cellular and molecular genetic features associated with MDS due to MPV17 gene mutations. We identified 12 pathogenic mutations in the MPV17 gene, of which 11 are novel, in 17 patients from 12 families. All patients manifested liver disease. Poor feeding, hypoglycaemia, raised serum lactate, hypotonia and faltering growth were common presenting features. mtDNA depletion in liver was demonstrated in all seven cases where liver tissue was available. Mosaic mtDNA depletion was found in primary fibroblasts by PicoGreen staining. These results confirm that MPV17 mutations are an important cause of hepatocerebral mtDNA depletion syndrome, and provide the first demonstration of mosaic mtDNA depletion in human MPV17 mutant fibroblast cultures. We found that a severe clinical phenotype was associated with profound tissue-specific mtDNA depletion in liver, and, in some cases, mosaic mtDNA depletion in fibroblasts.