NONO Detects the Nuclear HIV Capsid to Promote cGAS-Mediated Innate Immune Activation.

Detection of viruses by innate immune sensors induces protective antiviral immunity. The viral DNA sensor cyclic GMP-AMP synthase (cGAS) is necessary for detection of HIV by human dendritic cells and macrophages. However, synthesis of HIV DNA during infection is not sufficient for immune activation. The capsid protein, which associates with viral DNA, has a pivotal role in enabling cGAS-mediated immune activation. We now find that NONO is an essential sensor of the HIV capsid in the nucleus. NONO protein directly binds capsid with higher affinity for weakly pathogenic HIV-2 than highly pathogenic HIV-1. Upon infection, NONO is essential for cGAS activation by HIV and cGAS association with HIV DNA in the nucleus. NONO recognizes a conserved region in HIV capsid with limited tolerance for escape mutations. Detection of nuclear viral capsid by NONO to promote DNA sensing by cGAS reveals an innate strategy to achieve distinction of viruses from self in the nucleus.

Bi-allelic HPDL Variants Cause a Neurodegenerative Disease Ranging from Neonatal Encephalopathy to Adolescent-Onset Spastic Paraplegia.

We report bi-allelic pathogenic HPDL variants as a cause of a progressive, pediatric-onset spastic movement disorder with variable clinical presentation. The single-exon gene HPDL encodes a protein of unknown function with sequence similarity to 4-hydroxyphenylpyruvate dioxygenase. Exome sequencing studies in 13 families revealed bi-allelic HPDL variants in each of the 17 individuals affected with this clinically heterogeneous autosomal-recessive neurological disorder. HPDL levels were significantly reduced in fibroblast cell lines derived from more severely affected individuals, indicating the identified HPDL variants resulted in the loss of HPDL protein. Clinical presentation ranged from severe, neonatal-onset neurodevelopmental delay with neuroimaging findings resembling mitochondrial encephalopathy to milder manifestation of adolescent-onset, isolated hereditary spastic paraplegia. All affected individuals developed spasticity predominantly of the lower limbs over the course of the disease. We demonstrated through bioinformatic and cellular studies that HPDL has a mitochondrial localization signal and consequently localizes to mitochondria suggesting a putative role in mitochondrial metabolism. Taken together, these genetic, bioinformatic, and functional studies demonstrate HPDL is a mitochondrial protein, the loss of which causes a clinically variable form of pediatric-onset spastic movement disorder.

MYT1L-associated neurodevelopmental disorder: description of 40 new cases and literature review of clinical and molecular aspects.

Pathogenic variants of the myelin transcription factor-1 like (MYT1L) gene include heterozygous missense, truncating variants and 2p25.3 microdeletions and cause a syndromic neurodevelopmental disorder (OMIM#616,521). Despite enrichment in de novo mutations in several developmental disorders and autism studies, the data on clinical characteristics and genotype-phenotype correlations are scarce, with only 22 patients with single nucleotide pathogenic variants reported. We aimed to further characterize this disorder at both the clinical and molecular levels by gathering a large series of patients with MYT1L-associated neurodevelopmental disorder. We collected genetic information on 40 unreported patients with likely pathogenic/pathogenic MYT1L variants and performed a comprehensive review of published data (total = 62 patients). We confirm that the main phenotypic features of the MYT1L-related disorder are developmental delay with language delay (95%), intellectual disability (ID, 70%), overweight or obesity (58%), behavioral disorders (98%) and epilepsy (23%). We highlight novel clinical characteristics, such as learning disabilities without ID (30%) and feeding difficulties during infancy (18%). We further describe the varied dysmorphic features (67%) and present the changes in weight over time of 27 patients. We show that patients harboring highly clustered missense variants in the 2-3-ZNF domains are not clinically distinguishable from patients with truncating variants. We provide an updated overview of clinical and genetic data of the MYT1L-associated neurodevelopmental disorder, hence improving diagnosis and clinical management of these patients.

eIF2γ mutation that disrupts eIF2 complex integrity links intellectual disability to impaired translation initiation.

Together with GTP and initiator methionyl-tRNA, translation initiation factor eIF2 forms a ternary complex that binds the 40S ribosome and then scans an mRNA to select the AUG start codon for protein synthesis. Here, we show that a human X-chromosomal neurological disorder characterized by intellectual disability and microcephaly is caused by a missense mutation in eIF2γ (encoded by EIF2S3), the core subunit of the heterotrimeric eIF2 complex. Biochemical studies of human cells overexpressing the eIF2γ mutant and of yeast eIF2γ with the analogous mutation revealed a defect in binding the eIF2ß subunit to eIF2γ. Consistent with this loss of eIF2 integrity, the yeast eIF2γ mutation impaired translation start codon selection and eIF2 function in vivo in a manner that was suppressed by overexpressing eIF2ß. These findings directly link intellectual disability to impaired translation initiation, and provide a mechanistic basis for the human disease due to partial loss of eIF2 function.

Mutations in TBCK, Encoding TBC1-Domain-Containing Kinase, Lead to a Recognizable Syndrome of Intellectual Disability and Hypotonia.

Through an international multi-center collaboration, 13 individuals from nine unrelated families and affected by likely pathogenic biallelic variants in TBC1-domain-containing kinase (TBCK) were identified through whole-exome sequencing. All affected individuals were found to share a core phenotype of intellectual disability and hypotonia, and many had seizures and showed brain atrophy and white-matter changes on neuroimaging. Minor non-specific facial dysmorphism was also noted in some individuals, including multiple older children who developed coarse features similar to those of storage disorders. TBCK has been shown to regulate the mammalian target of rapamycin (mTOR) signaling pathway, which is also stimulated by exogenous leucine supplementation. TBCK was absent in cells from affected individuals, and decreased phosphorylation of phospho-ribosomal protein S6 was also observed, a finding suggestive of downregulation of mTOR signaling. Lastly, we demonstrated that activation of the mTOR pathway in response to L-leucine supplementation was retained, suggesting a possible avenue for directed therapies for this condition.

Disruption of POGZ Is Associated with Intellectual Disability and Autism Spectrum Disorders.

Intellectual disability (ID) and autism spectrum disorders (ASD) are genetically heterogeneous, and a significant number of genes have been associated with both conditions. A few mutations in POGZ have been reported in recent exome studies; however, these studies do not provide detailed clinical information. We collected the clinical and molecular data of 25 individuals with disruptive mutations in POGZ by diagnostic whole-exome, whole-genome, or targeted sequencing of 5,223 individuals with neurodevelopmental disorders (ID primarily) or by targeted resequencing of this locus in 12,041 individuals with ASD and/or ID. The rarity of disruptive mutations among unaffected individuals (2/49,401) highlights the significance (p = 4.19 × 10(-13); odds ratio = 35.8) and penetrance (65.9%) of this genetic subtype with respect to ASD and ID. By studying the entire cohort, we defined common phenotypic features of POGZ individuals, including variable levels of developmental delay (DD) and more severe speech and language delay in comparison to the severity of motor delay and coordination issues. We also identified significant associations with vision problems, microcephaly, hyperactivity, a tendency to obesity, and feeding difficulties. Some features might be explained by the high expression of POGZ, particularly in the cerebellum and pituitary, early in fetal brain development. We conducted parallel studies in Drosophila by inducing conditional knockdown of the POGZ ortholog row, further confirming that dosage of POGZ, specifically in neurons, is essential for normal learning in a habituation paradigm. Combined, the data underscore the pathogenicity of loss-of-function mutations in POGZ and define a POGZ-related phenotype enriched in specific features.

Exome sequencing findings in 27 patients with myoclonic-atonic epilepsy: Is there a major genetic factor?

Myoclonic-atonic epilepsy (MAE) is thought to have a genetic etiology. Mutations in CHD2, SLC2A1 and SLC6A1 genes have been reported in few patients showing often intellectual disability prior to MAE onset. We aimed to explore putative causal genetic factors in MAE. We performed array-CGH and whole-exome sequencing in 27 patients. We considered non-synonymous variants, splice acceptor, donor site mutations, and coding insertions/deletions. A gene was causal when its mutations have been already linked to epilepsy or other brain diseases or when it has a putative function in neuronal excitability or brain development. We identified candidate disease-causing variants in 11 patients (41%). Single variants were found in some known epilepsy-associated genes (namely CHD2, KCNT1, KCNA2 and STXBP1) but not in others (SLC2A1 and SLC6A1). One new candidate gene SUN1 requires further validation. MAE shows underlying genetic heterogeneity with only few cases linked to mutations in genes reported in developmental and epileptic encephalopathies.

De novo mutation screening in childhood-onset cerebellar atrophy identifies gain-of-function mutations in the CACNA1G calcium channel gene.

Cerebellar atrophy is a key neuroradiological finding usually associated with cerebellar ataxia and cognitive development defect in children. Unlike the adult forms, early onset cerebellar atrophies are classically described as mostly autosomal recessive conditions and the exact contribution of de novo mutations to this phenotype has not been assessed. In contrast, recent studies pinpoint the high prevalence of pathogenic de novo mutations in other developmental disorders such as intellectual disability, autism spectrum disorders and epilepsy. Here, we investigated a cohort of 47 patients with early onset cerebellar atrophy and/or hypoplasia using a custom gene panel as well as whole exome sequencing. De novo mutations were identified in 35% of patients while 27% had mutations inherited in an autosomal recessive manner. Understanding if these de novo events act through a loss or a gain of function effect is critical for treatment considerations. To gain a better insight into the disease mechanisms causing these cerebellar defects, we focused on CACNA1G, a gene not yet associated with the early-onset form. This gene encodes the Cav3.1 subunit of T-type calcium channels highly expressed in Purkinje neurons and deep cerebellar nuclei. We identified four patients with de novo CACNA1G mutations. They all display severe motor and cognitive impairment, cerebellar atrophy as well as variable features such as facial dysmorphisms, digital anomalies, microcephaly and epilepsy. Three subjects share a recurrent c.2881G>A/p.Ala961Thr variant while the fourth patient has the c.4591A>G/p.Met1531Val variant. Both mutations drastically impaired channel inactivation properties with significantly slower kinetics (∼5 times) and negatively shifted potential for half-inactivation (>10 mV). In addition, these two mutations increase neuronal firing in a cerebellar nuclear neuron model and promote a larger window current fully inhibited by TTA-P2, a selective T-type channel blocker. This study highlights the prevalence of de novo mutations in early-onset cerebellar atrophy and demonstrates that A961T and M1531V are gain of function mutations. Moreover, it reveals that aberrant activity of Cav3.1 channels can markedly alter brain development and suggests that this condition could be amenable to treatment.

Distal duplication of chromosome 16q22.1q23.1 in a Vietnamese patient with midface hypoplasia and intellectual disability.

The clinical presentation of distal duplications of the long arm of chromosome (chr) 16 is currently not well described. Only one case of microduplication of chr16q22.1 and another involving the chr16q22.1q23.1 region have been reported so far. Here, using array comparative genomic hybridization, we identified a second case of chr16q22.1q23.1 duplication in a Vietnamese boy, who shares significant clinical phenotype with the previously described case. Aside from developmental delay, intellectual disability and midface hypoplasia, our patient also displays a forked tongue, visual impairment and external ptosis. Our report further expands the clinical spectrum associated with duplication of this region.

Novel de novo ZBTB20 mutations in three cases with Primrose syndrome and constant corpus callosum anomalies.

Corpus callosum (CC) is the major brain commissure connecting homologous areas of cerebral hemispheres. CC anomalies (CCAs) are the most frequent brain anomalies leading to variable neurodevelopmental outcomes making genetic counseling difficult in the absence of a known etiology that might inform the prognosis. Here, we used whole exome sequencing, and a targeted capture panel of syndromic CCA known causal and candidate genes to screen a cohort of 64 fetuses with CCA observed upon autopsy, and 34 children with CCA and intellectual disability. In one fetus and two patients, we identified three novel de novo mutations in ZBTB20, which was previously shown to be causal in Primrose syndrome. In addition to CCA, all cases presented with additional features of Primrose syndrome including facial dysmorphism and macrocephaly or megalencephaly. All three variations occurred within two out of the five zinc finger domains of the transcriptional repressor ZBTB20. Through homology modeling, these variants are predicted to result in local destabilization of each zinc finger domain suggesting subsequent abnormal repression of ZBTB20 target genes. Neurohistopathological analysis of the fetal case showed abnormal regionalization of the hippocampal formation as well as a reduced density of cortical upper layers where originate most callosal projections. Here, we report novel de novo ZBTB20 mutations in three independent cases with characteristic features of Primrose syndrome including constant CCA. Neurohistopathological findings in fetal case corroborate the observed key role of ZBTB20 during hippocampal and neocortical development. Finally, this study highlights the crucial role of ZBTB20 in CC development in human.

Mutations in QARS, encoding glutaminyl-tRNA synthetase, cause progressive microcephaly, cerebral-cerebellar atrophy, and intractable seizures.

Progressive microcephaly is a heterogeneous condition with causes including mutations in genes encoding regulators of neuronal survival. Here, we report the identification of mutations in QARS (encoding glutaminyl-tRNA synthetase [QARS]) as the causative variants in two unrelated families affected by progressive microcephaly, severe seizures in infancy, atrophy of the cerebral cortex and cerebellar vermis, and mild atrophy of the cerebellar hemispheres. Whole-exome sequencing of individuals from each family independently identified compound-heterozygous mutations in QARS as the only candidate causative variants. QARS was highly expressed in the developing fetal human cerebral cortex in many cell types. The four QARS mutations altered highly conserved amino acids, and the aminoacylation activity of QARS was significantly impaired in mutant cell lines. Variants p.Gly45Val and p.Tyr57His were located in the N-terminal domain required for QARS interaction with proteins in the multisynthetase complex and potentially with glutamine tRNA, and recombinant QARS proteins bearing either substitution showed an over 10-fold reduction in aminoacylation activity. Conversely, variants p.Arg403Trp and p.Arg515Trp, each occurring in a different family, were located in the catalytic core and completely disrupted QARS aminoacylation activity in vitro. Furthermore, p.Arg403Trp and p.Arg515Trp rendered QARS less soluble, and p.Arg403Trp disrupted QARS-RARS (arginyl-tRNA synthetase 1) interaction. In zebrafish, homozygous qars loss of function caused decreased brain and eye size and extensive cell death in the brain. Our results highlight the importance of QARS during brain development and that epilepsy due to impairment of QARS activity is unusually severe in comparison to other aminoacyl-tRNA synthetase disorders.

Refining the phenotype associated with CASC5 mutation.

Autosomal recessive primary microcephaly is a neurodevelopmental disorder characterized by congenitally reduced head circumference by at least two standard deviations (SD) below the mean for age and gender. It is associated with nonprogressive mental retardation of variable degree, minimal neurological deficit with no evidence of architectural anomalies of the brain. So far, 12 genetic loci (MCPH1-12) and corresponding genes have been identified. Most of these encode centrosomal proteins. CASC5 is one the most recently unravelled genes responsible for MCPH with mutations reported in three consanguineous families of Moroccan origin, all of whom harboured the same CASC5 homozygous mutation (c.6125G>A; p.Met2041Ile). Here, we report the identification, by whole exome sequencing, of the same missense mutation in a consanguineous Algerian family. All patients exhibited a similar clinical phenotype, including congenital microcephaly with head circumferences ranging from -3 to -4 standard deviations (SD) after age 5 years, moderate to severe cognitive impairment, short stature (adult height -3 SD), dysmorphic features included a sloping forehead, thick eyebrows, synophris and a low columella. Severe vermis hypoplasia and a large cyst of the posterior fossa were observed in one patient. Close microsatellite markers showed identical alleles in the Algerian the previously and Moroccan patients. This study confirms the involvement of CASC5 in autosomal recessive microcephaly and supports the hypothesis of a founder effect of the c.6125G>A mutation. In addition, this report refines the phenotype of this newly recognized form of primary microcephaly.

Loss of function of KIAA2022 causes mild to severe intellectual disability with an autism spectrum disorder and impairs neurite outgrowth.

Existence of a discrete new X-linked intellectual disability (XLID) syndrome due to KIAA2022 deficiency was questioned by disruption of KIAA2022 by an X-chromosome pericentric inversion in a XLID family we reported in 2004. Three additional families with likely pathogenic KIAA2022 mutations were discovered within the frame of systematic parallel sequencing of familial cases of XLID or in the context of routine array-CGH evaluation of sporadic intellectual deficiency (ID) cases. The c.186delC and c.3597dupA KIAA2022 truncating mutations were identified by X-chromosome exome sequencing, while array CGH discovered a 70 kb microduplication encompassing KIAA2022 exon 1 in the third family. This duplication decreased KIAA2022 mRNA level in patients' lymphocytes by 60%. Detailed clinical examination of all patients, including the two initially reported, indicated moderate-to-severe ID with autistic features, strabismus in all patients, with no specific dysmorphic features other than a round face in infancy and no structural brain abnormalities on magnetic resonance imaging (MRI). Interestingly, the patient with decreased KIAA2022 expression had only mild ID with severe language delay and repetitive behaviors falling in the range of an autism spectrum disorder (ASD). Since little is known about KIAA2022 function, we conducted morphometric studies in cultured rat hippocampal neurons. We found that siRNA-mediated KIAA2022 knockdown resulted in marked impairment in neurite outgrowth including both the dendrites and the axons, suggesting a major role for KIAA2022 in neuron development and brain function.

Phenotype-genotype correlations in 17 new patients with an Xp11.23p11.22 microduplication and review of the literature.

Array comparative genomic hybridization (array CGH) has proven its utility in uncovering cryptic rearrangements in patients with X-linked intellectual disability. In 2009, Giorda et al. identified inherited and de novo recurrent Xp11.23p11.22 microduplications in two males and six females from a wide cohort of patients presenting with syndromic intellectual disability. To date, 14 females and 5 males with an overlapping microduplication have been reported in the literature. To further characterize this emerging syndrome, we collected clinical and microarray data from 17 new patients, 10 females, and 7 males. The Xp11.23p11.2 microduplications detected by array CGH ranged in size from 331 Kb to 8.9 Mb. Five patients harbored 4.5 Mb recurrent duplications mediated by non-allelic homologous recombination between segmental duplications and 12 harbored atypical duplications. The chromosomal rearrangement occurred de novo in eight patients and was inherited in six affected males from three families. Patients shared several common major characteristics including moderate to severe intellectual disability, early onset of puberty, language impairment, and age related epileptic syndromes such as West syndrome and focal epilepsy with activation during sleep evolving in some patients to continuous spikes-and-waves during slow sleep. Atypical microduplications allowed us to identify minimal critical regions that might be responsible for specific clinical findings of the syndrome and to suggest possible candidate genes: FTSJ1 and SHROOM4 for intellectual disability along with PQBP1 and SLC35A2 for epilepsy. Xp11.23p11.22 microduplication is a recently-recognized syndrome associated with intellectual disability, epilepsy, and early onset of puberty in females. In this study, we propose several genes that could contribute to the phenotype.

Adaptor protein complex 4 deficiency causes severe autosomal-recessive intellectual disability, progressive spastic paraplegia, shy character, and short stature.

Intellectual disability inherited in an autosomal-recessive fashion represents an important fraction of severe cognitive-dysfunction disorders. Yet, the extreme heterogeneity of these conditions markedly hampers gene identification. Here, we report on eight affected individuals who were from three consanguineous families and presented with severe intellectual disability, absent speech, shy character, stereotypic laughter, muscular hypotonia that progressed to spastic paraplegia, microcephaly, foot deformity, decreased muscle mass of the lower limbs, inability to walk, and growth retardation. Using a combination of autozygosity mapping and either Sanger sequencing of candidate genes or next-generation exome sequencing, we identified one mutation in each of three genes encoding adaptor protein complex 4 (AP4) subunits: a nonsense mutation in AP4S1 (NM_007077.3: c.124C>T, p.Arg42(∗)), a frameshift mutation in AP4B1 (NM_006594.2: c.487_488insTAT, p.Glu163_Ser739delinsVal), and a splice mutation in AP4E1 (NM_007347.3: c.542+1_542+4delGTAA, r.421_542del, p.Glu181Glyfs(∗)20). Adaptor protein complexes (AP1-4) are ubiquitously expressed, evolutionarily conserved heterotetrameric complexes that mediate different types of vesicle formation and the selection of cargo molecules for inclusion into these vesicles. Interestingly, two mutations affecting AP4M1 and AP4E1 have recently been found to cause cerebral palsy associated with severe intellectual disability. Combined with previous observations, these results support the hypothesis that AP4-complex-mediated trafficking plays a crucial role in brain development and functioning and demonstrate the existence of a clinically recognizable syndrome due to deficiency of the AP4 complex.

Multiple congenital anomalies-intellectual disability (MCA-ID) and neuroblastoma in a patient harboring a de novo 14q23.1q23.3 deletion.

Neuroblastoma is the most frequent extra cranial solid tumor in infants and children. Genetic predisposition to neuroblastoma has been suspected previously due to familial cases of sporadic NB and predisposition to NB in several syndromes. Here, we report on a de novo 14q23.1-q23.3 microdeletion in a male presenting with a neuroblastoma diagnosed at 9 months, and spherocytosis, congenital heart defect, cryptorchidism, hypoplasia of corpus callosum, epilepsy, and developmental delay. Myc-associated-factor X (MAX) haploinsufficiency could be regarded as the predisposing factor to NB. Indeed 14q deletion is a recurrent somatic rearrangement in NB and MAX somatic and germline loss of function mutation have recently been described in pheochromocytoma and paraganglioma. However, MAX was expressed in the tumor of the patient we report on and, accordingly, loss of heterozygosity, mutation, or promoter methylation were excluded. In addition, we discuss the potential involvement in the clinical spectrum presented by the patient of five of the deleted genes, namely DAAM1, PLEKHG3, SPTB, AKAP5, and ARID4A.

Between hope and reality: treatment of genetic diseases through nucleic acid-based drugs.

Rare diseases (RD) affect a small number of people compared to the general population and are mostly genetic in origin. The first clinical signs often appear at birth or in childhood, and patients endure high levels of pain and progressive loss of autonomy frequently associated with short life expectancy. Until recently, the low prevalence of RD and the gatekeeping delay in their diagnosis have long hampered research. The era of nucleic acid (NA)-based therapies has revolutionized the landscape of RD treatment and new hopes arise with the perspectives of disease-modifying drugs development as some NA-based therapies are now entering the clinical stage. Herein, we review NA-based drugs that were approved and are currently under investigation for the treatment of RD. We also discuss the recent structural improvements of NA-based therapeutics and delivery system, which overcome the main limitations in their market expansion and the current approaches that are developed to address the endosomal escape issue. We finally open the discussion on the ethical and societal issues that raise this new technology in terms of regulatory approval and sustainability of production.

Inhibition of poly(ADP-Ribosyl)ation reduced vascular smooth muscle cells loss and improves aortic disease in a mouse model of human accelerated aging syndrome.

Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare genetic disorder associated with features of accelerated aging. HGPS is an autosomal dominant disease caused by a de novo mutation of LMNA gene, encoding A-type lamins, resulting in the truncated form of pre-lamin A called progerin. While asymptomatic at birth, patients develop symptoms within the first year of life when they begin to display accelerated aging and suffer from growth retardation, and severe cardiovascular complications including loss of vascular smooth muscle cells (VSMCs). Recent works reported the loss of VSMCs as a major factor triggering atherosclerosis in HGPS. Here, we investigated the mechanisms by which progerin expression leads to massive VSMCs loss. Using aorta tissue and primary cultures of murine VSMCs from a mouse model of HGPS, we showed increased VSMCs death associated with increased poly(ADP-Ribosyl)ation. Poly(ADP-Ribosyl)ation is recognized as a post-translational protein modification that coordinates the repair at DNA damage sites. Poly-ADP-ribose polymerase (PARP) catalyzes protein poly(ADP-Ribosyl)ation by utilizing nicotinamide adenine dinucleotide (NAD+). Our results provided the first demonstration linking progerin accumulation, augmented poly(ADP-Ribosyl)ation and decreased nicotinamide adenine dinucleotide (NAD+) level in VSMCs. Using high-throughput screening on VSMCs differentiated from iPSCs from HGPS patients, we identified a new compound, trifluridine able to increase NAD+ levels through decrease of PARP-1 activity. Lastly, we demonstrate that trifluridine treatment in vivo was able to alleviate aortic VSMCs loss and clinical sign of progeria, suggesting a novel therapeutic approach of cardiovascular disease in progeria.

Mutation in TTI2 reveals a role for triple T complex in human brain development.

Tel2-interacting proteins 1 and 2 (TTI1 and TTI2) physically interact with telomere maintenance 2 (TEL2) to form a conserved trimeric complex called the Triple T complex. This complex is a master regulator of phosphoinositide-3-kinase-related protein kinase (PIKKs) abundance and DNA damage response signaling. Using a combination of autozygosity mapping and high-throughput sequencing in a large consanguineous multiplex family, we found that a missense c.1307T>A/p.I436N mutation in TTI2 causes a human autosomal recessive condition characterized by severe cognitive impairment, microcephaly, behavioral troubles, short stature, skeletal anomalies, and facial dysmorphic features. Immunoblotting experiment showed decreased amount of all Triple T complex components in the patient skin fibroblasts. Consistently, a drastically reduced steady-state level of all PIKKs tested was also observed in the patient cells. Combined with previous observations, these findings emphasises the role of the TTI2 gene in the etiology of intellectual disability and further support the role of PIKK signaling in brain development and functioning.

Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome.

By using a combination of array comparative genomic hybridization and a candidate gene approach, we identified nuclear factor I/X (NFIX) deletions or nonsense mutation in three sporadic cases of a Sotos-like overgrowth syndrome with advanced bone age, macrocephaly, developmental delay, scoliosis, and unusual facies. Unlike the aforementioned human syndrome, Nfix-deficient mice are unable to gain weight and die in the first 3 postnatal weeks, while they also present with a spinal deformation and decreased bone mineralization. These features prompted us to consider NFIX as a candidate gene for Marshall-Smith syndrome (MSS), a severe malformation syndrome characterized by failure to thrive, respiratory insufficiency, accelerated osseous maturation, kyphoscoliosis, osteopenia, and unusual facies. Distinct frameshift and splice NFIX mutations that escaped nonsense-mediated mRNA decay (NMD) were identified in nine MSS subjects. NFIX belongs to the Nuclear factor one (NFI) family of transcription factors, but its specific function is presently unknown. We demonstrate that NFIX is normally expressed prenatally during human brain development and skeletogenesis. These findings demonstrate that allelic NFIX mutations trigger distinct phenotypes, depending specifically on their impact on NMD.