Human 'knockouts' of CSF3 display severe congenital neutropenia.

Colony-stimulating factor 3 (CSF3) is a key factor in neutrophil production and function, and recombinant forms have been used clinically for decades to treat congenital and acquired neutropenia. Although biallelic inactivation of its receptor CSF3R is a well-established cause of severe congenital neutropenia (SCN), no corresponding Mendelian disease has been ascribed to date to CSF3. Here, we describe three patients from two families each segregating a different biallelic inactivating variant in CSF3 with SCN. Complete deficiency of CSF3 as a result of nonsense-mediated decay (NMD) could be demonstrated on RT-PCR using skin fibroblasts-derived RNA. The phenotype observed in this cohort mirrors that documented in mouse and zebrafish models of CSF3 deficiency. Our results suggest that CSF3 deficiency in humans causes a novel autosomal recessive form of SCN.

IFT27, encoding a small GTPase component of IFT particles, is mutated in a consanguineous family with Bardet-Biedl syndrome.

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy with multisystem involvement. So far, 18 BBS genes have been identified and the majority of them are essential for the function of BBSome, a protein complex involved in transporting membrane proteins into and from cilia. Yet defects in the identified genes cannot account for all the BBS cases. The genetic heterogeneity of this disease poses significant challenge to the identification of additional BBS genes. In this study, we coupled human genetics with functional validation in zebrafish and identified IFT27 as a novel BBS gene (BBS19). This is the first time an intraflagellar transport (IFT) gene is implicated in the pathogenesis of BBS, highlighting the genetic complexity of this disease.

Mutation in ADAT3, encoding adenosine deaminase acting on transfer RNA, causes intellectual disability and strabismus.

BACKGROUND: Intellectual disability (ID) is one of the most common forms of disability worldwide, displaying a wide range of aetiologies and affecting nearly 2% of the global population. OBJECTIVE: To describe a novel autosomal recessive form of ID with strabismus and its underlying aetiology. MATERIALS AND METHODS: Autozygosity mapping, linkage analysis and exome sequencing were performed in a large multiplex consanguineous family that segregates ID and strabismus. Exome sequencing was independently performed in three other consanguineous families segregating the same disease. Direct sequencing of the resulting candidate gene was performed in four additional families with the same phenotype. RESULTS: A single missense mutation was identified in ADAT3 in all studied families on an ancient ancestral haplotype. This gene encodes one of two eukaryotic proteins that are necessary for the deamination of adenosine at position 34 to inosine in t-RNA. Our results show the first human mutation in the t-RNA editing machinery and expand the landscape of pathways involved in the pathogenesis of ID.

Mutations in FKBP10 cause both Bruck syndrome and isolated osteogenesis imperfecta in humans.

Bruck syndrome (BS) is an autosomal recessive syndromic form of osteogenesis imperfecta (OI) that is characterized by the additional presence of pterygium formation. We have recently shown that FKBP10 previously reported as a novel autosomal recessive OI gene also defines a novel Bruck syndrome locus (BKS3). In this manuscript, we extend our analysis to describe a mutation previously described in isolated OI patients and show that it results in BS phenotype in a Saudi family. More interestingly, we describe a novel FKBP10 mutation that results in isolated OI as well as BS phenotype in the same family. These results, combined with recently published work, confirm that FKBP10 is a bonafide BS locus and lay the foundation for future research into modifiers that underlie the phenotypic heterogeneity of FKBP10 mutations.

Categorized Genetic Analysis in Childhood-Onset Cardiomyopathy.

BACKGROUND: Childhood-onset cardiomyopathy is a heterogeneous group of conditions the cause of which is largely unknown. The influence of consanguinity on the genetics of cardiomyopathy has not been addressed at a large scale. METHODS: To unravel the genetic cause of childhood-onset cardiomyopathy in a consanguineous population, a categorized approach was adopted. Cases with childhood-onset cardiomyopathy were consecutively recruited. Based on the likelihood of founder mutation and on the clinical diagnosis, genetic test was categorized to either (1) targeted genetic test with targeted mutation test, single-gene test, or multigene panel for Noonan syndrome, or (2) untargeted genetic test with whole-exome sequencing or whole-genome sequencing. Several bioinformatics tools were used to filter the variants. RESULTS: Two-hundred five unrelated probands with various forms of cardiomyopathy were evaluated. The median age of presentation was 10 months. In 30.2% (n=62), targeted genetic test had a yield of 82.7% compared with 33.6% for whole-exome sequencing/whole-genome sequencing (n=143) giving an overall yield of 53.7%. Strikingly, 96.4% of the variants were homozygous, 9% of which were found in 4 dominant genes. Homozygous variants were also detected in 7 novel candidates (ACACB, AASDH, CASZ1, FLII, RHBDF1, RPL3L, ULK1). CONCLUSIONS: Our work demonstrates the impact of consanguinity on the genetics of childhood-onset cardiomyopathy, the value of adopting a categorized population-sensitive genetic approach, and the opportunity of uncovering novel genes. Our data suggest that if a founder mutation is not suspected, adopting whole-exome sequencing/whole-genome sequencing as a first-line test should be considered.

Loss-of-function mutations in UDP-Glucose 6-Dehydrogenase cause recessive developmental epileptic encephalopathy.

Developmental epileptic encephalopathies are devastating disorders characterized by intractable epileptic seizures and developmental delay. Here, we report an allelic series of germline recessive mutations in UGDH in 36 cases from 25 families presenting with epileptic encephalopathy with developmental delay and hypotonia. UGDH encodes an oxidoreductase that converts UDP-glucose to UDP-glucuronic acid, a key component of specific proteoglycans and glycolipids. Consistent with being loss-of-function alleles, we show using patients' primary fibroblasts and biochemical assays, that these mutations either impair UGDH stability, oligomerization, or enzymatic activity. In vitro, patient-derived cerebral organoids are smaller with a reduced number of proliferating neuronal progenitors while mutant ugdh zebrafish do not phenocopy the human disease. Our study defines UGDH as a key player for the production of extracellular matrix components that are essential for human brain development. Based on the incidence of variants observed, UGDH mutations are likely to be a frequent cause of recessive epileptic encephalopathy.