Mark C. Hannibal

Exome sequencing identifies MLL2 mutations as a cause of Kabuki syndrome

We demonstrate the successful application of exome sequencing to discover a gene for an autosomal dominant disorder, Kabuki syndrome (OMIM%147920). We subjected the exomes of ten unrelated probands to massively parallel sequencing. After filtering against existing SNP databases, there was no compelling candidate gene containing previously unknown variants in all affected individuals. Less stringent filtering criteria allowed for the presence of modest genetic heterogeneity or missing data but also identified multiple candidate genes. However, genotypic and phenotypic stratification highlighted MLL2, which encodes a Trithorax-group histone methyltransferase: seven probands had newly identified nonsense or frameshift mutations in this gene. Follow-up Sanger sequencing detected MLL2 mutations in two of the three remaining individuals with Kabuki syndrome (cases) and in 26 of 43 additional cases. In families where parental DNA was available, the mutation was confirmed to be de novo (n = 12) or transmitted (n = 2) in concordance with phenotype. Our results strongly suggest that mutations in MLL2 are a major cause of Kabuki syndrome.

Connexin 43 (GJA1) Mutations Cause the Pleiotropic Phenotype of Oculodentodigital Dysplasia

Gap junctions are assemblies of intercellular channels that regulate a variety of physiologic and developmental processes through the exchange of small ions and signaling molecules. These channels consist of connexin family proteins that allow for diversity of channel composition and conductance properties. The human connexin 43 gene, or GJA1, is located at human chromosome 6q22-q23 within the candidate region for the oculodentodigital dysplasia locus. This autosomal dominant syndrome presents with craniofacial (ocular, nasal, and dental) and limb dysmorphisms, spastic paraplegia, and neurodegeneration. Syndactyly type III and conductive deafness can occur in some cases, and cardiac abnormalities are observed in rare instances. We found mutations in the GJA1 gene in all 17 families with oculodentodigital dysplasia that we screened. Sixteen different missense mutations and one codon duplication were detected. These mutations may cause misassembly of channels or alter channel conduction properties. Expression patterns and phenotypic features of gja1 animal mutants, reported elsewhere, are compatible with the pleiotropic clinical presentation of oculodentodigital dysplasia.

Mutations in SEPT9 cause hereditary neuralgic amyotrophy

Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant recurrent neuropathy affecting the brachial plexus. HNA is triggered by environmental factors such as infection or parturition. We report three mutations in the gene septin 9 (SEPT9) in six families with HNA linked to chromosome 17q25. HNA is the first monogenetic disease caused by mutations in a gene of the septin family. Septins are implicated in formation of the cytoskeleton, cell division and tumorigenesis.

Phenotypic spectrum and management issues in Kabuki syndrome.

OBJECTIVE To report the phenotypic spectrum and management issues of children with Kabuki syndrome (Niikawa-Kuroki syndrome) from North America. DESIGN A case series of children (n = 18) with clinical findings of Kabuki syndrome. SETTING Medical genetics clinics in Washington, Alaska, and Arizona. RESULTS Most patients had postnatal growth retardation, and all had developmental delay and hypotonia. Feeding difficulties, with or without cleft palate, were common; 5 patients required gastrostomy tube placement. Developmental quotients/IQs in all but 2 were 60 or less. Seizures were seen in less than half of the patients, but ophthalmologic and otologic problems were common, particularly recurrent otitis media. Congenital heart defects were present in 7 (39%); 3 patients underwent repair of coarctation of the aorta. Other features included urinary tract anomalies, malabsorption, joint hypermobility and dislocation, congenital hypothyroidism, idiopathic thrombocytopenic purpura, and in one patient, autoimmune hemolytic anemia and hypogammaglobulinemia. All patients had negative family histories for Kabuki syndrome. CONCLUSIONS Kabuki syndrome is a mental retardation-malformation syndrome affecting multiple organ systems, with a broad spectrum of neuromuscular dysfunction and mental ability. Given that 18 ethnically diverse patients were identified from 2 genetics programs, it appears that this syndrome is more common in North American non-Japanese patients than previously appreciated.

Spectrum of MLL2 (ALR) mutations in 110 cases of Kabuki syndrome.

Kabuki syndrome is a rare, multiple malformation disorder characterized by a distinctive facial appearance, cardiac anomalies, skeletal abnormalities, and mild to moderate intellectual disability. Simplex cases make up the vast majority of the reported cases with Kabuki syndrome, but parent‐to‐child transmission in more than a half‐dozen instances indicates that it is an autosomal dominant disorder. We recently reported that Kabuki syndrome is caused by mutations in MLL2, a gene that encodes a Trithorax‐group histone methyltransferase, a protein important in the epigenetic control of active chromatin states. Here, we report on the screening of 110 families with Kabuki syndrome. MLL2 mutations were found in 81/110 (74%) of families. In simplex cases for which DNA was available from both parents, 25 mutations were confirmed to be de novo, while a transmitted MLL2 mutation was found in two of three familial cases. The majority of variants found to cause Kabuki syndrome were novel nonsense or frameshift mutations that are predicted to result in haploinsufficiency. The clinical characteristics of MLL2 mutation‐positive cases did not differ significantly from MLL2 mutation‐negative cases with the exception that renal anomalies were more common in MLL2 mutation‐positive cases. These results are important for understanding the phenotypic consequences of MLL2 mutations for individuals and their families as well as for providing a basis for the identification of additional genes for Kabuki syndrome.

TBX5 genetic testing validates strict clinical criteria for Holt-Oram syndrome

Holt-Oram syndrome (HOS) is an autosomal dominant heart-hand syndrome characterized by congenital heart disease (CHD) and upper limb deformity, and caused by mutations in the TBX5 gene. To date, the sensitivity of TBX5 genetic testing for HOS has been unclear. We now report mutational analyses of a nongenetically selected population of 54 unrelated individuals who were consecutively referred to our center with a clinical diagnosis of HOS. TBX5 mutational analyses were performed in all individuals, and clinical histories and findings were reviewed for each patient without reference to the genotypes. Twenty-six percent of the complete cohort was shown to have mutations of the TBX5 gene. However, among those subjects for whom clinical review demonstrated that their presentations met strict diagnostic criteria for HOS, TBX5 mutations were identified in 74%. No mutations were identified in those subjects who did not meet these criteria. Thus, these studies validate our clinical diagnostic criteria for HOS including an absolute requirement for preaxial radial ray upper limb malformation. Accordingly, TBX5 genotyping has high sensitivity and specificity for HOS if stringent diagnostic criteria are used in assigning the clinical diagnosis.

TGFBR2 mutations alter smooth muscle cell phenotype and predispose to thoracic aortic aneurysms and dissections

AIMS Transforming growth factor-β (TGF-β) signaling is critical for the differentiation of smooth muscle cells (SMCs) into quiescent cells expressing a full repertoire of contractile proteins. Heterozygous mutations in TGF-β receptor type II (TGFBR2) disrupt TGF-β signaling and lead to genetic conditions that predispose to thoracic aortic aneurysms and dissections (TAADs). The aim of this study is to determine the molecular mechanism by which TGFBR2 mutations cause TAADs. METHODS AND RESULTS Using aortic SMCs explanted from patients with TGFBR2 mutations, we show decreased expression of SMC contractile proteins compared with controls. Exposure to TGF-β1 fails to increase expression of contractile genes in mutant SMCs, whereas control cells further increase expression of these genes. Analysis of fixed and frozen aortas from patients with TGFBR2 mutations confirms decreased in vivo expression of contractile proteins relative to unaffected aortas. Fibroblasts explanted from patients with TGFBR2 mutations fail to transform into mature myofibroblasts with TGF-β1 stimulation as assessed by expression of contractile proteins. CONCLUSIONS These data support the conclusion that heterozygous TGFBR2 mutations lead to decreased expression of SMC contractile protein in both SMCs and myofibroblasts. The failure of TGFBR2-mutant SMCs to fully express SMC contractile proteins predicts defective contractile function in these cells and aligns with a hypothesis that defective SMC contractile function contributes to the pathogenesis of TAAD.

Mutations in FKBP10, which result in Bruck syndrome and recessive forms of osteogenesis imperfecta, inhibit the hydroxylation of telopeptide lysines in bone collagen

Although biallelic mutations in non-collagen genes account for <10% of individuals with osteogenesis imperfecta, the characterization of these genes has identified new pathways and potential interventions that could benefit even those with mutations in type I collagen genes. We identified mutations in FKBP10, which encodes the 65 kDa prolyl cis-trans isomerase, FKBP65, in 38 members of 21 families with OI. These include 10 families from the Samoan Islands who share a founder mutation. Of the mutations, three are missense; the remainder either introduce premature termination codons or create frameshifts both of which result in mRNA instability. In four families missense mutations result in loss of most of the protein. The clinical effects of these mutations are short stature, a high incidence of joint contractures at birth and progressive scoliosis and fractures, but there is remarkable variability in phenotype even within families. The loss of the activity of FKBP65 has several effects: type I procollagen secretion is slightly delayed, the stabilization of the intact trimer is incomplete and there is diminished hydroxylation of the telopeptide lysyl residues involved in intermolecular cross-link formation in bone. The phenotype overlaps with that seen with mutations in PLOD2 (Bruck syndrome II), which encodes LH2, the enzyme that hydroxylates the telopeptide lysyl residues. These findings define a set of genes, FKBP10, PLOD2 and SERPINH1, that act during procollagen maturation to contribute to molecular stability and post-translational modification of type I procollagen, without which bone mass and quality are abnormal and fractures and contractures result.

Investigation of NRXN1 deletions: clinical and molecular characterization.

Deletions at 2p16.3 involving exons of NRXN1 are associated with susceptibility for autism and schizophrenia, and similar deletions have been identified in individuals with developmental delay and dysmorphic features. We have identified 34 probands with exonic NRXN1 deletions following referral for clinical microarray‐based comparative genomic hybridization. To more firmly establish the full phenotypic spectrum associated with exonic NRXN1 deletions, we report the clinical features of 27 individuals with NRXN1 deletions, who represent 23 of these 34 families. The frequency of exonic NRXN1 deletions among our postnatally diagnosed patients (0.11%) is significantly higher than the frequency among reported controls (0.02%; P = 6.08 × 10−7), supporting a role for these deletions in the development of abnormal phenotypes. Generally, most individuals with NRXN1 exonic deletions have developmental delay (particularly speech), abnormal behaviors, and mild dysmorphic features. In our cohort, autism spectrum disorders were diagnosed in 43% (10/23), and 16% (4/25) had epilepsy. The presence of NRXN1 deletions in normal parents and siblings suggests reduced penetrance and/or variable expressivity, which may be influenced by genetic, environmental, and/or stochastic factors. The pathogenicity of these deletions may also be affected by the location of the deletion within the gene. Counseling should appropriately represent this spectrum of possibilities when discussing recurrence risks or expectations for a child found to have a deletion in NRXN1.

Novel mutations in the gene SALL4 provide further evidence for acro-renal-ocular and Okihiro syndromes being allelic entities, and extend the phenotypic spectrum

The SALL genes, similar to the Drosophila gene spalt ,1 encode likely zinc finger transcription factors. In humans, four such genes have been identified to date. Mutations in the gene SALL1 on chromosome 16q12.1 have been associated with Townes-Brocks syndrome and related phenotypes,2,3 and mutations in the gene SALL4 have been shown to be causative in patients with Okihiro syndrome.4,5 SALL2 6 and SALL3 7 remain to be associated with human disease. We previously reported frameshift and nonsense mutations in SALL4 in five of eight families segregating the Okihiro syndrome phenotype.5 A further report4 identified two frameshift mutations and one nonsense mutation in three affected kindreds, including the family reported by Okihiro et al .8 In a recent study of patients with a clinical diagnosis of Holt-Oram syndrome, one additional frameshift mutation and an unclear missense change were reported from a family who turned out to have Okihiro syndrome rather than Holt-Oram.9 Furthermore, we reported one previously identified and three novel SALL4 mutations in patients originally diagnosed as having either Holt-Oram syndrome (later revised to Okihiro syndrome based on the observation of a Duane anomaly in at least one of the affected family members in each family), acro-renal-ocular syndrome, or Holt-Oram syndrome versus thalidomide embryopathy.10 While our findings suggested that acro-renal-ocular syndrome and Okihiro syndrome are allelic, evidence so far has come only from one family in which no gross structural eye defects were seen.11 In order to further substantiate our findings we sought to perform mutation analysis in additional patients diagnosed with acro-renal-ocular syndrome, especially those who presented with structural eye defects. We were also interested in extending our studies to further patients with Okihiro syndrome in order to allow a genotype–phenotype correlation. Here we report five novel …