Mohnish Suri

Mutations in the Human Laminin β2 (LAMB2) Gene and the Associated Phenotypic Spectrum

Mutations of LAMB2 typically cause autosomal recessive Pierson syndrome, a disorder characterized by congenital nephrotic syndrome, ocular and neurologic abnormalities, but may occasionally be associated with milder or oligosymptomatic disease variants. LAMB2 encodes the basement membrane protein laminin β2, which is incorporated in specific heterotrimeric laminin isoforms and has an expression pattern corresponding to the pattern of organ manifestations in Pierson syndrome. Herein we review all previously reported and several novel LAMB2 mutations in relation to the associated phenotype in patients from 39 unrelated families. The majority of disease‐causing LAMB2 mutations are truncating, consistent with the hypothesis that loss of laminin β2 function is the molecular basis of Pierson syndrome. Although truncating mutations are distributed across the entire gene, missense mutations are clearly clustered in the N‐terminal LN domain, which is important for intermolecular interactions. There is an association of missense mutations and small in frame deletions with a higher mean age at onset of renal disease and with absence of neurologic abnormalities, thus suggesting that at least some of these may represent hypomorphic alleles. Nevertheless, genotype alone does not appear to explain the full range of clinical variability, and therefore hitherto unidentified modifiers are likely to exist. Hum Mutat 31:992–1002, 2010.

Activating mutations in STIM1 and ORAI1 cause overlapping syndromes of tubular myopathy and congenital miosis

Significance Stormorken syndrome is a rare autosomal-dominant genetic condition characterized by congenital miosis, bleeding diathesis, thrombocytopenia, and proximal muscle weakness. Other manifestations include functional or anatomical asplenia, ichthyosis, headaches, and dyslexia. A milder form of Stormorken syndrome is associated with muscle weakness and congenital miosis, but without hematologic abnormalities. Here we identify the gene mutations responsible for these syndromes and show that both conditions are caused by the hyperactivation of the Ca2+ release-activated Ca2+ (CRAC) channel. These findings contrast the pathologies associated with loss or diminished function of the CRAC channel, provide new molecular insights of the function of the CRAC channel, and suggest new approaches to combat these conditions by blocking CRAC channel activity. Signaling through the store-operated Ca2+ release-activated Ca2+ (CRAC) channel regulates critical cellular functions, including gene expression, cell growth and differentiation, and Ca2+ homeostasis. Loss-of-function mutations in the CRAC channel pore-forming protein ORAI1 or the Ca2+ sensing protein stromal interaction molecule 1 (STIM1) result in severe immune dysfunction and nonprogressive myopathy. Here, we identify gain-of-function mutations in the cytoplasmic domain of STIM1 (p.R304W) associated with thrombocytopenia, bleeding diathesis, miosis, and tubular myopathy in patients with Stormorken syndrome, and in ORAI1 (p.P245L), associated with a Stormorken-like syndrome of congenital miosis and tubular aggregate myopathy but without hematological abnormalities. Heterologous expression of STIM1 p.R304W results in constitutive activation of the CRAC channel in vitro, and spontaneous bleeding accompanied by reduced numbers of thrombocytes in zebrafish embryos, recapitulating key aspects of Stormorken syndrome. p.P245L in ORAI1 does not make a constitutively active CRAC channel, but suppresses the slow Ca2+-dependent inactivation of the CRAC channel, thus also functioning as a gain-of-function mutation. These data expand our understanding of the phenotypic spectrum of dysregulated CRAC channel signaling, advance our knowledge of the molecular function of the CRAC channel, and suggest new therapies aiming at attenuating store-operated Ca2+ entry in the treatment of patients with Stormorken syndrome and related pathologic conditions.

Autosomal dominant inheritance of a heterozygous mutation in SAMHD1 causing familial chilblain lupus

Autosomal Dominant Inheritance of a Heterozygous Mutation in SAMHD1 Causing Familial Chilblain Lupus Jane C. Ravenscroft, Mohnish Suri, Gillian I. Rice, Marcin Szynkiewicz, and Yanick J. Crow* Department of Dermatology, Nottingham University Hospitals NHS Trust, Nottingham, UK Nottingham Clinical Genetics Service, Nottingham University Hospitals NHS Trust, Nottingham, UK Genetic Medicine, University of Manchester, Manchester Academic Heath Science Centre, Central Manchester Foundation Trust University Hospitals, Manchester, UK

An overview of L‐2‐hydroxyglutarate dehydrogenase gene (L2HGDH) variants: a genotype–phenotype study

L‐2‐Hydroxyglutaric aciduria (L2HGA) is a rare, neurometabolic disorder with an autosomal recessive mode of inheritance. Affected individuals only have neurological manifestations, including psychomotor retardation, cerebellar ataxia, and more variably macrocephaly, or epilepsy. The diagnosis of L2HGA can be made based on magnetic resonance imaging (MRI), biochemical analysis, and mutational analysis of L2HGDH. About 200 patients with elevated concentrations of 2‐hydroxyglutarate (2HG) in the urine were referred for chiral determination of 2HG and L2HGDH mutational analysis. All patients with increased L2HG (n=106; 83 families) were included. Clinical information on 61 patients was obtained via questionnaires. In 82 families the mutations were detected by direct sequence analysis and/or multiplex ligation dependent probe amplification (MLPA), including one case where MLPA was essential to detect the second allele. In another case RT‐PCR followed by deep intronic sequencing was needed to detect the mutation. Thirty‐five novel mutations as well as 35 reported mutations and 14 nondisease‐related variants are reviewed and included in a novel Leiden Open source Variation Database (LOVD) for L2HGDH variants (http://www.LOVD.nl/L2HGDH). Every user can access the database and submit variants/patients. Furthermore, we report on the phenotype, including neurological manifestations and urinary levels of L2HG, and we evaluate the phenotype–genotype relationship. Hum Mutat 30:1–11, 2010.

Missense mutations in the beta strands of the single A-domain of matrilin-3 result in multiple epiphyseal dysplasia.

Multiple epiphyseal dysplasia (MED) is a relatively mild and clinically variable osteochondrodysplasia in which the hip and knee joints are most frequently affected. Both autosomal dominant and autosomal recessive forms of MED are recognised. The more severe forms of MED are often described as the “Fairbank type”, whereas the milder cases are known as the “Ribbing type”. However, this classification belies a much greater clinical spectrum in which characteristics such as radiographic features, age of onset, degree of lower limb deformity, stature, and long term morbidity such as osteoarthritis are extremely variable.1–4 It is therefore not surprising that MED is genetically heterogeneous, and to date mutations in six different genes have been shown to cause MED.5,6 Mutations in the genes encoding cartilage oligomeric matrix protein ( COMP ), the α1, α2, and α3 chains of type IX collagen ( COL9A1, COL9A2 , and COL9A3 ) and matrilin-3 ( MATN3 ) all result in autosomal dominant MED,7–11 whereas specific mutations in the sulphate transporter 26A2 ( SLC26A2 /DTDST ) have been shown to result in an autosomal recessive form of MED.12,13 Preliminary genotype–phenotype correlations have suggested that the more severe forms of autosomal dominant MED often result from COMP mutations, while the milder forms are more probably caused by mutations in the genes encoding type IX collagen or matrilin-3.14,15 However, the number of patients available for comparative study, particularly those with type IX collagen and matrilin-3 defects, has remained too limited to date to allow any in depth correlations to be derived. The matrilins are a four member family of extracellular matrix proteins; matrilin-1 and -3 are specifically expressed in cartilaginous tissues, while matrilin-2 and -4 have a wider pattern of expression in a variety of extracellular matrices including non-skeletal tissues.16,17 Each member of the family comprise …

Clinical and radiographic findings in multiple epiphyseal dysplasia caused by MATN3 mutations: Description of 12 patients

Multiple epiphyseal dysplasia (MED) is characterized by pain and stiffness in joints and delayed and irregular ossification of epiphyses. Causative mutations have been recognized in six different genes. We have identified disease‐causing mutations in the gene encoding matrilin‐3, an extracellular matrix protein, in seven families with autosomal dominant MED. Review of the clinical and radiographic features in 12 of the affected family members shows a uniform pattern of skeletal anomalies in all patients with considerable degree of variability in severity, both between and within families. The characteristic clinical findings are onset of symptoms in early childhood with predominance of knee and hip related complaints, normal stature, and early‐onset osteoarthritis. Radiographs show small and irregular epiphyses and mild metaphyseal irregularities and striations, especially at the knees and hips and mild spinal changes. Despite overlap, both clinically and radiographically, with other forms of MED, the described features may help to differentiate this particular form from other entities within the MED spectrum.

EXOSC3 mutations in pontocerebellar hypoplasia type 1: novel mutations and genotype-phenotype correlations

BackgroundPontocerebellar hypoplasia (PCH) represents a group of neurodegenerative disorders with prenatal onset. Eight subtypes have been described thus far (PCH1-8) based on clinical and genetic features. Common characteristics include hypoplasia and atrophy of the cerebellum, variable pontine atrophy, and severe mental and motor impairments. PCH1 is distinctly characterized by the combination with degeneration of spinal motor neurons. Recently, mutations in the exosome component 3 gene (EXOSC3) have been identified in approximately half of the patients with PCH subtype 1.MethodsWe selected a cohort of 99 PCH patients (90 families) tested negative for mutations in the TSEN genes, RARS2, VRK1 and CASK. Patients in this cohort were referred with a tentative diagnose PCH type 1, 2, 4, 7 or unclassified PCH. Genetic analysis of the EXOSC3 gene was performed using Sanger sequencing. Clinical data, MR images and autopsy reports of patients positive for EXOSC3 mutations were analyzed.ResultsEXOSC3 mutations were found in twelve families with PCH subtype 1, and were not found in patients with other PCH subtypes. Identified mutations included a large deletion, nonsense and missense mutations. Examination of clinical data reveals a prolonged disease course in patients with a homozygous p.D132A mutation. MRI shows variable pontine hypoplasia in EXOSC3 mediated PCH, where the pons is largely preserved in patients with a homozygous p.D132A mutation, but attenuated in patients with other mutations. Additionally, bilateral cerebellar cysts were found in patients compound heterozygous for a p.D132A mutation and a nonsense allele.ConclusionsEXOSC3 mediated PCH shows clear genotype-phenotype correlations. A homozygous p.D132A mutation leads to PCH with possible survival into early puberty, and preservation of the pons. Compound heterozygosity for a p.D132A mutation and a nonsense or p.Y109N allele, a homozygous p.G31A mutation or a p.G135E mutation causes a more rapidly progressive course leading to death in infancy and attenuation of the ventral pons.Our findings imply a clear correlation between genetic mutation and clinical outcome in EXOSC3 mediated PCH, including variable involvement of the pons.

A Study of the Clinical and Radiological Features in a Cohort of 93 Patients with a COL2A1 Mutation Causing Spondyloepiphyseal Dysplasia Congenita or a Related Phenotype

Type 2 collagen disorders encompass a diverse group of skeletal dysplasias that are commonly associated with orthopedic, ocular, and hearing problems. However, the frequency of many clinical features has never been determined. We retrospectively investigated the clinical, radiological, and genotypic data in a group of 93 patients with molecularly confirmed SEDC or a related disorder. The majority of the patients (80/93) had short stature, with radiological features of SEDC (n = 64), others having SEMD (n = 5), Kniest dysplasia (n = 7), spondyloperipheral dysplasia (n = 2), or Torrance‐like dysplasia (n = 2). The remaining 13 patients had normal stature with mild SED, Stickler‐like syndrome or multiple epiphyseal dysplasia. Over 50% of the patients had undergone orthopedic surgery, usually for scoliosis, femoral osteotomy or hip replacement. Odontoid hypoplasia was present in 56% (95% CI 38–74) and a correlation between odontoid hypoplasia and short stature was observed. Atlanto‐axial instability, was observed in 5 of the 18 patients (28%, 95% CI 10–54) in whom flexion‐extension films of the cervical spine were available; however, it was rarely accompanied by myelopathy. Myopia was found in 45% (95% CI 35–56), and retinal detachment had occurred in 12% (95% CI 6–21; median age 14 years; youngest age 3.5 years). Thirty‐two patients complained of hearing loss (37%, 95% CI 27–48) of whom 17 required hearing aids. The ophthalmological features and possibly also hearing loss are often relatively frequent and severe in patients with splicing mutations. Based on clinical findings, age at onset and genotype–phenotype correlations in this cohort, we propose guidelines for the management and follow‐up in this group of disorders.

Genetic Heterogeneity and Clinical Variability in Musculocontractural Ehlers–Danlos Syndrome Caused by Impaired Dermatan Sulfate Biosynthesis

Bi‐allelic variants in CHST14, encoding dermatan 4‐O‐sulfotransferase‐1 (D4ST1), cause musculocontractural Ehlers–Danlos syndrome (MC‐EDS), a recessive disorder characterized by connective tissue fragility, craniofacial abnormalities, congenital contractures, and developmental anomalies. Recently, the identification of bi‐allelic variants in DSE, encoding dermatan sulfate epimerase‐1 (DS‐epi1), in a child with MC‐EDS features, suggested locus heterogeneity for this condition. DS‐epi1 and D4ST1 are crucial for biosynthesis of dermatan sulfate (DS) moieties in the hybrid chondroitin sulfate (CS)/DS glycosaminoglycans (GAGs). Here, we report four novel families with severe MC‐EDS caused by unique homozygous CHST14 variants and the second family with a homozygous DSE missense variant, presenting a somewhat milder MC‐EDS phenotype. The glycanation of the dermal DS proteoglycan decorin is impaired in fibroblasts from D4ST1‐ as well as DS‐epi1‐deficient patients. However, in D4ST1‐deficiency, the decorin GAG is completely replaced by CS, whereas in DS‐epi1‐deficiency, still some DS moieties are present. The multisystemic abnormalities observed in our patients support a tight spatiotemporal control of the balance between CS and DS, which is crucial for multiple processes including cell differentiation, organ development, cell migration, coagulation, and connective tissue integrity.

Further delineation of the KAT6B molecular and phenotypic spectrum

KAT6B sequence variants have been identified previously in both patients with the Say-Barber-Biesecker type of blepharophimosis mental retardation syndromes (SBBS) and in the more severe genitopatellar syndrome (GPS). We report on the findings in a previously unreported group of 57 individuals with suggestive features of SBBS or GPS. Likely causative variants have been identified in 34/57 patients and were commonly located in the terminal exons of KAT6B. Of those where parental samples could be tested, all occurred de novo. Thirty out of thirty-four had truncating variants, one had a missense variant and the remaining three had the same synonymous change predicted to affect splicing. Variants in GPS tended to occur more proximally to those in SBBS patients, and genotype/phenotype analysis demonstrated significant clinical overlap between SBBS and GPS. The de novo synonymous change seen in three patients with features of SBBS occurred more proximally in exon 16. Statistical analysis of clinical features demonstrated that KAT6B variant-positive patients were more likely to display hypotonia, feeding difficulties, long thumbs/great toes and dental, thyroid and patella abnormalities than KAT6B variant-negative patients. The few reported patients with KAT6B haploinsufficiency had a much milder phenotype, though with some features overlapping those of SBBS. We report the findings in a previously unreported patient with a deletion of the KAT6B gene to further delineate the haploinsufficiency phenotype. The molecular mechanisms giving rise to the SBBS and GPS phenotypes are discussed.