Emma Burkitt Wright

Mutations in PRDM5 in Brittle Cornea Syndrome Identify a Pathway Regulating Extracellular Matrix Development and Maintenance

Extreme corneal fragility and thinning, which have a high risk of catastrophic spontaneous rupture, are the cardinal features of brittle cornea syndrome (BCS), an autosomal-recessive generalized connective tissue disorder. Enucleation is frequently the only management option for this condition, resulting in blindness and psychosocial distress. Even when the cornea remains grossly intact, visual function could also be impaired by a high degree of myopia and keratoconus. Deafness is another common feature and results in combined sensory deprivation. Using autozygosity mapping, we identified mutations in PRDM5 in families with BCS. We demonstrate that regulation of expression of extracellular matrix components, particularly fibrillar collagens, by PRDM5 is a key molecular mechanism that underlies corneal fragility in BCS and controls normal corneal development and maintenance. ZNF469, encoding a zinc finger protein of hitherto undefined function, has been identified as a quantitative trait locus for central corneal thickness, and mutations in this gene have been demonstrated in Tunisian Jewish and Palestinian kindreds with BCS. We show that ZNF469 and PRDM5, two genes that when mutated cause BCS, participate in the same regulatory pathway.

Constitutive activation of B-Raf in the mouse germ line provides a model for human cardio-facio-cutaneous syndrome

RASopathies are a class of developmental syndromes that result from congenital mutations in key elements of the RAS/RAF/MEK signaling pathway. A well-recognized RASopathy is the cardio-facio-cutaneous (CFC) syndrome characterized by a distinctive facial appearance, heart defects, and mental retardation. Clinically diagnosed CFC patients carry germ-line mutations in four different genes, B-RAF, MEK1, MEK2, and K-RAS. B-RAF is by far the most commonly mutated locus, displaying mutations that most often result in constitutive activation of the B-RAF kinase. Here, we describe a mouse model for CFC generated by germ-line expression of a B-RafLSLV600E allele. This targeted allele allows low levels of expression of B-RafV600E, a constitutively active B-Raf kinase first identified in human melanoma. B-Raf+/LSLV600E mice are viable and display several of the characteristic features observed in CFC patients, including reduced life span, small size, facial dysmorphism, cardiomegaly, and epileptic seizures. These mice also show up-regulation of specific catecholamines and cataracts, two features detected in a low percentage of CFC patients. In addition, B-Raf+/LSLV600E mice develop neuroendocrine tumors, a pathology not observed in CFC patients. These mice may provide a means of better understanding the pathophysiology of at least some of the clinical features present in CFC patients. Moreover, they may serve as a tool to evaluate the potential therapeutic efficacy of B-RAF inhibitors and establish the precise window at which they could be effective against this congenital syndrome.

Contributions of intrinsic mutation rate and selfish selection to levels of de novo HRAS mutations in the paternal germline.

Significance Harvey rat sarcoma viral oncogene homolog (HRAS) occupies an important place in medical history, because it was the first gene in which acquired mutations that led to activation of a normal protein were associated with cancer, making it the prototype of the now canonical oncogene mechanism. Here, we explore what happens when similar HRAS mutations occur in male germ cells, an issue of practical importance because the mutations cause a serious congenital disorder, Costello syndrome, if transmitted to offspring. We provide evidence that the mutant germ cells are positively selected, leading to an increased burden of the mutations as men age. Although there are many parallels between this germline process and classical oncogenesis, there are interesting differences of detail, which are explored in this paper. The RAS proto-oncogene Harvey rat sarcoma viral oncogene homolog (HRAS) encodes a small GTPase that transduces signals from cell surface receptors to intracellular effectors to control cellular behavior. Although somatic HRAS mutations have been described in many cancers, germline mutations cause Costello syndrome (CS), a congenital disorder associated with predisposition to malignancy. Based on the epidemiology of CS and the occurrence of HRAS mutations in spermatocytic seminoma, we proposed that activating HRAS mutations become enriched in sperm through a process akin to tumorigenesis, termed selfish spermatogonial selection. To test this hypothesis, we quantified the levels, in blood and sperm samples, of HRAS mutations at the p.G12 codon and compared the results to changes at the p.A11 codon, at which activating mutations do not occur. The data strongly support the role of selection in determining HRAS mutation levels in sperm, and hence the occurrence of CS, but we also found differences from the mutation pattern in tumorigenesis. First, the relative prevalence of mutations in sperm correlates weakly with their in vitro activating properties and occurrence in cancers. Second, specific tandem base substitutions (predominantly GC>TT/AA) occur in sperm but not in cancers; genomewide analysis showed that this same mutation is also overrepresented in constitutional pathogenic and polymorphic variants, suggesting a heightened vulnerability to these mutations in the germline. We developed a statistical model to show how both intrinsic mutation rate and selfish selection contribute to the mutational burden borne by the paternal germline.

RAS-MAPK pathway disorders: important causes of congenital heart disease, feeding difficulties, developmental delay and short stature

The disorders described as the neuro-cardio-facio-cutaneous conditions (NCFCs) may all present with symptoms that are common in paediatric practice. They result from germline mutations in genes encoding kinases and other proteins interacting in the RAS-MAPK pathway. This review summarises these disorders, discussing their presenting features and clinical course, identifying overarching similarities and, conversely, features that can help to discriminate one condition from another. The genetic basis and importance of precise clinical diagnosis and molecular diagnostic confirmation when possible is discussed, given each conditions different prognosis, and the need to remain vigilant for specific complications.

X-linked isolated growth hormone deficiency: Expanding the phenotypic spectrum of SOX3 polyalanine tract expansions

Isolated growth hormone (GH) deficiency (IGHD) resulting in short stature has an estimated birth incidence of 1 out of 4000-10000 (Millar et al., 2003) and is usually sporadic, but monogenic forms are known. Involvement of a number of autosomal genes has been demonstrated, including the GH-1 (Wainrajch et al., 1996; Hess et al., 2007), GH releasing hormone receptor (Wainrajch et al., 1996), and HESX1 (Thomas et al., 2001) genes. Further molecular heterogeneity is suggested, as the inherited basis of many familial cases remains unclear (Dattani, 2005). X-linked combined pituitary hormone deficiency has been demonstrated previously to show linkage to two loci. One locus at Xq21.3q22 is associated with agammaglobulinemia (Conley et al., 1991), whereas another is associated with the learning disability and spina bifida and results from duplication of Xq26.1-q27.3 (Solomon et al., 2002). SOX3 lies in this duplicated segment, and is an SRY-related high mobility group box transcription factor expressed in the developing pituitary (Collignon et al., 1996). In a single large family, Laumonnier et al., (2002) identified an in-frame 33bp duplication, encoding 11 additional alanines, in the polyalanine tract in SOX3 as the cause for X-linked learning disability associated with GH deficiency. Woods et al., (2005) subsequently demonstrated that a seven alanine repeat duplication in SOX3 could cause congenital hypopituitarism.

Brittle cornea syndrome: recognition, molecular diagnosis and management.

Brittle cornea syndrome (BCS) is an autosomal recessive disorder characterised by extreme corneal thinning and fragility. Corneal rupture can therefore occur either spontaneously or following minimal trauma in affected patients. Two genes, ZNF469 and PRDM5, have now been identified, in which causative pathogenic mutations collectively account for the condition in nearly all patients with BCS ascertained to date. Therefore, effective molecular diagnosis is now available for affected patients, and those at risk of being heterozygous carriers for BCS. We have previously identified mutations in ZNF469 in 14 families (in addition to 6 reported by others in the literature), and in PRDM5 in 8 families (with 1 further family now published by others). Clinical features include extreme corneal thinning with rupture, high myopia, blue sclerae, deafness of mixed aetiology with hypercompliant tympanic membranes, and variable skeletal manifestations. Corneal rupture may be the presenting feature of BCS, and it is possible that this may be incorrectly attributed to non-accidental injury. Mainstays of management include the prevention of ocular rupture by provision of protective polycarbonate spectacles, careful monitoring of visual and auditory function, and assessment for skeletal complications such as developmental dysplasia of the hip. Effective management depends upon appropriate identification of affected individuals, which may be challenging given the phenotypic overlap of BCS with other connective tissue disorders.

Can the diagnosis of NF1 be excluded clinically? A lack of pigmentary findings in families with spinal neurofibromatosis demonstrates a limitation of clinical diagnosis

Background Consensus clinical diagnostic criteria for neurofibromatosis type I (NF1) include café-au-lait macules and skinfold freckling. The former are frequently the earliest manifestation of NF1, and as such are of particular significance when assessing young children at risk of the condition. A phenotype of predominantly spinal neurofibromatosis has been identified in a small minority of families with NF1, often in association with a relative or absolute lack of cutaneous manifestations. An association with splicing and missense mutations has previously been reported for spinal neurofibromatosis, but on the basis of molecular results in only a few families. Method Patients with spinal NF1 were identified through the Manchester nationally commissioned service for complex NF1. Results Five families with spinal NF1 were identified, with a broad spectrum of NF1 mutations, providing further evidence that this phenotype may arise in association with any genre of mutation in this gene. Pigmentary manifestations were absent or very mild in affected individuals. Several further affected individuals, some with extensive spinal root tumours, were ascertained when additional family members were assessed. Conclusions Clinical NF1 consensus criteria cannot be used to exclude the diagnosis of spinal NF1, especially in childhood. This emphasises the importance of molecular confirmation in individuals and families with atypical presentations of NF1.

Cutaneous features in 17q21.31 deletion syndrome: a differential diagnosis for cardio-facio-cutaneous syndrome

Microdeletion of 17q21.31 causes a recurrent recognizable dysmorphic syndrome. A further four patients with 17q21.31 microdeletions are reported here in whom an earlier diagnosis of cardio–facio–cutaneous syndrome was suggested. These patients have significant similarities of facial gestalt to earlier reported 17q21.31 microdeletion patients, but a striking feature that has not been emphasized previously is the large number of naevi and other pigmentary skin abnormalities that may be present. These features, together with a coarse facial appearance, relative macrocephaly and significant learning disabilities, were what had led to the earlier diagnostic suggestion of cardio–facio–cutaneous syndrome in each of these four cases.

Pierpont syndrome: a collaborative study

Pierpont syndrome is a multiple congenital anomaly syndrome with learning disability first described in 1998. There are only three patients with Pierpont syndrome who have previously been published in the literature. Details of a series of patients with features of this condition were therefore obtained retrospectively to better characterize its key features. These patients were noted to have distinctive shared facial characteristics, in addition to plantar fat pads and other limb abnormalities. Further individuals with equally striking hand and foot findings were identified whose facies were less characteristic, and hence we considered them unlikely to be affected with the same condition. Despite several patients with possible Pierpont syndrome having had high‐resolution array CGH or SNP array, the etiology of this phenotype remains unknown. Whilst it is as yet unclear whether it is a single entity, there appears to be a group of patients in whom Pierpont syndrome may be a recognizable condition, with typical facies, particularly when smiling, and characteristic hand and foot findings.

VSX2 in microphthalmia: a novel splice site mutation producing a severe microphthalmia phenotype

Microphthalmia shows great genetic and clinical heterogeneity, whether as part of a syndrome or an isolated ocular phenotype. Chromosomal or single-gene disorders and teratogens may all cause microphthalmia. Associated syndromic features include cardiac problems, clefting, microcephaly and learning disabilities.1 Microphthalmia is frequently bilateral, but commonly asymmetrical in severity. Homozygous mutations in VSX2/CHX10 have been demonstrated in human and murine microphthalmia.2 3 VSX2 is thought to act principally as a repressor of transcription, particularly of the genes encoding cyclin-dependent kinase inhibitor (p27kip1) and microphthalmia transcription factor (MITF).4 These repressive roles enable cell proliferation by preventing retinal progenitor cells from exiting the cell cycle, and by maintaining neuroretinal cell identity. Loss of these functions therefore causes failures in eye development. Other genes implicated in microphthalmia include SOX2 , PAX6 , sonic hedgehog ( SHH ), RAX , OTX2 , CRYBA …