David Markie

A serine/threonine kinase gene defective in Peutz-Jeghers syndrome

Studies of hereditary cancer syndromes have contributed greatly to our understanding of molecular events involved in tumorigenesis. Here we investigate the molecular background of the Peutz–Jeghers syndrome, (PJS), a rare hereditary disease in which there is predisposition to benign and malignant tumours of many organ systems. A locus for this condition was recently assigned to chromosome 19p (ref. 3). We have identified truncating germline mutations in a gene residing on chromosome 19p in multiple individuals affected by PJS. This previously identified but unmapped gene, LKB1 (ref. 4), has strong homology to a cytoplasmic Xenopus serine/threonine protein kinase XEEK1 (ref. 5), and weaker similarity to many other protein kinases. Peutz–Jeghers syndrome is therefore the first cancer-susceptibility syndrome to be identified that is due to inactivating mutations in a protein kinase.

Germline mutations in BMPR1A/ALK3 cause a subset of cases of juvenile polyposis syndrome and of cowden and bannayan-riley-ruvalcaba syndromes

Juvenile polyposis syndrome (JPS) is an inherited hamartomatous-polyposis syndrome with a risk for colon cancer. JPS is a clinical diagnosis by exclusion, and, before susceptibility genes were identified, JPS could easily be confused with other inherited hamartoma syndromes, such as Bannayan-Riley-Ruvalcaba syndrome (BRRS) and Cowden syndrome (CS). Germline mutations of MADH4 (SMAD4) have been described in a variable number of probands with JPS. A series of familial and isolated European probands without MADH4 mutations were analyzed for germline mutations in BMPR1A, a member of the transforming growth-factor beta-receptor superfamily, upstream from the SMAD pathway. Overall, 10 (38%) probands were found to have germline BMPR1A mutations, 8 of which resulted in truncated receptors and 2 of which resulted in missense alterations (C124R and C376Y). Almost all available component tumors from mutation-positive cases showed loss of heterozygosity (LOH) in the BMPR1A region, whereas those from mutation-negative cases did not. One proband with CS/CS-like phenotype was also found to have a germline BMPR1A missense mutation (A338D). Thus, germline BMPR1A mutations cause a significant proportion of cases of JPS and might define a small subset of cases of CS/BRRS with specific colonic phenotype.

Mutations in PYCR1 cause cutis laxa with progeroid features.

Autosomal recessive cutis laxa (ARCL) describes a group of syndromal disorders that are often associated with a progeroid appearance, lax and wrinkled skin, osteopenia and mental retardation. Homozygosity mapping in several kindreds with ARCL identified a candidate region on chromosome 17q25. By high-throughput sequencing of the entire candidate region, we detected disease-causing mutations in the gene PYCR1. We found that the gene product, an enzyme involved in proline metabolism, localizes to mitochondria. Altered mitochondrial morphology, membrane potential and increased apoptosis rate upon oxidative stress were evident in fibroblasts from affected individuals. Knockdown of the orthologous genes in Xenopus and zebrafish led to epidermal hypoplasia and blistering that was accompanied by a massive increase of apoptosis. Our findings link mutations in PYCR1 to altered mitochondrial function and progeroid changes in connective tissues.

Mutations in genes encoding the cadherin receptor-ligand pair DCHS1 and FAT4 disrupt cerebral cortical development.

The regulated proliferation and differentiation of neural stem cells before the generation and migration of neurons in the cerebral cortex are central aspects of mammalian development. Periventricular neuronal heterotopia, a specific form of mislocalization of cortical neurons, can arise from neuronal progenitors that fail to negotiate aspects of these developmental processes. Here we show that mutations in genes encoding the receptor-ligand cadherin pair DCHS1 and FAT4 lead to a recessive syndrome in humans that includes periventricular neuronal heterotopia. Reducing the expression of Dchs1 or Fat4 within mouse embryonic neuroepithelium increased progenitor cell numbers and reduced their differentiation into neurons, resulting in the heterotopic accumulation of cells below the neuronal layers in the neocortex, reminiscent of the human phenotype. These effects were countered by concurrent knockdown of Yap, a transcriptional effector of the Hippo signaling pathway. These findings implicate Dchs1 and Fat4 upstream of Yap as key regulators of mammalian neurogenesis.

Analysis of genetic and phenotypic heterogeneity in juvenile polyposis

BACKGROUND Juvenile polyposis syndrome (JPS) is characterised by gastrointestinal (GI) hamartomatous polyposis and an increased risk of GI malignancy. Juvenile polyps also occur in the Cowden (CS), Bannayan-Ruvalcaba-Riley (BRRS) and Gorlin (GS) syndromes. Diagnosing JPS can be problematic because it relies on exclusion of CS, BRRS, and GS. Germline mutations in the PTCH, PTENand DPC4 (SMAD4)genes can cause GS, CS/BRRS, and JPS, respectively. AIMS To examine the contribution of mutations in PTCH,PTEN, and DPC4(SMAD4) to JPS. METHODS Forty seven individuals from 15 families and nine apparently sporadic cases with JPS were screened for germline mutations inDPC4, PTEN, andPTCH. RESULTS No patient had a mutation in PTEN orPTCH. Five different germline mutations were detected in DPC4; three of these were deletions, one a single base substitution creating a stop codon, and one a missense change. None of these patients had distinguishing clinical features. CONCLUSIONS Mutations in PTEN and PTCHare unlikely to cause juvenile polyposis in the absence of clinical features indicative of CS, BRRS, or GS. A proportion of JPS patients harbour DPC4 mutations (21% in this study) but there remains uncharacterised genetic heterogeneity in JPS.

Peutz-Jeghers disease: most, but not all, families are compatible with linkage to 19p13.3.

A locus for Peutz-Jeghers syndrome (PJS) was recently mapped to chromosome 19p13.3. Each of 12 families studied was compatible with linkage to the marker D19S886. We have analysed 20 further families and found that the majority of these are consistent with a PJS gene on 19p13.3. Three families were, however, unlinked to 19p13.3 and none of the available PJS polyps from these families showed allele loss at D19S886. There were no obvious clinicopathological or ethnic differences between the 19p13.3 linked and unlinked families. There appears, therefore, to be a major PJS locus on chromosome 19p13.3 and the possibility exists of a minor locus (or loci) elsewhere.

Screening SMAD1 , SMAD2 , SMAD3 , and SMAD5 for germline mutations in juvenile polyposis syndrome

BACKGROUND AND AIMS Juvenile polyps occur in several Mendelian disorders, whether in association with gastrointestinal cancer alone (juvenile polyposis syndrome, JPS) or as part of known syndromes (Cowden, Gorlin, and Bannayan-Zonana) in association with developmental abnormalities, dysmorphic features, or extraintestinal tumours. Recently, some JPS families were shown to harbour germline mutations in theSMAD4 (DPC4) gene, providing further evidence for the importance of the TGFβ signalling pathway in colorectal cancer. There remains, however, considerable, unexplained genetic heterogeneity in JPS. Other members of the SMAD family are excellent candidates for JPS, especiallySMAD2 (which, likeSMAD4, is mutated somatically in colorectal cancers), SMAD3 (which causes colorectal cancer when “knocked out” in mice),SMAD5, and SMAD1. METHODS SMAD1,SMAD2, SMAD3, andSMAD5 were screened for germline mutations in 30 patients with JPS and without SMAD4mutations. RESULTS No mutations were found in any of these genes. A G–A C89Y polymorphism with possible effects on protein function was found in SMAD3, but the frequencies of the G and A alleles did not differ between patients with JPS and controls. CONCLUSIONS It remains to be determined whether or not this polymorphism is involved in a minor predisposition to colorectal or other carcinomas.SMAD4 may be the only member of the SMAD family which causes JPS when mutant in the germline. The other genes underlying JPS remain to be identified.

Heat shock induces chromosome loss in the yeast Candida albicans.

SummaryThe heat shock protocol described in this paper causes mitotic instability in log phase Candida albicans cells. Such instability is induced in diploid, aneuploid and tetraploid strains. The strains analysed are multiple heterozygotes which facilitates the detection of mitotic instability as manifested by the formation of homozygotes. Strains previously shows to be carrying cis linked mutant alleles show coincident segregation of the linked alleles. Conversely, strains which carry unlinked mutant alleles display no such coincident segregation. This segregation of complete linkage groups suggests that heat shock is inducing chromosome some loss in C. albicans. The application of this protocol to the genetics of the imperfect fungus C. albicans has produced evidence of at least three chromosomes.

Evidence of associations between bipolar disorder and the brain‐derived neurotrophic factor (BDNF) gene

Sears C, Markie D, Olds R, Fitches A. Evidence of associations between bipolar disorder and the brain‐derived neurotropic factor (BDNF) gene. Bipolar Disord 2011: 13: 630–637.

A pericentric inversion of chromosome six in a patient with Peutz-Jeghers' syndrome and the use of FISH to localise the breakpoints on a genetic map

Abstract Karyotypic analysis in a patient with Peutz-Jeghers’ syndrome demonstrated a pericentric inversion on chromosome 6. Further investigation was undertaken using fluorescence in situ hybridisation (FISH) with yeast artificial chromosome clones selected to contain genetic markers from chromosome 6, and a probe for the centromeric alphoid repeat array. This analysis located one inversion breakpoint within the alphoid array, in a 1-cM interval between D6S257 and D6S402, and the other in a 4-cM interval between D6S403 and D6S311. The oestrogen receptor gene locus (ESR) is excluded from the latter interval.