Peter Bridge

The T-786→C Mutation in Endothelial Nitric Oxide Synthase Is Associated With Hypertension

Although the pathogenic mechanisms involved in predisposing individuals to hypertension are not well defined, evidence is accumulating that suggests a strong genetic transmission. Animal studies and some clinical investigations have revealed that aberrant NO production may be an important contributing factor. Indeed, a missense mutation in the endothelial NO gene caused by a Glu298Asp alteration has been strongly associated with essential hypertension, coronary artery spasm, and myocardial infarction. Recently, another point mutation caused by a T-786→C transition in the 5′-flanking region of the endothelial NO synthase gene has been identified and, like the Glu298Asp mutation, is associated with coronary artery spasm. The present study was conducted to determine the effect of the T-786→C point mutation on hypertension. We investigated the interaction between the endothelial NO synthase T-786→C polymorphism and blood pressure in a large (n=705) clinically healthy population. Allele frequencies for the T and C alleles were 62% and 38%, translating into 39%, 46% and 15% of the population having the T/T, T/C, and C/C genotypes, respectively, for the T-786→C point mutation. Subjects with the C/C genotype had significantly higher systolic blood pressures and were 2.16(95% confidence interval, 1.3 to 3.7) more likely to be hypertensive. Therefore, the −786 C/C genotype in NO synthase is a significant contributing factor for increasing the risk of essential hypertension.

Evaluation of BRCA1 and BRCA2 mutation prevalence, risk prediction models and a multistep testing approach in French-Canadian families with high risk of breast and ovarian cancer

Background and objective: In clinical settings with fixed resources allocated to predictive genetic testing for high-risk cancer predisposition genes, optimal strategies for mutation screening programmes are critically important. These depend on the mutation spectrum found in the population under consideration and the frequency of mutations detected as a function of the personal and family history of cancer, which are both affected by the presence of founder mutations and demographic characteristics of the underlying population. The results of multistep genetic testing for mutations in BRCA1 or BRCA2 in a large series of families with breast cancer in the French-Canadian population of Quebec, Canada are reported. Methods: A total of 256 high-risk families were ascertained from regional familial cancer clinics throughout the province of Quebec. Initially, families were tested for a panel of specific mutations known to occur in this population. Families in which no mutation was identified were then comprehensively tested. Three algorithms to predict the presence of mutations were evaluated, including the prevalence tables provided by Myriad Genetics Laboratories, the Manchester Scoring System and a logistic regression approach based on the data from this study. Results: 8 of the 15 distinct mutations found in 62 BRCA1/BRCA2-positive families had never been previously reported in this population, whereas 82% carried 1 of the 4 mutations currently observed in ⩾2 families. In the subset of 191 families in which at least 1 affected individual was tested, 29% carried a mutation. Of these 27 BRCA1-positive and 29 BRCA2-positive families, 48 (86%) were found to harbour a mutation detected by the initial test. Among the remaining 143 inconclusive families, all 8 families found to have a mutation after complete sequencing had Manchester Scores ⩾18. The logistic regression and Manchester Scores provided equal predictive power, and both were significantly better than the Myriad Genetics Laboratories prevalence tables (p<0.001). A threshold of Manchester Score ⩾18 provided an overall sensitivity of 86% and a specificity of 82%, with a positive predictive value of 66% in this population. Conclusion: In this population, a testing strategy with an initial test using a panel of reported recurrent mutations, followed by full sequencing in families with Manchester Scores ⩾18, represents an efficient test in terms of overall cost and sensitivity.

Impact of gender and parent of origin on the phenotypic expression of hereditary nonpolyposis colorectal cancer in a large Newfoundland kindred with a common MSH2 mutation.

AbstractPURPOSE: This study was designed to provide precise estimates of death and cancer risks, by gender and parent of origin, in hereditary nonpolyposis colorectal cancer independent of mutation, geographic variation, and ascertainment bias. METHODS: A group of 12 families with a founder MSH2 mutation (nucleotide 943+3, A → T) causing hereditary nonpolyposis colorectal cancer was identified in Newfoundland. Genetic testing was offered to those at 50 percent risk of inheriting the mutation. Medical records were reviewed to identify cancer types, age at onset of cancer, and age at death. Ascertainment bias was limited by analyzing only sibships with good ascertainment of genetic status (≥50 percent of sibships had known genetic status). RESULTS: Of 302 family members with hereditary nonpolyposis colorectal cancer or at 50 percent risk, 151 (50 percent) were considered to be mutation carriers, 96 (32 percent) were mutation negative, and 55 (18 percent) were of unknown mutation status. By age 50 years, 72 percent of males and 72 percent of females who were hereditary nonpolyposis colorectal cancer mutation carriers had developed cancer. The age-related risks of colorectal cancer or of death of cancer were significantly higher in males than in females (relative risk = 2.8, P = 0.0001 and relative risk = 2.1, P = 0.01, respectively). The mutation was transmitted by the mother more frequently than the father. Females who inherited the mutation from their father had an increased risk of developing colorectal cancer (relative risk = 2.5, P = 0.05) and of dying of cancer (relative risk = 2.7, P = 0.04) compared with females who inherited the mutation from their mother. CONCLUSIONS: Investigation of large kindreds from the same geographic area who share the same MSH2 mutation and in whom family members have been identified with little ascertainment bias suggests that the risks for colorectal cancer and death of cancer are higher for male mutation carriers than for females and that females who inherit the mutation from their father are at higher risk of colorectal cancer than females who inherit the mutation from their mother.

Molecular and genealogical characterization of the R1443X BRCA1 mutation in high-risk French-Canadian breast/ovarian cancer families

The Quebec population contains about six-million French Canadians, descended from the French settlers who colonized “Nouvelle-France” between 1608 and 1765. Although the relative genetic contribution of each of these founders is highly variable, altogether they account for the major part of the contemporary French-Canadian gene pool. This study was designed to analyze the role of this founder effect in the introduction and diffusion of the BRCA1 recurrent R1443X mutant allele. A highly conserved haplotype, observed in 18 French-Canadian families and generated using 17 microsatellite markers surrounding the BRCA1 locus, supports the fact that the R1443X mutation is a founder mutation in the Quebec population. We also performed haplotyping analysis of R1443X carriers on 19 other families from seven different nationalities; although the same alleles are shared for three markers surrounding the BRCA1 gene, distinct haplotypes were obtained in four families, suggesting multiple origins for the R1443X mutation. Ascending genealogies of the 18 French Canadian families and of controls were reconstructed on an average depth of 10 generations. We identified the founder couple with the highest probability of having introduced the mutation in the population. Based on the descending genealogy of this couple, we detected the presence of geographical concentration in the diffusion pattern of the mutation. This study demonstrates how molecular genetics and demogenetic analyses can complement each other to provide findings that could have an impact on public health. Moreover, this approach is certainly not unique to breast cancer genetics and could be used to understand other complex traits.

No Evidence of False Reassurance among Women with an Inconclusive BRCA1/2 Genetic Test Result

Background: Little is known about how women who receive an inconclusive result from BRCA1/2 testing interpret their result. Clinical observations suggest that some of them may be falsely reassured and, consequently, may not adhere to recommended surveillance. The purpose of this study is to evaluate whether women with inconclusive BRCA1/2 test results are falsely reassured. Methods: Participants were adult women with a family history suggestive of a germ-line mutation in either the BRCA1 or the BRCA2 gene who underwent genetic testing in the context of the interdisciplinary research program INHERIT BRCAs. Data were collected using selfadministered questionnaires at genetic counseling and 1 month after result disclosure. Reassurance was assessed through indicators of cancer risk perception, cancer worry, relief following result disclosure, painfulness of the test result, and its effect on quality of life. Results: Five-hundred women (105 carriers, 140 noncarriers, and 255 inconclusive) were included in this analysis. Compared to noncarriers, women with inconclusive results had higher cancer risk perception, were more worried about cancer, were less relieved by their test result, and perceived their quality of life as being more adversely affected by it. Conclusion: The differences observed between noncarriers and women who received an inconclusive result run counter to the hypothesis that the latter are falsely reassured following BRCA1/2 testing. For clinicians, our findings show the value of taking precautions to fully explain to women that inconclusive results do not rule out the possibility that they still may face a higher risk of developing breast and/or ovarian cancer. (Cancer Epidemiol Biomarkers Prev 2005;14(12):2862–7)

Effect of heterozygosity for the methionine synthase 2756 A→G mutation on the risk for recurrent cardiovascular events

Increased dietary intake of folate has been shown to significantly reduce the risk for fatal myocardial infarction, possibly by lowering homocysteine levels. We therefore investigated the association between recurrent cardiovascular events and a mutation in methionine synthase (2756 A-->G)--an enzyme directly involved in folate and homocysteine metabolism. This mutation significantly reduced the risk for recurrent cardiovascular events and elevated red blood cell folate levels.

Further evidence for genetic heterogeneity in familial hemophagocytic lymphohistiocytosis (FHLH).

Familial hemophagocytic lymphohistiocytosis (FHLH; MIM #267700) is an autosomal recessive disorder of immune regulation characterized by fever, hepatosplenomegaly, and cytopenia that is fatal without bone marrow transplantation. Recent studies have suggested the existence of FHLH loci at 9q21.3–22 and 10q21–22 in Asian and European/African/Australian families, respectively. We studied two unrelated Canadian families in which first cousins were affected with FHLH. In an effort to localize the causative gene, we completed a genome-wide screen for homozygosity by descent by using an automated system to genotype 400 highly polymorphic dinucleotide repeat markers covering the genome with an average resolution of 10 centiMorgans (cM). We identified a total of three candidate loci that met the combined criteria for homozygosity by descent in one family and shared maternal alleles in the other family. One of these, D9S1690, had a cytogenetic localization (9q22.33) proximal to a previously reported inversion of chromosome 9 in an FHLH patient. However, additional closely linked flanking markers within 1–2 cM of all three candidates did not conform to the criteria for linkage in either family. Similarly, we excluded the linked 9q21.3-q22 and 10q21–22 regions recently reported in Asian and European/African/Australian families, respectively. The two families were then analyzed independently to encompass the possibility that they were segregating separate genes. Six additional candidate loci were identified on the basis of homozygosity for the same allele in all affected members of one family, but further analysis of closely linked flanking markers did not demonstrate similar homozygosity. Our data provide further evidence of genetic heterogeneity in FHLH and suggest the existence of at least a third locus for this disease.

Large kindred with Ehlers-Danlos syndrome type IV due to a point mutation (G571S) in the COL3A1 gene of type III procollagen: low risk of pregnancy complications and unexpected longevity in some affected relatives.

Ehlers-Danlos syndrome (EDS) type IV is an autosomal dominant connective tissue disorder. Early morbidity and mortality results from rupture of vessels and internal organs. A large kindred with EDS type IV was studied clinically, and the biochemical defects and underlying mutation in the COL3A1 gene that encodes the chains of type III procollagen were identified. A G-->A transition results in a single amino acid substitution, G571S, in the triple helical domain of the products of one COL3A1 allele. Although the clinical findings seen on examination are characteristic of EDS type IV, longevity is longer than that seen in many families and there is less pregnancy-associated morbidity or mortality than in some families. This suggests that some clinical aspects of EDS type IV may be related to the nature of the mutation and its effect on the behavior of the protein.

Maternal uniparental heterodisomy for chromosome 16 : Case report

A patient with uniparental heterodisomy for chromosome 16 presented initially at prenatal diagnosis with a karyotype of 47, XX + 16 on chorionic villus sampling at 11 weeks gestation. The pregnancy was proceeding normally and follow up amniocentesis showed a normal female karyotype. At birth, the child was healthy, but had intrauterine growth retardation. She had unilateral talipes equinovarus and unilateral renal agenesis. Her growth had improved to within the normal range by age three years. On examination, she has epicanthic folds, a flat midface and almond shaped eyes. While these characteristics are not frankly abnormal, they are significantly different from other relatives in her family.

Adult onset spinocerebellar ataxia in a Canadian movement disorders clinic

BACKGROUND The spinocerebellar ataxias (SCAs) are a genetically and clinically heterogeneous group of neurodegenerative disorders. Relative frequencies vary within different ethnic groups and geographical locations. OBJECTIVES 1) To determine the frequencies of hereditary and sporadic adult onset SCAs in the Movement Disorders population; 2) to assess if the fragile X mental retardation gene 1 (FMR1) premutation is found in this population. METHODS A retrospective chart review of individuals with a diagnosis of adult onset SCA was carried out. Testing for SCA types 1, 2, 3, 6, 7, and 8, Dentatorubral-pallidoluysian atrophy (DRPLA), Friedreich ataxia and the FMR1 expansion was performed. RESULTS A total of 69 patients in 60 families were identified. Twenty-one (35%) of the families displayed autosomal dominant and two (3.3%) showed autosomal recessive (AR) pattern of inheritance. A positive but undefined family history was noted in nine (15%). The disorder appeared sporadic in 26 patients (43.3%). In the AD families, the most common mutation was SCA3 (23.8%) followed by SCA2 (14.3%) and SCA6 (14.3%). The SCA1 and SCA8 were each identified in 4.8%. FA was found in a pseudodominant pedigree, and one autosomal recessive pedigree. One sporadic patient had a positive test (SCA3).Dentatorubral-pallidoluysian atrophy and FMR1 testing was negative. CONCLUSION A positive family history was present in 53.3% of our adult onset SCA patients. A specific genetic diagnosis could be given in 61.9% of dominant pedigrees with SCA3 being the most common mutation, followed by SCA2 and SCA6. The yield in sporadic cases was low. The fragile X premutation was not found to be responsible for SCA.