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

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.

Neonatal lethal Costello syndrome and unusual dinucleotide deletion/insertion mutations in HRAS predicting p.Gly12Val.

De novo heterozygous mutations in HRAS cause Costello syndrome (CS), a condition with high mortality and morbidity in infancy and early childhood due to cardiac, respiratory, and muscular complications. HRAS mutations predicting p.Gly12Val, p.Gly12Asp, and p.Gly12Cys substitutions have been associated with severe, lethal, CS. We report on molecular, clinical, and pathological findings in patients with mutations predicting HRAS p.Gly12Val that were identified in our clinical molecular genetic testing service. Such mutations were identified in four patients. Remarkably, three were deletion/insertion mutations affecting coding nucleotides 35 and 36. All patients died within 6 postnatal weeks, providing further evidence that p.Gly12Val mutations predict a very poor prognosis. High birth weight, polyhydramnios (and premature birth), cardiac hypertrophy, respiratory distress, muscle weakness, and postnatal growth failure were present. Dysmorphism was subtle or non-specific, with edema, coarsened facial features, prominent forehead, depressed nasal bridge, anteverted nares, and low-set ears. Proximal upper limb shortening, a small bell-shaped chest, talipes, and fixed flexion deformities of the wrists were seen. Neonatal atrial arrhythmia, highly suggestive of CS, was also present in two patients. One patient had congenital alveolar dysplasia, and another, born after 36 weeks' gestation, bronchopulmonary dysplasia. A rapidly fatal disease course, and the difficulty of identifying subtle dysmorphism in neonates requiring intensive care, suggest that this condition remains under-recognized, and should enter the differential diagnosis for very sick infants with a range of clinical problems including cardiac hypertrophy and disordered pulmonary development. Clinical management should be informed by knowledge of the poor prognosis of this condition.

Genetic variants in XRCC2: new insights into colorectal cancer tumorigenesis.

Polymorphisms in DNA double-strand break repair gene XRCC2 may play an important role in colorectal cancer etiology, specifically in disease subtypes. Associations of XRCC2 variants and colorectal cancer were investigated by tumor site and tumor instability status in a four-center collaboration including three U.K. case-control studies (Sheffield, Leeds, and Dundee) and a U.S. case-control study of cases from high-risk Utah pedigrees (total: 1,252 cases and 1,422 controls). The 14 variants studied were tagging single nucleotide polymorphisms (SNP) selected from National Institute of Environmental Health Sciences/HapMap data supplemented with SNPs identified from sequencing of 125 cases chosen to represent multiple colorectal cancer groups (familial, metastatic disease, and tumor subsite). Monte Carlo significance testing using Genie software provided valid meta-analyses of the total resource that includes family-based data. Similar to reports of colorectal cancer and other cancer sites, the rs3218536 R188H allele was not associated with increased risk. However, we observed a novel, highly significant association of a common SNP, rs3218499G>C, with increased risk of rectal tumors (odds ratio, 2.1; 95% confidence interval, 1.3-3.3; P(chi2) = 0.0006) versus controls, with the largest risk found for female rectal cases (odds ratio, 3.1; 95% confidence interval, 1.6-6.1; P(chi2) = 0.0006). This difference was significantly different to that for proximal and distal colon cancers (P(chi2) = 0.02). Our investigation supports a role for XRCC2 in colorectal cancer tumorigenesis, conferring susceptibility to rectal tumors.

Meta association of colorectal cancer confirms risk alleles at 8q24 and 18q21.

BACKGROUND: Genome-wide association studies of colorectal cancer (CRC) have identified genetic variants that reproducibly associate with CRC. Associations of 12 single nucleotide polymorphisms at 8q24, 9p24, and 18q21 (SMAD7) and CRC were investigated in a three-center collaborative study including two U.K. case-control cohorts (Sheffield and Leeds) and a U.S. case-control study of CRC cases from high-risk Utah pedigrees. METHODS: Our combined resource included 1,092 CRC case subjects and 1,060 age- and sex-matched controls. Meta statistics and Monte Carlo significance testing using Genie software provided a valid combined analysis of our mixed independent and related case-control resource. We also evaluated whether these associations differed by sex, age at diagnosis, family history, or tumor site. RESULTS: At 8q24, we observed two independent significant associations at single nucleotide polymorphisms located in two different risk regions of 8q24: rs6983267 in region 3 [P(trend) = 0.01; per allele odds ratio (OR), 1.17; 95% confidence intervals (95% CI), 1.03-1.32] and rs10090154 in region 5 (P(trend) = 0.05; per allele OR, 1.24; 95% CI, 1.01-1.51). At 18q21, associations were observed in distal colon tumors but not in proximal or rectal cancers: rs4939827 (P(trend) = 0.007; per allele OR, 0.77; 95% CI, 0.64-0.93; case-case p(diff) = 0.03) and rs12953717 (P(trend) = 0.01; per allele OR, 1.27; 95% CI, 1.06-1.52). We were unable to detect any associations at 9p24 with CRC. CONCLUSIONS: Our investigation confirms that variants across multiple risk regions of 8q24 are associated with CRC, and that associations at 18q21 differ by tumor site.

MLH1 -93G>A promoter polymorphism and risk of mismatch repair deficient colorectal cancer.

Rare inherited mutations in the mutL homolog 1 (MLH1) DNA mismatch repair gene can confer an increased susceptibility to colorectal cancer (CRC) with high penetrance where disease frequently develops in the proximal colon. The core promoter of MLH1 contains a common single nucleotide polymorphism (SNP) (-93G>A, dbSNP ID:rs1800734) located in a region essential for maximum transcriptional activity. We used logistic regression analysis to examine the association between this variant and risk of CRC in patients in the United Kingdom. All statistical tests were 2 sided. In an analysis of 1,518 patients with CRC, homozygosity for the MLH1 -93A variant was associated with a significantly increased 3-fold risk of CRC negative for MLH1 protein by immunohistochemistry (odds ratio (OR): AA vs GG = 3.30, 95% CI 1.46-7.47, n = 1392, p = 0.004, MLH1 negative vs MLH1 positive CRC) and with a 68% excess of proximal CRC (OR: AA vs GG=1.68, 95% confidence interval (CI) 1.00-2.83, n = 1,518, p = 0.05, proximal vs distal CRC). These findings suggest that the MLH1 -93G>A polymorphism defines a low penetrance risk allele for CRC.