Deciphering the contribution of genetic instability in somatic cells is critical to our understanding of many human disorders. Myotonic dystrophy type 1 (DM1) is one such disorder that is caused by the expansion of a CTG repeat that shows extremely high levels of somatic instability. This somatic instability has compromised attempts to measure intergenerational repeat dynamics and infer genotype-phenotype relationships. Using single-molecule PCR, we have characterized more than 17 000 de novo somatic mutations from a large cohort of DM1 patients. These data reveal that the estimated progenitor allele length is the major modifier of age of onset. We find no evidence for a threshold above which repeat length does not contribute toward age at onset, suggesting pathogenesis is not constrained to a simple molecular switch such as nuclear retention of the DMPK transcript or haploinsufficiency for DMPK and/or SIX5. Importantly, we also show that age at onset is further modified by the level of somatic instability; patients in whom the repeat expands more rapidly, develop the symptoms earlier. These data establish a primary role for somatic instability in DM1 severity, further highlighting it as a therapeutic target. In addition, we show that the level of instability is highly heritable, implying a role for individual-specific trans-acting genetic modifiers. Identifying these trans-acting genetic modifiers will facilitate the formulation of novel therapies that curtail the accumulation of somatic expansions and may provide clues to the role these factors play in the development of cancer, aging and inherited disease in the general population.

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Several risk factors have been associated with gastric cancer, among them Helicobacter pylori infection. This bacterium yields inflammation, the degree of which depends on the bacterial strain and the severity of the host response. The inflammatory response involves a complex cytokine network. Recently, polymorphisms of the genes coding for interleukin-1beta (IL-1B), interleukin-1Ra (ILRN) and interleukin-10 have been associated with an increased risk of gastric cancer. In order to determine the association of the IL-1B, IL-1RN and IL-10 polymorphisms with gastric cancer in a high-risk Costa Rican population, we analysed purified DNA of 58 gastric cancer patients, 99 controls and 41 patients classified as group I or II, according to the Japanese classification. Genotyping was carried out by PCR, PCR-RFLP and pyrosequencing analysis. We did not find any association of the IL-1B-31, IL-1B-511 and IL-10 polymorphisms with the risk for developing gastric cancer in the studied population. Carriers of the IL-1B+3954T/- had an increased risk for developing gastric cancer (OR 3.7; 95%CI: 1.34-10.2). Also we found an increased risk for developing gastric cancer for allele 2 heterozygotes of the IL-1RN (OR 2.94; 95%CI: 1.09-7.93). This is the first time that IL-1B+3954 has been associated with gastric cancer. This is one of the first studies trying to describe the role played by IL-1B, IL-1RN and IL-10 genetic polymorphisms in gastric cancer in one of the highest risk American countries. Further investigation on American countries is needed.

Myotonic dystrophy type 1 (DM1) is caused by the expansion of an unstable CTG repeat (g.17294_17296(45_1000)) with more repeats associated with increased disease severity and reduced age at onset. Expanded disease-associated alleles are highly unstable in both the germline and soma. Germline instability is expansion biased, providing a molecular explanation for anticipation. Somatic instability is expansion biased, size-and age-dependent, features that have compromised genotypephenotype correlations and intergenerational studies. We corrected these confounding factors by estimating the progenitor allele length in 54 father-offspring and 52 mother-offspring pairs in Costa Rican DM1 families. Not surprisingly, we found major parental allele length effects on the size of the allele transmitted, the magnitude of the intergenerational length change, the age at onset in the next generation and the degree of anticipation in both male and female transmissions. We also detected, for the first time, an age-of-parent effect for both male and female transmission. Interestingly, we found no evidence for an intrauterine effect in the transmission of congenital DM1, suggesting previous reports may have been an artefact of agedependent somatic instability and sampling bias. These data provide new insights into the germline dynamics of the CTG repeat and opportunities for providing additional advice and more accurate risk assessments to prospective parents in DM1 families.

Mutations in the gene coding for the skeletal muscle Cl(-) channel (CLCN1) lead to dominant or recessive myotonia. Here, we identified and characterized CLCN1 mutations in Costa Rican patients, who had been clinically diagnosed with myotonic dystrophy type 1 but who were negative for DM1 mutations. CLCN1 mutations c.501C>G, p.F167L and c.1235A>C, p.Q412P appeared to have recessive inheritance but patients had atypical clinical phenotypes; c.313C>T, p.R105C was found in combination with c.501C>G, p.F167L in an apparently recessive family and the c.461A>G, p.Q154R variant was associated with a less clear clinical picture. In Xenopus oocytes, none of the mutations exhibited alterations of fast or slow gating parameters or single channel conductance, and mutations p.R105C, p.Q154R, and p.F167L were indistinguishable from wild-type (WT). p.Q412P displayed a dramatically reduced current density, surface expression and exerted no dominant negative effect in the context of the homodimeric channel. Fluorescently tagged constructs revealed that p.Q412P is expressed inefficiently. Our study confirms p.F167L and p.R105C as myotonia mutations in the Costa Rican population, whereas p.Q154R may be a benign variant. p.Q412P most likely induces a severe folding defect, explaining the lack of dominance in patients and expression systems, but has WT properties once expressed in the plasma membrane.