Acid-induced disassembly of the Haemophilus ducreyi cytolethal distending toxin.

The cytolethal distending toxins (CDTs) produced by many Gram-negative pathogens are tripartite genotoxins with a single catalytic subunit (CdtB) and two cell-binding subunits (CdtA + CdtC). CDT moves by vesicle carriers from the cell surface to the endosomes and through the Golgi apparatus en route to the endoplasmic reticulum (ER). CdtA dissociates from the rest of the toxin before reaching the Golgi apparatus, and CdtB separates from CdtC in the ER. The free CdtB subunit, which is only active after holotoxin disassembly, then crosses the ER membrane and enters the nucleus where it generates DNA breaks. We hypothesized that the acidified lumen of the endosomes is responsible for separating CdtA from the CdtB/CdtC heterodimer. To test this prediction, possible acid-induced disruptions to the CDT holotoxin were monitored by size exclusion chromatography and surface plasmon resonance. We found that CDT could not efficiently assemble from its individual subunits at the early endosome pH of 6.3. Partial disassembly of the CDT holotoxin also occurred at pH 6.3, with complete separation of CdtA from an intact CdtB/CdtC heterodimer occurring at both pH 6.0 and the late endosome pH of 5.6. Acidification caused the precipitation of CdtA at pH 6.5 and below, but neither CdtB nor CdtC were affected by a pH as low as 5.2. Circular dichroism further showed that the individual CdtB subunit adopts a different secondary structure as compared to its structure in the holotoxin. We conclude the first stage of CDT disassembly occurs in the early endosomes, where an acid-induced alteration to CdtA releases it from the CdtB/CdtC heterodimer.

Common variants of the thyroglobulin gene are associated with differentiated thyroid cancer risk.

BACKGROUND: Genetic factors are important in thyroid cancer susceptibility. Recently, it has been reported that there are associations of certain chromosome regions with thyroid cancer. In this case-control study, we sought to determine whether there is an association between differentiated thyroid cancer (DTC) and variants in regions of chromosome 8q. METHODS: We used a case-control association design in a population of 877 individuals (398 patients with sporadic DTC and 479 healthy controls). The iPLEX technology was applied to analyze seven single-nucleotide polymorphisms (SNPs) in chromosome 8q: two SNPs that map at 8q24, previously reported as risk markers in different types of cancer, two SNPs in the thyrotropin-releasing hormone receptor gene (TRHR), and three SNPs in the thyroglobulin gene (TG). Risk assessment was done by unconditional regression analysis. RESULTS: The two SNPs that map at 8q24, rs6983267 and rs1447295, and the two TRHR polymorphisms showed no association with DTC. No association was also found for the exon 33 TG polymorphism. The two TG polymorphisms in the exon 10-12 cluster, however, were associated with an increased risk of DTC (dominant model odds ratio = 1.80, 95% confidence interval = 1.30-2.50, p < 0.001). CONCLUSIONS: In this study, we show for the first time that the TG gene is a susceptibility factor for thyroid cancer. Although these conclusions are based on a large population, additional studies are warranted to support these data.