Proofreading exonuclease activity of human DNA polymerase delta and its effects on lesion-bypass DNA synthesis.

Replicative DNA polymerases possess 3' --> 5' exonuclease activity to reduce misincorporation of incorrect nucleotides by proofreading during replication. To examine if this proofreading activity modulates DNA synthesis of damaged templates, we constructed a series of recombinant human DNA polymerase delta (Pol delta) in which one or two of the three conserved Asp residues in the exonuclease domain are mutated, and compared their properties with that of the wild-type enzyme. While all the mutant enzymes lost more than 95% exonuclease activity and severely decreased the proofreading activity than the wild-type, the bypass efficiency of damaged templates was varied: two mutant enzymes, D515V and D402A/D515A, gave higher bypass efficiencies on templates containing an abasic site, but another mutant, D316N/D515A, showed a lower bypass efficiency than the wild-type. All the enzymes including the wild-type inserted an adenine opposite the abasic site, whereas these enzymes inserted cytosine and adenine opposite an 8-oxoguanine with a ratio of 6:4. These results indicate that the exonuclease activity of human Pol delta modulates its intrinsic bypass efficiency on the damaged template, but does not affect the choice of nucleotide to be inserted.

Loss of p53 enhances the induction of colitis-associated neoplasia by dextran sulfate sodium.

Loss of p53 function is an early event in colitis-associated neoplasia in humans. We assessed the role of p53 in a mouse model of colitis-associated neoplasia. Colitis was induced in p53-/-, p53+/- and p53+/+ mice using three or four cycles of dextran sulfate sodium (DSS) followed by 120 days of water. Mice were examined for incidence, multiplicity and types of neoplastic lesions. Lesions were examined for mutations in beta-catenin (exon 3), K-ras (codons 12/13) and p53 (exons 5-8) by sequencing and for cellular localization of beta-catenin by immunohistochemistry. The incidence of neoplastic lesions was 57, 20 and 20% in p53-/-, p53+/- and p53+/+ mice, respectively (P = 0.013). p53-/- mice had a greater number of total lesions (P < 0.0001), cancers (P = 0.001) and dysplasias (P = 0.009) per mouse than either p53+/- or p53+/+ mice. Flat lesions were associated with the p53-/- genotype, whereas polypoid lesions were associated with the p53+/- and p53+/+ genotypes (P < 0.0001). beta-Catenin mutations were present in 75% of lesions of p53+/+ mice and absent in lesions from p53-/- mice (P = 0.055). Nuclear expression of beta-catenin was seen only in polypoid lesions (91%). No K-ras or p53 mutations were detected. These data indicate that loss of p53 enhances the induction of colitis-associated neoplasia, particularly flat lesions, and dysregulation of beta-catenin signaling plays an important role in the formation of polypoid lesions in this mouse model. As observed in humans, p53 plays a protective role in colitis-associated neoplasia in the DSS model.

Serum antimüllerian hormone in healthy premenopausal women.

Antimüllerian hormone (AMH) is extensively studied in ovarian aging and pathology; however, little is known about correlates in healthy premenopausal women. We found that AMH levels are strongly inversely associated with age and differed significantly between oral contraceptive pill users and nonusers, whereas no significant associations were seen between AMH and other clinical, behavioral, and anthropometric characteristics and laboratory variables, making it an attractive hormone for clinical applications.

Assay reproducibility and within-person variation of Müllerian inhibiting substance.

OBJECTIVES: To assess reproducibility of a commercial müllerian inhibiting substance (MIS) assay and evaluate within-person variation in serum MIS levels. DESIGN: Assay reproducibility was evaluated by measuring MIS in multiple serum aliquots from the same blood collection. Within-person variation was assessed by measuring MIS in serum collected twice from the same individuals. SETTING: Cancer Prevention Biomarker and Genotyping Facility, fox Chase Cancer Center, Philadelphia, Pennsylvania. PATIENT(S): Assay reproducibility was evaluated using serum from five volunteers with regular menstrual cycles. Within-person variation was evaluated in serum from 20 premenopausal women who donated blood twice at least 1 year apart. INTERVENTION(S): For both studies, samples were randomly ordered in batches and laboratory personnel were blinded to which aliquots were from the same subject. MAIN OUTCOME MEASURE(S): The MIS was measured by ELISA. RESULT(S): Within- and between-batch coefficients of variation (CVs) of the assay were 7.9% and 12.3%, respectively. After deleting one subject with extreme values, these CVs decreased to 7.6% and 7.7%, respectively. Within- and between-subject variance in MIS measurements were 2.19 and 0.31, respectively, and the intraclass correlation coefficient was 0.88 (95% confidence interval .77-.98). CONCLUSION(S): The MIS serum concentration is relatively stable over 1 year in premenopausal women and can be measured with good reproducibility using a commercial kit.

Prospective case-control study of serum mullerian inhibiting substance and breast cancer risk.

BACKGROUND: Müllerian inhibiting substance (MIS) is a member of the transforming growth factor beta family of growth and differentiation factors that inhibits elongation and branching of mammary ducts and has been shown to inhibit mammary tumor growth in vitro and in animal models. The objective of this study was to determine whether serum MIS levels are associated with breast cancer risk. METHODS: We conducted a prospective case-control study of 309 participants who were registered in the Columbia, Missouri Serum Bank. Each of 105 in situ or invasive breast cancer case patients with prediagnostic serum collected before menopause was matched to two control subjects by age, date, menstrual cycle day, and time of day of blood collection. MIS was measured in serum by using an enzyme-linked immunosorbent assay, and estradiol and testosterone concentrations were quantified by using specific radioimmunoassays. Data were analyzed using conditional logistic regression. All tests of statistical significance were two-sided. RESULTS: The relative odds ratio of breast cancer for women in increasing MIS quartiles were 1, 2.8 (95% confidence interval [CI] = 1.0 to 7.4), 5.9 (95% CI = 2.4 to 14.6), and 9.8 (95% CI = 3.3 to 28.9, P(trend) < .001). The association of MIS with breast cancer was weaker in women who were not taking oral contraceptives at the time of blood collection, but adjustment for estradiol and testosterone levels did not materially alter results for these women. The association of MIS with breast cancer did not vary by age at blood collection but was stronger among women who were diagnosed with breast cancer at an older age than among those who were diagnosed at a younger age. CONCLUSION: MIS may be a novel biomarker of increased breast cancer risk. Additional research including confirmatory epidemiological studies and mechanistic studies is needed.

mTOR is activated in the majority of malignant melanomas.

The objective of this study was to determine whether activation of the kinase mammalian target of rapamycin (mTOR) is associated with human melanoma. We found moderate or strong hyperphosphorylation of ribosomal protein S6 in 78/107 melanomas (73%). In contrast, only 3/67 benign nevi (4%) were moderately positive, and none were strongly positive. These data indicate that mTOR activation is very strongly associated with malignant, compared to benign, melanocytic lesions. Next, we tested six melanoma-derived cell lines for evidence of mTOR dysregulation. Five of the six lines showed persistent phosphorylation of S6 after 18 hours of serum deprivation, and four had S6 phosphorylation after 30 minutes of amino-acid withdrawal, indicating inappropriate mTOR activation. The proliferation of three melanoma-derived lines was blocked by the mTOR inhibitor rapamycin, indicating that mTOR activation is a growth-promoting factor in melanoma-derived cells. mTOR is directly activated by the small guanosine triphosphatase Ras homolog enriched in brain (Rheb), in a farnesylation-dependent manner. Therefore, to investigate the mechanism of mTOR activation, we used the farnesyl transferase inhibitor FTI-277, which partially blocked the growth of three of the six melanoma cell lines. Together, these data implicate activation of mTOR in the pathogenesis of melanoma, and suggest that Rheb and mTOR may be targets for melanoma therapy.

Generation of a unique strain of multiple intestinal neoplasia (Apc(+/Min-FCCC)) mice with significantly increased numbers of colorectal adenomas.

The relevance of the Apc(+/Min) mouse model in the study of human colorectal cancer remains uncertain due to the predominance of small intestinal adenomas and few, if any, colorectal adenomas. A new strain of Apc(+/Min) mice (Apc(+/Min-FCCC)) with significantly greater numbers of colorectal adenomas has been generated and characterized. Male C57BL/6J-Apc(+/Min) mice (the Jackson Laboratory, Bar Harbor, ME) were crossed with wild-type (Apc(+/+)) C57BL/6J females from an independent colony at this institution (offspring=Apc(+/Min-FCCC)) and 233 animals were evaluated over 20 generations. In order to determine the contribution of genetics to the enhanced colorectal adenoma phenotype, breeding pairs (Apc(+/Min) male x wild type female C57BL/6J) were purchased from the Jackson Laboratory and offspring (Apc(+/Min-JAX)) were maintained in our facility under identical conditions (n=98). Animals were fed Purina Rodent chow (#5013) diet containing 5% fat. The entire intestinal tract was examined histopathologically in both strains. Both the Apc and Pla2g2a (candidate for Mom1) genes were sequenced and found to be identical for both the Apc(+/Min-FCCC) and Apc(+/Min-JAX) mouse strains. The multiplicity of colorectal adenomas in the Apc(+/Min-FCCC) mice was much higher than reported in the literature and significantly greater than the multiplicity of colorectal adenomas in Apc(+/Min-JAX) mice maintained in our facility (P=0.01). Apc(+/Min-FCCC) had a significantly greater incidence of rectal prolapse (P = 0.02) and small intestinal adenocarcinomes (P=0.001), and multiplicity of small intestinal adenocarcinomas (P=0.001) compared to Apc(+/Min-JAX) mice. Male Apc(+/Min-FCCC) mice had significantly greater numbers of colorectal adenomas compared to female Apc(+/Min-FCCC) mice (P=0.0002), as did male Apc(+/Min-JAX) mice vs. female Apc(+/Min-JAX) mice (P< 0.0001). These results allow us to conclude: (1) Apc(+/Min-FCCC) mice are unique in that they develop significantly greater numbers of colorectal adenomas and small intestinal cancers, and a significantly greater incidence of small intestinal cancers and rectal prolapse than Apc(+/Min-JAX) mice. (2) This study represents the first report of a significant gender difference in multiplicity of colorectal adenomas. (3) Differences between Apc(+/Min-FCCC) and Apc(+/Min-JAX) mice in currently undefined genetic modifiers may contribute to the enhanced colorectal phenotype. (4) The Apc(+/Min-FCCC) strain is highly suited for the investigation of colorectal neoplastic disease and chemoprevention studies.