Papers of the Month
By Yu-Wei Chen, Douglas Ganini da Silva, Gabriel Knudsen, Ketrell McWhorter, and Rajneesh Pathania
NTP assesses potential for transgenerational inheritance of health effects
National Toxicology Program (NTP) researchers developed an interactive evidence map as part of a literature review of transgenerational inheritance. Transgenerational inheritance refers to how exposure to a chemical agent or other stressor can result in adverse effects in later generations that were never directly exposed to the stressor. The systematic evidence map allows researchers to explore published literature by broad health effect categories, exposures, and the evidence stream to illustrate areas of consistency, uncertainty, data gaps, and research needs. Although more than 250 studies were mapped, the team identified relatively few bodies of evidence in which more than one study evaluated the same exposure and the same or similar outcomes.
A number of studies reported some evidence of potential reproductive, metabolic, or neurological transgenerational effects. However, the review found that risk of bias and heterogeneity in exposure presented limitations for drawing conclusions. To improve the usability of future studies, the authors strongly supported minimizing bias through best practices in study design and reporting. These practices include randomization of treatment, blinding of outcome assessors to study groups, and controlling for effects within a litter when evaluating potential transgenerational effects. (GK)
Citation: Walker VR, Boyles AL, Pelch KE, Holmgren SD, Shapiro AJ, Blystone CR, Devito MJ, Newbold RR, Blain R, Hartman P, Thayer KA, Rooney AA. 2018. Human and animal evidence of potential transgenerational inheritance of health effects: an evidence map and state-of-the-science evaluation. Environ Int 115:48−69.
Pol delta contributes to leading strand DNA replication
Researchers from NIEHS and Brandeis University provided new evidence showing that DNA polymerase delta (Pol delta), an enzyme required for DNA synthesis, is important in initiating leading strand DNA replication in yeast. The findings help scientists understand the functional roles of different enzymes in DNA replication and may ultimately shed light on how changes in the process affect human health.
Before this research, scientists knew DNA polymerase epsilon (Pol epsilon) and Pol delta were involved in replication of the undamaged nuclear genome, but their precise functions were unclear. Using the yeast Saccharomyces cerevisiae, the researchers first compared the phenotypes of mutants that lack the catalytic activities of Pol epsilon with those that lack only the proofreading ability of Pol epsilon. The catalytic mutants demonstrated severe growth defects, confirming that Pol epsilon is essential for the synthesis of the leading strand.
Results from ribonucleotide incorporation analysis further revealed that in the absence of Pol epsilon, Pol delta can replicate both DNA strands, although replication was inefficient and negatively affected genome stability. These analyses also suggested that Pol delta participated in leading strand replication at the origins. The scientists described two models to explain the involvement of Pol delta in the initiation of leading strand replication. (YWC)
Citation: Garbacz MA, Lujan SA, Burkholder AB, Cox PB, Wu Q, Zhou ZX, Haber JE, Kunkel TA. 2018. Evidence that DNA polymerase delta contributes to initiating leading strand DNA replication in Saccharomyces cerevisiae. Nat Commun 9(1):858.
Transient increase in NTHL1 leads to cancer traits in cells
Research conducted by an NIEHS scientist and his collaborators showed that cells developed genomic instability and became cancerous when they transiently accumulated NTHL1, a protein regularly involved in the repair of damaged DNA. The team discovered that increased NTHL1 negatively affected the homologous recombination (HR) pathway, an essential DNA repair mechanism.
NTHL1 is involved in the repair of oxidized DNA by the base excision repair mechanism. However, cells with transiently high NTHL1 lost some ability to repair strand-broken DNA by HR. Those cells accumulated damaged DNA and often formed a micronucleus, a small nucleus produced when a chromosome fragment is not incorporated into a daughter cell. It is a characteristic of genomic instability and carcinogenesis. Interestingly, when using an enzymatically dead NTHL1, the researchers demonstrated that cellular alterations were independent of the DNA damage repair function of NTHL1. They also found that the protein directly binds and probably sequesters a protein known as XPG, which is essential for HR.
The researchers determined that compared with normal cell lines, those from nonsmall cell lung cancer had elevated levels of NTHL1. They hypothesized that their findings could be fundamental to understanding carcinogenesis and, more importantly, might offer new strategies for destroying human cancers with high NTHL1 levels, such as some nonsmall cell lung cancers. (DGS)
Citation: Limpose KL, Trego KS, Li Z, Leung SW, Sarker AH, Shah JA, Ramalingam SS, Werner EM, Dynan WS, Cooper PK, Corbett AH, Doetsch PW. 2018. Overexpression of the base excision repair NTHL1 glycosylase causes genomic instability and early cellular hallmarks of cancer. Nucleic Acids Res; doi: 10.1093/nar/gky162 [Online 7 March 2018].
Specific mutations in GATA3 affect breast cancer outcome
NIEHS scientists and their collaborators at the University of North Carolina at Chapel Hill identified GATA3-specific genetic mutations in breast cancer cells that are associated with poor survival of breast cancer patients. GATA3 is the third most frequently mutated gene in breast cancer, and the research demonstrated that the loss of two nucleotides in one allele of GATA3 drastically changed the transcriptional program and biology of the cells. The work provided additional insight into one of the most important causes of mortality in women.
By creating a cellular model and mouse model system, the researchers discovered that the GATA3 mutation in zinc-finger 2 has a large impact on the gene expression program, which is reflected in a growth advantage for mutant cells when they are implanted into the mammary glands of mice. At the molecular level, the GATA3 mutation led to the loss of expression of the progesterone receptor (PR), which accompanies the loss of the biological program downstream of the PR.
The findings demonstrated that GATA3 mutations in zinc-finger 2 reprogram breast cancer cell properties. This reprogramming might contribute to poor health outcomes in breast cancer patients. (RP)
Citation: Takaku M, Grimm SA, Roberts JD, Chrysovergis K, Bennett BD, Myers P, Perera L, Tucker CJ, Perou CM, Wade PA. 2018. GATA3 zinc finger 2 mutations reprogram the breast cancer transcriptional network. Nat Commun 9(1):1059.
Glucocorticoids promote fatty liver disease in estrogen-deficient mice
NIEHS scientists found that glucocorticoids (GCs), or stress hormones, are responsible for the development of fatty liver in estrogen-depleted mice. The lack of estrogen correlates with changes that happen during menopause. The researchers also uncovered a regulatory loop in which the presence of estrogen limited the production of follicle stimulating hormone (FSH), which subsequently constrained liver stress hormone signaling. By targeting the stress hormone pathway, the authors found they could block and also reverse the metabolic syndrome following estrogen depletion. These findings suggest that the glucocorticoid receptor (GR) pathway may be a good target for the treatment of metabolic complications, such as fatty liver, during menopause in women.
The authors observed that the estrogen-depleted mice had increased liver weights, excess fasting glucose, and the presence fatty liver disease. Further analysis by transcriptome profiling revealed that estrogen deficiency promoted stress hormone hypersensitivity, particularly on genes involved in lipid storage, which promotes fatty liver disease. They found that stress hormone hypersensitivity in the liver of estrogen-depleted female mice was due to excessive FSH production in the pituitary, which in turn mediated hormone-induced hyperphosphorylation of GR.
This researcher proposed that the GR pathway plays a key role in the development of fatty liver during menopause and could potentially replace classical hormone replacement therapy as treatment. (KM)
Citation: Quinn MA, Xu X, Ronfani M, Cidlowski JA. 2018. Estrogen deficiency promotes hepatic steatosis via a glucocorticoid receptor-dependent mechanism in mice. Cell Rep 22(10):2690–2701.