A systematic evidence map for the evaluation of noncancer health effects and exposures to polychlorinated biphenyl mixtures.

Assessing health outcomes associated with exposure to polychlorinated biphenyls (PCBs) is important given their persistent and ubiquitous nature. PCBs are classified as a Group 1 carcinogen, but the full range of potential noncancer health effects from exposure to PCBs has not been systematically summarized and evaluated. We used systematic review methods to identify and screen the literature using combined manual review and machine learning approaches. A protocol was developed that describes the literature search strategy and Populations, Exposures, Comparators, and Outcomes (PECO) criteria used to facilitate subsequent screening and categorization of literature into a systematic evidence map of PCB exposure and noncancer health endpoints across 15 organs/systems. A comprehensive literature search yielded 62,599 records. After electronic prioritization steps, 17,037 studies were manually screened at the title and abstract level. An additional 900 studies identified by experts or supplemental searches were also included. After full-text screening of 3889 references, 1586 studies met the PECO criteria. Relevant study details such as the endpoints assessed, exposure duration, and species were extracted into literature summary tables. This review compiles and organizes the human and mammalian studies from these tables into an evidence map for noncancer health endpoints and PCB mixture exposure to identify areas of robust research as well as areas of uncertainty that would benefit from future investigation. Summary data are available online as interactive visuals with downloadable metadata. Sufficient research is available to inform PCB hazard assessments for most organs/systems, but the amount of data to inform associations with specific endpoints differs. Furthermore, despite many years of research, sparse data exist for inhalation and dermal exposures, which are highly relevant human exposure routes. This evidence map provides a foundation for future systematic reviews and noncancer hazard assessments of PCB mixtures and for strategic planning of research to inform areas of greater uncertainty.

Integrating data gap filling techniques: A case study predicting TEFs for neurotoxicity TEQs to facilitate the hazard assessment of polychlorinated biphenyls.

The application of toxic equivalency factors (TEFs) or toxic units to estimate toxic potencies for mixtures of chemicals which contribute to a biological effect through a common mechanism is one approach for filling data gaps. Toxic Equivalents (TEQ) have been used to express the toxicity of dioxin-like compounds (i.e., dioxins, furans, and dioxin-like polychlorinated biphenyls (PCBs)) in terms of the most toxic form of dioxin: 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). This study sought to integrate two data gap filling techniques, quantitative structure-activity relationships (QSARs) and TEFs, to predict neurotoxicity TEQs for PCBs. Simon et al. (2007) previously derived neurotoxic equivalent (NEQ) values for a dataset of 87 PCB congeners, of which 83 congeners had experimental data. These data were taken from a set of four different studies measuring different effects related to neurotoxicity, each of which tested overlapping subsets of the 83 PCB congeners. The goals of the current study were to: (i) evaluate an alternative neurotoxic equivalent factor (NEF) derivations from an expanded dataset, relative to those derived by Simon et al. and (ii) develop QSAR models to provide NEF estimates for the large number of untested PCB congeners. The models used multiple linear regression, support vector regression, k-nearest neighbor and random forest algorithms within a 5-fold cross validation scheme and position-specific chlorine substitution patterns on the biphenyl scaffold as descriptors. Alternative NEF values were derived but the resulting QSAR models had relatively low predictivity (RMSE ∼0.24). This was mostly driven by the large uncertainties in the underlying data and NEF values. The derived NEFs and the QSAR predicted NEFs to fill data gaps should be applied with caution.

Maternal Malignancy Evaluation After Discordant Cell-Free DNA Results.

Cell-free DNA screening for fetal aneuploidy is a commonly used testing strategy in pregnancies at high risk for fetal aneuploidy. The use of cell-free DNA screening is expanding to the low-risk population, because the detection rate for trisomy 21 surpasses that of traditional screening modalities. Although the sensitivity and specificity of cell-free DNA are superior to traditional screening, false-positive results do occur and may indicate an adverse maternal health condition, including maternal mosaicism or, rarely, malignancy. The risk of maternal cancer is significantly elevated when more than one aneuploidy is detected that is discordant from fetal karyotype. Given this risk as well as the rising incidence of cancer in pregnancy, patient counseling and malignancy evaluation should be considered in women when more than one aneuploidy is detected. We reviewed the published literature and developed an algorithm to evaluate women when these results are identified.

Prenatal Diagnosis: Screening and Diagnostic Tools.

The American Congress of Obstetricians and Gynecologists recommends that all pregnant women be offered aneuploidy screening or diagnostic testing. A myriad of screening and testing options are available to patients based on their risk profile and gestational age. Screening options include traditional serum analyte screening, such as first-trimester screening or quadruple screening, and more recently, cell-free DNA. Diagnostic testing choices include chorionic villus sampling and amniocentesis. The number of screening and diagnostic modalities complicates prenatal counseling for physicians and can be difficult for patients to grasp. Appropriate pretest and posttest counseling is important to ensure adequate understanding of results and ensure testing strategy is concordant with patient goals.