Papers of the Month
By Sara Amolegbe
Early-life phthalates linked with motor skill deficiencies
An NIEHS study linked phthalate exposure early in life to motor skill problems among 11-year old children, with differences between boys and girls. Phthalates are widely used as plasticizers and are present in a wide variety of personal care products, plastic containers, and children’s toys.
In a study of 209 New York City mothers and their children, researchers measured metabolites of six phthalates in urine from women late in their pregnancy and from their children at ages 3, 5, and 7 years. At age 11 years, they administered a motor proficiency screening test to the children to assess motor skills.
They found that higher exposure to phthalates before birth was associated with lower motor function among 11-year-old girls, whereas higher exposure during childhood was associated with lower scores among boys. Almost one-third of the children in the study had below average motor skills.
The findings suggest that phthalate exposure during certain periods may lead to long-lasting adverse effects of motor function in children, with different susceptibility windows before or after birth, depending on sex. According to the authors, the study emphasizes the need to reduce early-life exposure to phthalates because children use motor skills to navigate their physical and social environments.
Citation: Balalian AA, Whyatt RM, Liu X, Insel BJ, Rauh VA, Herbstman J, Factor-Litvak P. 2019. Prenatal and childhood exposure to phthalates and motor skills at age 11 years. Environ Res 171:416–427.
New tool rapidly evaluates chemical effects on cells
NIEHS grantees have developed a new toxicity test that can measure the effects of chemicals on cell survival. The tool is much faster than the gold standard cell survival tool and more sensitive than other rapid cell toxicity tests.
With their new test, the MicroColonyChip, cells are grown in small wells on a plate and the number of cells per colony is quantified by staining the DNA with a fluorescent tag and measuring the level of fluorescence. The researchers developed a publicly available companion software program, which was used to measure the amount of fluorescent DNA in each well and calculated the amount of cell growth. The researchers could estimate the toxicity of a compound being studied by comparing growth of treated and untreated cells.
The researchers compared their new test to the gold standard colony formation assay and found that the results were the same, and the sensitivity was nearly identical. With MicroColonyChip, the test took 3-5 days, instead of the 2-3 weeks required for the gold standard test. Colonies were assessed using their automated image analysis software instead of counting by eye. The new test used a small 96-well plate format, instead of large dishes, which required approximately 250 times less surface area than the gold standard test.
Compared with other more rapid cell toxicity tests, the MicroColonyChip is more sensitive and less prone to inaccurate cell counts due to changes in experimental conditions, such as pH. According to the authors, the new test is a powerful tool for many applications, including screens for drug development, epidemiological studies, and chemical safety studies.
Citation: Ngo LP, Chan TK, Ge J, Samson LD, Engelward BP. 2019. Microcolony size distribution assay enables high-throughput cell survival quantitation. Cell Rep 26(6):1668–1678.e4.
Ribosomal DNA can predict biological age
NIEHS grantees have discovered a new way to determine biological age using ribosomal DNA (rDNA), which is the DNA segment responsible for providing genetic code required to produce new protein molecules. The study introduces a biological age clock that measures the status of DNA methylation, a type of chemical change to DNA that alters its expression, within the rDNA to estimate biological age.
Biological age is based on changes that accumulate over time in DNA and has been shown to be a more accurate way to predict the onset of some aging-related diseases. However, these DNA methylation site clocks across the genome have varied between studies of the same species and are often not the same across species. The new rDNA clock could serve as a more universal marker to gauge individual age in humans as well as laboratory and other organisms.
The researchers found that rDNA methylation status could predict age in humans and dogs, often much more accurately than a larger group of methylation sites in non-rDNA areas of the genome. They also found that the sites were the same in both species. Looking at these rDNA clock sites, they also determined that putting mice on a low-calorie diet, which generally makes animals live longer, led to less methylation and thus made the mice appear to age more slowly. According to the authors, this new biological clock is a useful tool for future studies to determine whether certain behaviors or exposures may speed up or slow down the aging process across different organisms.
Citation: Wang M, Lemos B. 2019. Ribosomal DNA harbors an evolutionarily conserved clock of biological aging. Genome Res 29(3):325–333.
DDT exposure windows may affect breast cancer risk timing
An NIEHS study found that women exposed to high levels of the pesticide DDT early in life were at increased risk for breast cancer through age 54 years, with differences in risk depending on timing when they were first exposed. Women exposed during infancy were most likely to develop breast cancer before the age of 50 years, whereas those who were exposed after infancy were at increased risk of developing cancer later, at ages 50-54 years.
The researchers conducted a study within the Child Health and Development Studies cohort, which followed 15,528 women for nearly 60 years. Age at first DDT exposure was calculated by subtracting the woman’s birth year from 1945, the year DDT was first introduced into the United States. DDT exposure levels were estimated from stored cohort blood samples collected from 1959 to 1967. Researchers used state records to identify cases of breast cancer.
DDT exposure during childhood was a risk factor for both early and later breast cancer, but a higher risk for each of the outcomes was associated with exposure timing. Overall, they observed that the induction period from first exposure to breast cancer was about 40 years. According to the authors, measuring environmental exposures during windows of susceptibility are much more consistent in supporting a positive association with breast cancer risk.
Citation: Cohn BA, Cirillo PM, Terry MB. 2019. DDT and breast cancer: prospective study of induction time and susceptibility windows. J Natl Cancer Inst; doi:10.1093/jnci/djy198 [Online 13 Feb. 2019].