Scientists investigating the impact of pesticides on the DNA in lymphocytes
(a type of white blood cell) found that glyphosate may cause a type of
DNA damage that can lead to cancer. The finding is particularly important
in that glyphosate has been linked in several studies to non-Hodgkin lymphoma,
a type of cancer that affects lymphocytes.
What is a double strand break? Let’s start with the word “strand.”
You’ve probably seen diagrams of the structure of DNA, the complex
molecule found in all cells that many have compared to an instruction
book for building and operating each person’s body. Genes and chromosomes
– the “paragraphs” and “chapters” in the
genetic “how to” book – are made of DNA. Models of DNA
usually show two chains twisted around each other with stair steps joining
them like a spiral staircase. Those chains are the “strands”
that are severed in a double strand break.
Finding Double Strand Breaks in DNA After Herbicide Exposure
Evidence of glyphosate DNA damage comes in the form of proteins that are involved in the NHEJ repair process. The proteins are the “tools” left at the DNA repair worksite that provide evidence that a double strand break has occurred.
In this study, the researchers were looking for two key proteins. One is named gamma-H2AX. The location of gamma-H2AX at the NHEJ repair site is called a gamma-H2AX focus (plural foci). Another protein involved in the DNA repair process is called p-Ku80. Both proteins are considered “biomarkers” (biological evidence) for double strand breaks.
The researchers exposed lymphocyte cells to varying doses of each the eight pesticides. Five pesticides induced double strand breaks, but only two induced significant increases in p-Ku80: glyphosate and the chemical paraoxon.
Paraoxon is a metabolite of one of the most dangerous pesticides in the world – parathion. When parathion is ingested, it is chemically transformed (metabolized) into paraoxon in the body. Paraoxon is even more toxic than parathion. Glyphosate is clearly not in good company here.
According to the study, “[g]lyphosate was found to significantly induce the presence of p-Ku80 in a dose dependent manner.” In other words, increasing the dose appeared to increase the glyphosate DNA damage.
The degree of damage to the DNA was classified according to the percentage of cells that contained no gamma-H2AX foci (undamaged), between 1 and 10 H2AX foci (moderate damage), or more than 10 H2AX foci (severe damage). The authors reported that “glyphosate and paraoxon showed an increase of cells with more than 10 foci, related to the concentration [the dose of glyphosate the cell received].” Larger doses were tied to more gamma-H2AX foci. Each gamma-H2AX focus represents a double strand break.
“Chromosomal translocations are a hallmark of non-Hodgkin lymphoma (NHL) and can arise as a consequence of misrepair of DNA double-strand breaks.” — Deidre A. Hill et al., “Risk of non-Hodgkin lymphoma (NHL) in relation to germline variation in DNA repair and related genes,” Blood (2006);108: 3161-3167. (Note: A definition of chromosomal translocation can be found at reference.com.)
In discussing their results, the authors explain that the level of damage they observed may have been enough to induce a form of non-homologous end joining, which has been demonstrated to produce chromosomal alterations that are “of great concern in the development of leukemia, lymphoma and secondary cancers.” The authors stress that the effects they observed occurred in low concentrations of the pesticides and over relatively short time periods. This, they suggest, raises the possibility that “longer exposures in actual environmental settings” could produce “cumulative damage” over time.
It is this cumulative damage that is thought to be behind the accumulation of mutations that leads to cancer.
Summary Information
Title
Screening of Pesticides with the Potential of Inducing DSB and Successive
Recombinational Repair
Authors
Karen Suárez-Larios, Ana-María Salazar-Martínez, and
Regina Montero-Montoya
Department of Genomic Medicine and Environmental Toxicology, Institute for Biomedical Investigations, National Autonomous University of Mexico, Post Office Box 70228, 04510 Mexico City, Mexico
Journal
Journal of Toxicology (2017). Published 10 October 2017
Funding
This work was realized by funds from the Universitary Program to Support
Science and Technology (PAPIIT, Project no. IN203011-3) and a fellowship
by Consejo Nacional de Ciencia y Tecnología (no. 220284) for Karen
Suárez-Larios, M.S., a student at the Ph.D. Program of Biological Science,
UNAM. The authors acknowledge the valuable comments of Chemist Guillermina Vázquez and Biologist Juan Pablo Pánico.
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