Can the Pesticide Mancozeb Lead to Leukemia?

SAMITA MOHANASUNDARAM ’13

Figure 1: Results (a) SCGE shows no DNA Damage (identified by a lack of tail) for HSPCs untreated with mancozeb. (b) SCGE shows medium to high levels of DNA damage (as displayed by the presence of tails) in HSPCs treated with mancozeb. (c) CFU-GM colony size of a cell untreated with mancozeb, (d) CFU-GM colony size of a cell that is overproliferating after treatment with mancozeb, (e) example of FITC-Annexin V/PI flow cytometry plot; the lower left quadrant includes viable cells.
(a) SCGE shows no DNA Damage (identified by a lack of tail) for HSPCs untreated
with mancozeb. (b) SCGE shows medium to high levels of DNA damage (as displayed by the
presence of tails) in HSPCs treated with mancozeb. (c) CFU-GM colony size of a cell untreated
with mancozeb, (d) CFU-GM colony size of a cell that is overproliferating after treatment with
mancozeb, (e) example of FITC-Annexin V/PI flow cytometry plot; the lower left quadrant
includes viable cells.

These epidemiological studies lead to the question: does exposure to mancozeb cause tumorigenesis?

More than five billion tons of pesticide are used annually in the United States. Agricultural pesticides have become a significant health concern due to their toxicity and direct application on crops. Studies show that 35% of food in the United States has detectable pesticide residues. One of these pesticides is mancozeb, a broad range fungicide.  Shockingly, more than 5.6 million pounds of mancozeb are consumed annually. Sold in seventy countries and on the market for over fifty years, mancozeb is applied on lawns, fruits, and vegetables.

Epidemiologists have found a correlation between farm workers exposed to mancozeb and the development of certain cancers, especially leukemia. Leukemia is the cancer of white blood cells resulting from the oncogenic transformation of immature hematopoietic stem and progenitor cells (HSPCs), the precursor population to all committed blood cell types. These epidemiological studies lead to the question: does exposure to mancozeb cause tumorigenesis?

Using a combination of in vitro and in vivo studies, my study aims to determine whether exposure to mancozeb generates abnormalities in HSPCs that could result in cancer. Previous studies have shown that cells treated with high concentrations of mancozeb develop chromosomal abnormalities that result in apoptosis, or cell death. To test whether mancozeb induces tumorigenesis, the pesticide dose selected should induce DNA damage, but it should not severely impair cellular viability. For this purpose, HSPCs were isolated from bone marrow of mice, treated with different concentrations of mancozeb, and analyzed to determine DNA damage and cell survival rates associated with each concentration.

To look for DNA damage, Single Cell Gel Electrophoresis (SCGE) was performed. In general, exposure to mancozeb induced DNA breakages in HSPCs, thus corroborating previous studies. Interestingly, extremely low concentrations of mancozeb (500 ng/ml, which resulted from 1/740,000 dilution of the commercial product) were enough to induce DNA damage. To look at the effect of mancozeb on cell death, apoptosis assays were performed. As expected, higher concentrations of mancozeb led to higher rates of apoptosis. However, lower concentrations (250 ng/ml – 500 ng/ml) induced apoptosis at significantly lower rates, leaving cells relatively viable.

Finally, treated cells were cultured in semisolid media and allowed to form colonies, with the goal of observing both the viability of cells treated with different pesticide concentrations and the morphological differences in the colonies that formed. As predicted by the apoptosis assays, high concentrations of mancozeb resulted in low colony numbers, signifying that most cells did not grow or form colonies. Lower concentrations of mancozeb (500 ng/ml), however, gave rise to colony numbers comparable to those generated by the untreated control. Moreover, colonies with abnormalities were observed, even if with low frequency. This seems to indicate that DNA damage generated by exposure to low concentrations of mancozeb could result in an over-proliferative phenotype.

Currently, an ongoing long-term in vivo study is assessing whether HSPCs treated with low concentrations of mancozeb, determined through in vitro studies described above, generate abnormalities in the hematopoietic compartment that could result in cancer formation.

The writer can be reached at smohanas@college.harvard.edu.