Project 2: Identifying In Utero Exposures that Are Risk Factors for Childhood Leukemia

The Exposome Project (Project 2) will identify novel chemical risk factors for childhood leukemia, focusing on chemical exposures during the prenatal period.  The Exposome Project is led by Dr. Stephen Rappaport, a pioneer in the field of exposure biology. Dr. Rappaport has called for a paradigm shift in the way that researchers measure exposure to chemicals in epidemiological studies. He is a leading proponent of the concepts of the exposome and the exposome-wide assocation study.

What is the exposome?

The exposome refers to the totality of exposures (both external and internal) to which an individual is subjected from conception onwards. This concept was originally proposed by Christopher Wild and further expanded by CIRCLE Project Leader, Stephen Rappaport, and his colleague, Martyn Smith, who functionalized the exposome in terms of circulating chemicals in the body that reflect both exogenous and endogenous exposures.  In other words, the exposome represents the combined exposures from all sources that reach the internal chemical environment.  A few examples of the broad range of environmental agents encompassed by the exposome are shown in the figure to the right: radiation, stress, infections, drugs, diet, and pollutants.  Additionally, internal processes, such as inflammation, lipid peroxidation, oxidative stress, and gut flora also shape the exposome.  Signatures of these external and internal exposures can be detected as biomarkers in blood or serum.

Science is a bit like the joke about the drunk who is looking under a lamppost for a key that he has lost on the other side of the street, because that’s where the light is. It has no other choice.

-Noam Chomsky

An Untargeted Approach to Measuring Exposure to Chemicals

There is an allegory about a man searching for his keys under a lamppost that explains why many well-intentioned scientific efforts are limited or even biased.  In the context of an epidemiological study, the “keys” are important risk factors for disease and the “light from the lamppost” is a validated analytical assay.  Inherently, scientists tend to focus on the assays and chemicals that they know well in their efforts to identify important risk factors for disease.  However, the allegory teaches us that we should also be looking “across the street in the dark”, that is, in the universe of previously unidentified exposures that may be most relevant to disease.  To move away from the lamppost, CIRCLE will employ untargeted approaches for measuring children’s exposures to chemicals as part of the Exposome Project.

How CIRCLE Measures the Exposome

CIRCLE measures two types of biomarkers in blood specimens to characterize the exposome: small molecules with masses less than about 1,000 Da and adducts of reactive electrophiles that can be detected as modifications to the cysteine amino acid at position 34 (Cys34) of human serum albumin. Reactive electrophiles, including metabolites of pollutants, are inherently carcinogenic because they react with nucleophilic sites in functional macromolecules including DNA and proteins. Interestingly, Cys34 is the strongest nucleophile in serum, where it represents more than 80% of the antioxidant activity. As such, Cys34 is the ideal place to search for all of the chemicals to which a person has been exposed.  With state-of-the-science liquid chromatography-high resolution mass spectrometry, we can detect tens of thousands of chemical signatures representing a multitude of exposures.  Many of these signals come from agents which have not been evaluated as potential risk factors for childhood leukemia in the past.  Thus, these untargeted approaches give CIRCLE investigators the opportunity to identify novel risk factors for childhood leukemia.

Human serum albumin and reactive electrophiles that could potentially form adducts at the cysteine amino acid in position 34. Figure reproduced with permission from Stephen Rappaport.

Finding Causes of Childhood Leukemia

The ultimate goal of the CIRCLE Exposome Project is to identify agents that cause childhood leukemia, especially among those chemical exposures which have not been previously evaluated as risk factors for the disease.  To this end, CIRCLE investigators will perform an Exposome-Wide Association Study.  In this study, the in utero exposomes of children with leukemia will be compared to the in utero exposomes of healthy children using bioinformatics.  Chemical features that discriminate cases from controls will be identified and their sources determined.