Biodosimetry of ionizing radiation in humans using the glycophorin A genotoxicity assay

Our assay for determining somatic mutations in humans detects variant erythrocytes that occur as a result of in vivo allele loss at the glycophorin A (GPA) locus in erythroid precursor cells in the bone marrow. This gene codes for a cell surface sialoglycoprotein that occurs in two allelic forms, named the M and N forms, and is codominantly expressed on erythrocytes in peripheral blood of people who are heterozygous at the GPA locus. With our assay, which is performed only on GPA(MN) heterozygotes, we are able to detect rare variant erythrocytes that have lost expression of one of the two GPA alleles. Two different variant cell types are detected. one termed N OE variant cells, is hemizygous. Such cells might arise by mutation, deletion or inactivation of the GPA(M) allele or loss of chromosome carrying that allele in erythroid precursor cells. Our assay also detects homozygous variant erythrocytes that have lost expression of the GPA(M) allele and express the GPA(N) allele at twice the heterozygous level. These NN variant cells would be generated by chromosomal loss and duplication, gene conversion or mitotic recombination in erythroid precursor cells. The GPA assay requires expression of GPA(N) thus, guaranteeing that all variant cells are capable of normally expressing this cell surface antigen. The result of this assay is an enumeration of the frequency of N OE and NN variant cell types for each individual analyzed. Such variant cell frequencies provide a measure of the quantity of somatic cell mutations that have occurred at the GPA locus. If the relationship between the mutations expressed at the GPA locus and the onset of cancer induced by radiation can be developed, this assay may serve as an effective monitoring method for people at high risk for exposure to ionizing radiation with a cancer risk estimation linked to the results of this monitor