A woman who is heterozygous for glucose-6-phosphate dehydrogenase (G6PD), a polymorphic enzyme transcribed from the X chromosome, develops chronic myeloid leukemia. Restriction fragment length polymorphism (RFLP) studies on the tumor cells for G6PD reveal that only a single form of the enzyme is transcribed. This finding suppos which of the following features of neoplasia?
Correct Answer: Monoclonality
Description: G6PD is an enzyme involved in carbohydrate metabolism-it is neither an oncogene nor a tumor suppressor, and its altered expression in tumor cells is solely due to its location on the X chromosome. Each somatic cell expresses only one X chromosome, and thus only one of the two isotypes of G6PD. The presence of only one form of G6PD in the tumor cells reflects the monoclonal origin of the cells. Cells contained in the neoplasm express only a single isotype of G6PD because they are all derived from a single progenitor cell. Genetic mutation does not underlie the monotypic expression of G6PD in the tumor cells. The allele for G6PD not expressed in the monoclonal cell line was inactivated in embryogenesis (in the Barr body). Although genetic mutation is a requirement of neoplasia, it is not demonstrated by this phenomenon. Mosaicism is the expression of two or more different phenotypes heterogeneously throughout an organ or organism. Mosaicism usually reflects an asymmetry of mitotic division in embryogenesis. G6PD is not an oncogene, it is a metabolic enzyme expressed constitutively in all cells. Altered expression of G6PD does not produce any growth advantage in a cell. Ref: Van Solinge W.W., van Wijk R. (2010). Chapter 46. Disorders of Red Cells Resulting from Enzyme Abnormalities. In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds), Williams Hematology, 8e.
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