CONTRASTING TOXICITY OF D-GALACTOSE ON OOCYTES AND EMBRYOS

Abstract

A rare mutation in galactose-1-phosphate uridyltransferase, an enzyme in the Leloir pathway of galactose metabolism, causes classic galactosemia (CG), a metabolic disorder characterized by harmful accumulation of D-galactose and its metabolites galactose 1-phosphate (gal 1-P) and galactitol in physiological systems. One especially susceptible system is the female reproductive tract, specifically the ovaries, which often leads to primary ovarian insufficiency (POI) even with careful monitoring of diet. However, some number of pregnancies in CG patients has been reported even though the general perception is that they are infertile. Previous studies have delineated a mechanism of D-galactose toxicity in oocytes associated with apoptosis and excessive reactive oxygen species, but considering the incidence of pregnancy in CG patients, embryos could be less affected by D-galactose toxicity. Therefore, the aim of this study is to explicate the effects of D-galactose and its metabolites on embryos and subsequent development. This was an experimental case-control design. D-galactose (2 mM), galactitol (11 mM) and gal 1-P (0.1 mM), with concentrations corresponding to those typically found in plasma of CG patients under dietary restrictions, were used to treat one-cell mouse embryos (n = 31) for 4 h. After exposure, the embryos were then incubated in culture for 120 h. Similar procedures were used to treat metaphase II mouse oocytes (n = 120), which were then fertilized in vitro, and then cultured for 120 h. Cells were analyzed for development at 24, 48, and 96 h. Cleavage stage embryos were assessed for nuclear fragments, morphology, and blastomere count. Blastocyte stage embryos were assessed for blastocoel expansion, trophectoderm morphology, and inner cell mass quality. No differences were found between directly treated embryos and untreated controls. Conversely, there was significant decline in cleavage and blastocyst development in embryos derived from treated, fertilized oocytes compared to untreated controls. Considering the relatively high pregnancy rate observed in women with CG-associated POI compared to all women with POI, D-galactose exposure does not appear to be embryotoxic. Thus, oocytes that avoid the harmful effects of D-galactose or its metabolites have the potential for a normal developmental course even if they are then later exposed to D-galactose or its metabolites. These results should encourage women with CG to maintain optimism for childbearing and childbirth.