Imprinting: RNA expression for homocysteine recycling in the human oocyte

ObjectiveTo investigate whether homocysteine, a well known inhibitor of methylation, which is produced after imprinting and other methylation processes, can be recycled to methionine in the oocyte, at least until the stage of maternal to zygotic transition (i.e., four- to eight-cell stage); before this stage, most of the biochemical processes are carried out with the use of maternal stores of protein and mRNA.DesignA first approach using microarrays and then reverse-transcription polymerase chain reaction (RT-PCR) for methionine synthase (5-methyltetrahydrofolate-homocysteine methyltransferase [MTR]), betaine-homocysteine methyltransferase (BHMT), and cystathionine-beta synthase (CBS).SettingTwo private hospitals.Patient(s)Patients involved in IVF/ICSI procedures.Intervention(s)Germinal vesicle oocytes collected at the time of oocyte retrieval, RNA extraction amplification, RT-PCR, microarrays.Main Outcome Measure(s)mRNA expression of all the enzymes involved in the chain of methylation and recycling of homocysteine to methionine.Result(s)All of the enzymes required for methylation are present in the oocyte. Homocysteine can be recycled with BHMT and MTR.Conclusion(s)The human oocyte is able to regulate its Hcy level via remethylation using MTR and BHMT but not CBS. This aspect is important, because recent studies have shown that controlled ovarian hyperstimulation affects the homocysteine concentration in follicular fluid. This may regulate, at least in part, the risk of imprinting problems during IVF procedures.