Alternative splicing of the telomerase catalytic subunit in human oocytes and embryos.

The human telomerase catalytic subunit (hTCS) is a ribonucleoprotein which synthesizes telomere repeats on the ends of chromosomes. Telomerase activity is thought to be essential in maintaining normal telomere length in immortal (cancer) and germ cells. The objective of this study was to determine the gene expression of telomerase mRNA in human oocytes at different meiotic stages and in embryos. Normal and abnormal human oocytes, preimplantation embryos, and blastocysts were analysed for the presence and expression of the hTCS transcripts. Multiple telomerase mRNA products were identified by reverse transcription-polymerase chain reaction (RT-PCR) using primers within the reverse transcriptase domain. DNA sequencing of these amplicons suggest that there are alternative splicing variants which align to other telomerase reverse transcriptase (RT) consensus domains. Surprisingly, in unfertilized and immature gametes, as well as preimplantation embryos, hTCS expression revealed three different PCR product sizes, 457, 421 and 275 bp. The frequency of the 275 bp DNA product was 6.6% in oocytes (two out of 30) compared with 56.6% (17 out of 30) in poorly developing human preimplantation embryos (P < 0.005). The presence of alternately spliced mRNA variants in human preimplantation embryos may suggest a lack of telomerase activity and thus chromosomes associated with shortened telomeres.