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 activit y is thought to be essential in maintaining normal telomere length in immor tal (cancer) and germ cells. The objective of this study was to determine t he 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 t he hTCS transcripts. Multiple telomerase mRNA products were identified by r everse transcription-polymerase chain reaction (RT-PCR) using primers withi n the reverse transcriptase domain. DNA sequencing of these amplicons sugge st that there are alternative splicing variants which align to other telome rase reverse transcriptase (RT) consensus domains. Surprisingly, in unferti lized and immature gametes, as well as preimplantation embryos, hTCS expres sion revealed three different PCR product sizes, 457, 421 and 275 bp. The f requency of the 275 bp DNA product was 6.6% in oocytes (two out of 30) comp ared with 56.6% (17 out of 30) in poorly developing human preimplantation e mbryos (P < 0.005). The presence of alternately spliced mRNA variants in hu man preimplantation embryos may suggest a lack of telomerase activity and t hus chromosomes associated with shortened telomeres.