Expression of 11 members of the BCL-2 family of apoptosis regulatory molecules during human preimplantation embryo development and fragmentation

Apoptosis during preimplantationdevelopment has received much interest because of its potential role in eliminating defective cells. Although development in humans is characterised by a high degree of genetic abnormality, little is known of the regulation of apoptosis in embryos. By PolyA PCR we analysed expression of 11 BCL-2 genes in individual human embryos representative of normal development and in severely fragmented embryos. We demonstrate constitutive expression of BAX in virtually all embryos at all stages of development, and variable expression of BCL2, BCL-XL, BCL-W, MCL-1 BAK, BAD, BOKL, BID,BIK, and BCL-XS. The frequency of expression of pro- and anti-apoptotic BCL-2 members was similar throughout development, except at the two-cell stage where pro-apoptotic genes predominated. Protein expression was confirmed for BCL-2, MCL-1, BCL-X,BAX, BAD, and activated caspase 3. BCL-2 protein was associated with mitochondria but expressed inconsis- tently in the blastocyst inner cell mass. Consistent differences between morphologically intact and frag- mented embryos included the expression of BAK in fragmented but not intact four-cell embryos. Our study addresses the importance of examining single human embryos representative of the viable population for a large number of genes, in order to establish meaningful expression profiles and provide information on over-lapping function in a large gene family.