Eyes, feet, hands, and chromosomes – two of each seems to be the done thing. Indeed, two sets of each chromosome (diploidy) is a general feature in mammals, except when we produce eggs and sperm (meiosis) which contain only one set of chromosomes (haploidy). However, various studies have described the production of embryonic stem cells (ESCs) with only one set of chromosomes in some animals, but not as yet in humans…..
Mouse ESCs produced by artificial activation of unfertilized oocytes can lead to the production blastocyst stage embryos (parthenogenesis) and the derivation of so-called parthenogenetic ESCs (pESCs). Of interest to researchers from the laboratories of Dieter Egli and Nissim Benvenisty was the fact that some cells of the blastocyst stage embryo in mouse remain haploid, so human pESC cultures may potentially harbor human haploid ESCs.
So did their hypothesis work out?
- Using viable fluorescence-activated cell sorting (FACS) with Hoechst 33342 staining to isolate cells with fewer than two chromosomal copies of DNA content, the authors identified human pESC lines that contained a small percentage of haploid cells
- Cell enrichment and culture expansion of selected haploid ESCs using standard ESC culture techniques led to the generation of pure cultures of human haploid ESCs
- Haploid ESCs remained karyotypically normal and remained remarkably like diploid ESCs, including in their morphology, cell surface markers, pluripotency gene expression, and ability to differentiate into cells of the three embryonic germ layers.
- Differentiated cell cultures retained significant levels of haploid cells.
- Maintenance of haploidy required periodical sorting
- The study did, however, highlight some important differences between human haploid ESCs and diploid ESCs
- Small gene expression differences: 275 upregulated and 290 downregulated genes in haploid ESCs
- This included upregulation of X chromosome gene activation alongside a correlative lack of X chromosome inactivation
- Haploid ESCs expressed lower levels of total RNA and displayed a global reduction in gene expression levels.
- However, the study did note an increase in the expression of genes involved in oxidative phosphorylation (autosomal and mitochondrial)
- Small gene expression differences: 275 upregulated and 290 downregulated genes in haploid ESCs
The authors then demonstrated the potential of human haploid ESCs as a platform for loss-of-function genetic screening, although the researchers hope that human haploid ESCs could be put to other exciting uses – to decipher more about human genetics and human development and perhaps to create patient-specific germ cells for reproductive purposes.
Keep an eye (or two) on these exciting cells and their regenerative potential in Nature, March 2016.