Scientists create stem cells with half a genome

Scientists have created human stem cells with 23 chromosomes instead of 46, the normal number in a complete genome contained in almost all cells. The stem cells were capable of cell division despite only having one set of chromosomes.    

'This study has given us a new type of human stem cell that will have an important impact on human genetic and medical research,' said lead co-author of the study, Dr Nissim Benvenisty, Director of the Azrieli Center for Stem Cells and Genetic Research at the Hebrew University of Jerusalem. 'These cells will provide researchers with a novel tool for improving our understanding of human development, and the reasons why we reproduce sexually, instead of from a single parent.'

The study, published in Nature, created haploid cells which, like sperm and egg cells, contain only one set of chromosomes, rather than the two sets found in all other cells in the body – which are diploid cells. The haploid cells had mostly similar properties to diploid cells, but they were slightly smaller, regulated some genes differently, and had overall lower gene expression.  

The cells were created from an unfertilised human egg cell, which was stimulated to divide. As it divided, the egg cell duplicated its genes and made new diploid cells, as expected, but the researchers also suspected that some of newly created cells may be haploid.

To identify any haploid cells, the researchers used a fluorescent marker which binds to DNA. Their suspicions were proven correct when they found two haploid cells out of 2000 cells screened.

The researchers then investigated whether these cells could divide and differentiate, and compared their properties to diploid cells. They found that the new stem cells were able to divide and differentiate into different cell types, including heart, pancreatic and nerve cells, which remained haploid even after maturing.

The finding could be used to create sperm or eggs cells for infertile couples, or for screening to identify genes involved in disease. Often if only one copy of a gene is mutated, it can be difficult to identify, as the healthy copy can partly mask the effects of the mutation. However, in a haploid cell scientists could mutate the single copy of any gene and study the effect, without the other 'back-up' gene copy.  

Dr Dieter Egli, of Columbia University Medical Center in New York, and also co-author of the study, explained to the Guardian: 'We have two genes of everything and if one is mutated the effect is not so obvious. Because these cells reduce the number of possible combinations and reduce the number of variance, it should be easier to get the answers.'

Haploid mammalian stem cells were first created in 2011 (reported in BioNews 624) and have previously been created using cells from mice, rats and monkeys.