Haploid Stem Cells

August 10, 2017

Scientists have found a way to isolate human haploid embryonic stem cells. What are these cells, and do they avoid some of the ethical issues surrounding human embryonic stem cells?

Several press releases have reported on the isolation of a special type of embryonic stem cell that has half of the number of chromosomes as normal cells do. This stem cell was first found in mice in 2011 and later in primates. In 2016, researchers from Hebrew University and Columbia University isolated these special cells from human embryos.[1] (Researchers from China also independently reported the isolation of these cells in 2016).[2]

What Are Haploid Cells?

The cells are called haploid cells. They have half of the number of chromosomes of normal cells, which are known as diploid cells. If you took a cell from your body (say, a skin cell) and performed a test, you would find 46 chromosomes that make up your genetic library. These chromosomes contain the majority of your DNA.

Figure 1. Human (male) karyogram of a diploid cell. By Courtesy: National Human Genome Research Institute – Extracted image from www.genome.gov/glossary/resources/karyotype.pdf., Public Domain, commons.wikimedia.org/w/index.php?curid=583512%5B/caption%5D

Figure 1 shows a karyogram of a human cell. Notice that all of these chromosomes are in pairs. There are 23 pairs of chromosomes in a human being. Haploid cells, such as egg and sperm cells, only have 23 chromosomes total. They are missing the partner to the pair. Typically, haploid cells combine to make diploid cells.

Why Do Haploid Cells Matter?

The advantage of having pairs of chromosomes is that if a parent passes along a disease-causing mutation on the X-chromosome, the partner chromosome can make up for it. Think of it like having a back-up copy of your genes. This is why sex-linked diseases tend to appear in boys more than girls. The sex chromosomes are XX and XY. Boys do not have a back-up copy the way girls do if a parent passes harmful mutation to one of its offspring.

Haploid cells are helpful for understanding the effects of harmful mutations, particularly mutations that are recessive. In the past researchers have used yeast, which has naturally occurring haploid cells, or have bred organisms like zebrafish to have haploid cells. They would then “turn off” one gene or “turn on” another and see what happens. However, such organisms are not mammals, limiting how much information we can translate from these models to humans.

Even though haploid cells occur naturally in mammal gametes (i.e., egg and sperm), these cells do not survive very long in the lab. Scientists usually want cells that will divide and replicate many times so that they can do several experiments with the same cell line. Stem cells of any sort can divide and replicate many times (sometimes for months or years), and when they make more stem cells, those stem cells can divide and replicate many times. This allows for a seemingly endless supply of cells.[3]

Haploid Embryonic Stem Cells

In 2011, Martin Leeb and Anton Wutz reported in Nature that they isolated embryonic stem cells that only had one set of chromosomes.[4] These stem cells came from an embryo that they made from an unfertilized mouse egg using a type of cloning technique. When they studied the stem cells that were removed from the early embryo clone, they found that most of the stem cells had two pairs of chromosomes, as expected, but some had only one set of chromosomes. These are haploid embryonic stem cells (haploid ESCs).

These haploid embryonic stem cells were able to continue dividing into more cells, making a stem cell line. Researchers can take the haploid cells from this stem cell line and conduct genetic experiments. However, with the mouse haploid ESCs, some of the cells in the cell line became diploid cells during the cell division and replication process. This required frequent purification to ensure that the cell line remained haploid.

In 2016, Sagi et al. isolated haploid human embryonic stem cells from a human oocyte (haploid hESCs) using a cloning technique. Similar to the animal studies, they found that they could purify their stem cells so that their stem cell line only had haploid cells. But, also like the animal studies, every time the haploid hESCs replicated, about 9% become diploid. This is a large number that requires the group to purify their haploid hESCs every month.

One key feature of these haploid hESCs is that they can differentiate into mature haploid cells. This means that the hESCs became a specific cell type, such a neurological cells or cardiovascular cells, but with only half of the chromosomes typically found in these cell types. This means that scientists can potentially make genetic changes to the chromosomes in the embryonic stem cell and then see how those changes affect the cell’s development into particular cell types.

Ethical Issues with Haploid hESCs

From a bioethics standpoint, we need to address two main questions: 1) is the technique ethical, and 2) are the potential uses for these stem cells ethical?

The technique itself poses ethical issues, which Sagi, Egli, and Benvenisty acknowledge in their 2016 Nature Protocol paper.[5] First, this technique does not get around the ethical issues surrounding embryonic stem cell research. Embryonic stem cells are controversial because in retrieving the stem cells the embryo is typically destroyed in the process. In the process of creating the blastocyst, the researchers use one of two cloning techniques. One technique prompts the human oocyte to “think” it has been fertilized, converting it to an embryo with genetics from the mother. The other technique produces a “half-clone” because it removes the nucleus of the oocyte and adds the nucleus of the sperm cell. This means that more than 99% of the DNA in the embryo’s cells come from the father.

Secondly, both processes use an oocyte, or a woman’s egg. Obtaining eggs for experimental purposes is also controversial because of the known and unknown risks that a woman faces when injecting the medication needed to stimulate the release of eggs by the ovaries.

Third, this technology can potentially be used to create human eggs in the lab. The idea of making gametes in the laboratory is called gametogenesis. Even though the technique itself requires a gamete in the first place, one possibility is genetically modifying the haploid embryonic stem cells using newer techniques such as CRISPR-Cas9 and coaxing that cell into becoming a germ cell. This is still speculative for haploid human embryonic stem cells, although gametogenesis has been accomplished using mouse induced pluripotent stem cells.[6]

In sum, this technique produces haploid embryonic stem cells using gametes, which are the body’s naturally-occurring haploid cells. What is unique is that unlike gametes, which do not divide and replicate perpetually, haploid hESCs can make new cells in the lab perpetually, providing a stem cell line with only one set of chromosomes. However, this new type of stem cell has two major concerns: a practical concern that the cells need to be continually purified, and, these stem cells do not get around the ethical and legal concerns surrounding hESCs.

Notes

[1] Ido Sagi et al., “Derivation and Differentiation of Haploid Human Embryonic Cells,” Nature 532, no. 7597 (2016): 107–111. doi:10.1038/nature17408.

[2] Cuiqing Zhong et al., “Generation of Human Haploid Embryonic Stem Cells from Parthenogenetic Embryos Obtained by Microsurgical Removal of Male Pronucleus,” Cell Research 26, no. 8 (2016): 743–746. doi:10.1038/cr.2016.59.

[3] National Institutes of Health, U.S. Department of Health and Human Services, “NIH Stem Cell Information Home Page,” Stem Cell Information (2016), stemcells.nih.gov/info/basics/2.htm, (accessed July 20, 2017).

[4] Martin Leeb and Anton Wutz, “Derivation of Haploid Embryonic Stem Cells from Mouse Embryos,” Nature 479, no. 7371 (2011): 131–134. doi:10.1038/nature10448.

[5] Ido Sagi, Dieter Egli, and Nissim Benvenisty, “Identification and Propagation of Haploid Human Pluripotent Stem Cells,” Nature Protocols 11, no. 11 (2016): 2274–2286. doi:10.1038/nprot.2016.145.

[6] Inmaculada Moreno, Jose Manuel Miguez-Forjan, and Carlos Simon, “Artificial Gametes from Stem Cells,” Clinical and Experimental Reproductive Medicine 42, no. 2 (2015): 33–44. doi: 10.5653/cerm.2015.42.2.33.