Human parthenogenetic pluripotent stem cells (PSCs) originating from haploid unfertilized oocytes have only the maternal genome. As such they are imperative in studying imprinting disorders and haploidy. Parental imprinting is a form of epigenetic regulation by which genes are expressed exclusively according to their parent-of-origin. In humans, aberrations of imprinted genes are linked with several developmental disorders and malignancies. To analyze the role of parental imprinting in human embryogenesis, we generated parthenogenetic human PSCs, having only maternal chromosomes. By comparing the gene expression profile of parthenogenetic and normal PSCs, we have identified multiple novel imprinted genes, and uncovered their potential targets. One the targets led us to the study of the imprinting disorder Prader-Willi syndrome (PWS). Our analysis uncovered a crosstalk between the PWS region and another imprinted locus, suggesting a more complex regulation of imprinting than previously appreciated.
Diploidy is a fundamental genetic feature in mammals. However, haploid cells provide valuable tools for delineating genome function through loss-of-function genetic screening. We have recently generated and analyzed haplopid human PSCs originated from human haploid eggs. Although haploid human PSCs resembled their diploid counterparts by several aspects, they also displayed distinct properties including differential regulation of X-chromosome inactivation and genes involved in oxidative phosphorylation, alongside reduction in absolute gene expression levels and cell size. Most surprisingly, while studies on mouse haploid PSCs showed that haploidy is lost upon differentiation, we found that a haploid human genome is compatible not only with the undifferentiated pluripotent state, but also with differentiated somatic fates representing all three embryonic germ layers both in vitro and in vivo. Finally, we demonstrated the utility of haploid human PSCs for loss-of-function genetic screening by generating and analyzing a haploid gene-trap mutant library. Thus, haploid human PSCs hold a great potential for biomedically-relevant functional genomics by forward genetic screening, and will provide novel means for studying human genetics and development.