The common sex chromosome dosage in mammals is XX for females and XY for males. This implies that under normal circumstances each one of the X chromosomes in XX females comes from one of the parents, whereas in XY males the only X chromosome is always maternal, because the Y chromosome always comes from the father. Even though both X chromosomes in an XX individual behave as homologs, they may contain different variations of the same genes (alleles). Also, and because they come each from a different parent, they carry epigenetic information known as genetic imprinting, which allows to discriminate one from the other.
During spermatogenesis (sperm formation in the testis), an essential step called meiosis takes place. It consists of two rounds of cell divisions without DNA replication in between, separating first homologs and then chromatids, each into a single spermatid (the precursor of the spermatozoon). As a result, gametes with half the number of chromosomes are formed (haploid, as opposed to diploid somatic cells), hence the separation of X and Y chromosomes into different cells.
However, errors do occur, and so it happens that sex chromosomes do not always follow their normal behaviour. Instead, there are occasions in which both chromosomes may end up in the same spermatid (Fig. 1). If an XY-bearing spermatozoon fertilizes a normal X-bearing egg, the result is an XXY embryo. This sex chromosome trisomy (SCT) is one of the most common causes of infertility (in addition to other phenotypic traits), and it is known as Klinefelter syndrome. Similarly, other errors leading to nondisjunction of sex chromosomes can result in X0 females (Turner syndrome), XYY males (Jacobs syndrome) or even more complex combinations with multiple sex chromosomes (XXX, XXXX,…). Even though in most cases there are characteristic phenotypes associated to these chromosome abnormalities, mosaicism is also frequent (different cells containing different chromosome dosage), and so affected individuals may too have chromosomally normal children if their contributing gametes happen to be also genetically normal.
In a recent study led by sex chromosome expert Dr. James Turner (Francis Crick Institute in London, UK) (2), SCT-infertility has been overcome in mice. Researchers bred XXY and XYY infertile mice and used them to generate a line of cultured fibroblasts (with the original SCT complement), and then reprogrammed these into induced pluripotent stem cells (iPSCs). iPSCs were then differentiated into primordial germ cell-like cells (PGCLC) and later on into functional sperm (Fig. 2).
Interestingly, both in XXY and XYY iPSCs lines, spontaneous loss of a sex chromosome was observed. In both cases, a high percentage of cells had lost the extra duplicated sex chromosome, leaving the remaining complement as a normal XY. Such loss was also observed in normal XX and XY control lines, yet in a significantly much lower rate.
Following chromosome loss back to “normal”, XXY- and XYY-derived iPSC lines were differentiated into male germ cells (PGCLC), which were subsequently transplanted into testes from infertile mice lacking the germline. The result was the restoration of spermatogenesis for all cell lines used. Furthermore, functional sperm originated from these lines was successfully employed to fertilize eggs through ICSI, giving rise to genetically normal embryos and healthy, normal and fertile offspring.
The authors also showed promising results in preliminary experiments with human cells, demonstrating that returning cells to their original normal chromosome complement is possible. However, they warn about all risks associated with induced pluripotent stem cell manipulation and transplantation, as well as legal and ethical issues on these matters.
This said, it is undeniable that current technologies can help treat specific genetic conditions resulting in infertility, thus giving hope to patients who had found no alternative, so far. The future is becoming present, faster and faster every day.
- Maiburg M, Repping S, Giltay J. The genetic origin of Klinefelter syndrome and its effect on spermatogenesis. Fertil Steril. 2012;98:253-60.
- Hirota T, Ohta H, Powell BE, Mahadevaiah SK, Ojarikre OA, Saitou M, et al. Fertile offspring from sterile sex chromosome trisomic mice. Science. 2017;pii: eaam9046.