Sperm DNA methylation is predominantly stable in mice offspring born after transplantation of long-term cultured spermatogonial stem cells

BACKGROUND

Spermatogonial stem cell transplantation (SSCT) is proposed as a fertility therapy for childhood cancer survivors. SSCT starts with cryopreserving a testicular biopsy prior to gonadotoxic treatments such as cancer treatments. When the childhood cancer survivor reaches adulthood and desires biological children, the biopsy is thawed and SSCs are propagated in vitro and subsequently auto-transplanted back into their testis. However, culturing stress during long-term propagation can result in epigenetic changes in the SSCs, such as DNA methylation alterations, and might be inherited by future generations born after SSCT. Therefore, SSCT requires a detailed preclinical epigenetic assessment of the derived offspring before this novel cell therapy is clinically implemented. With this aim, the DNA methylation status of sperm from SSCT-derived offspring, with in vitro propagated SSCs, was investigated in a multi-generational mouse model using reduced-representation bisulfite sequencing.

RESULTS

Although there were some methylation differences, they represent less than 0.5% of the total CpGs and methylated regions, in all generations. Unsupervised clustering of all samples showed no distinct grouping based on their pattern of methylation differences. After selecting the few single genes that are significantly altered in multiple generations of SSCT offspring compared to control, we validated the results with quantitative Bisulfite Sanger sequencing and RT-qPCRin various organs. Differential methylation was confirmed only for Tal2, being hypomethylated in sperm of SSCT offspring and presenting higher gene expression in ovaries of SSCT F1 offspring compared to control F1.

CONCLUSIONS

We found no major differences in DNA methylation between SSCT-derived offspring and control, both in F1 and F2 sperm. The reassuring outcomes from our study are a prerequisite for promising translation of SSCT to the human situation.

Timing of spermatogonial stem cell transplantation affects the spermatogenic recovery outcome in mice

Spermatogonial stem cell transplantation (SSCT) is a strategy that has demonstrated to be feasible to restore spermatogenesis in animal models when it is performed shortly after the gonadotoxic onset to destroy their endogenous germ cells. However, in the case of boys subjected to fertility preservation, future transplantations will be performed with a delay of many years. In order to study how timing of SSCT affects donor-derived spermatogenic recovery in mice, we compared the percentage of spermatogenic tubule cross-sections within testes of 59 C57BL/6NCrl mice distributed in 6 groups: group 1, untreated mice controls (n = 9); group 2, mice that received a single dose of busulfan 40 mg/kg (n = 10); group 3, mice that received two additional doses of busulfan 10 mg/kg every 5 weeks (n = 10); group 4 (SSCT-A), mice subjected to a standard SSCT performed 5 weeks after a single injection of busulfan 40 mg/kg (n = 10); group 5 (SSCT-B), mice subjected to a delayed SSCT performed 15 weeks after a single injection of busulfan 40 mg/kg (n = 10); and group 6 (SSCT-C), mice subjected to a delayed SSCT with two additional doses of busulfan 10 mg/kg every 5 weeks (n = 10). Spermatogenic recovery in standard SSCT-A and SSCT-C groups ranged between 22.29 and 22.65%, compared with a lower recovery rate of 11.54% showed in the SSCT-B group. However, donor contribution resulted higher in standard SSCT-A, representing a 69.71% of cross-sections, compared with the rest of conditions ranging from 34.69 to 35.42%. Overall, we concluded that a delay in the SSCT from the gonadotoxic onset decreases the efficiency of donor-derived spermatogenic recovery in mice.

Establishment of a surgically induced cryptorchidism canine recipient model for spermatogonial stem cell transplantation

Transplantation of spermatogonial stem cells (SSCs) in experimental animal models has been used to study germ line stem cell biology and to produce transgenic animals. The species-specific recipient model preparation is important for the characterization of SSCs and the production of offspring. Here, we investigated the effects of surgically induced cryptorchidism in dog as a new recipient model for spermatogonial stem cell transplantation. Artificially unilateral or bilateral cryptorchidism was induced in ten mature male dogs by surgically returning the testis and epididymis to the abdominal cavity. The testes and epididymides were collected every week after the induction of artificial cryptorchidism (surgery) for one month. To determine the effect of surgical cryptorchidism, the seminiferous tubule diameter was measured and immunohistochemistry using PGP9.5 and GATA4 antibodies was analyzed. The diameters of the seminiferous tubules of abdominal testes were significantly reduced compared to those of the scrotal testes. Immunohistochemistry results showed that PGP9.5 positive undifferentiated spermatogonia were significantly reduced after surgical cryptorchidism induction, but there were no significant changes in GATA-4 positive sertoli cells. To evaluate the testis function recovery rate, orchiopexy was performed on two dogs after 30 days of bilateral cryptorchidism. In the orchiopexy group, SCP3 positive spermatocytes were detected, and spermatogenesis was recovered 8 weeks after orchiopexy. In this study, we provided optimum experimental conditions and time for surgical preparation of a recipient canine model for SSC transplantation. Additionally, our data will contribute to recipient preparation by using surgically induced cryptorchidism in non-rodent species.