Nov. 18, 2016
Advances in the survival of patients with cancer over the last three decades have been made through the work of cooperative protocol-driven clinical research, particularly in young patient categories. Now that the overall event-free survival rate for child, adolescent and young adult patients with cancer surpasses 75 percent, attention is being focused on quality of life and long-term consequences of therapy. In particular, patients receiving chemotherapy and radiotherapy for cancer or other conditions are often at risk of infertility, placing fertility preservation at the forefront of these concerns.
Progress to minimize the unwanted side effects of current treatment regimens without decreasing their effectiveness has allowed many cancer survivors to have children after spontaneous recovery of fertility. However, some oncological diseases require rigorous treatment regimens that will almost always lead to permanent infertility of the patient.
Diagnosis and treatment decisions in the event of a new diagnosis of cancer in young children have traditionally not focused heavily on fertility preservation options, partly due to the lack of any options being available for prepubertal children, until recently.
For children in puberty, the standard forms of fertility preservation (sperm cryopreservation for boys and egg and oocyte cryopreservation for girls) are offered in all circumstances wherein fertility may be adversely impacted due to gonadotoxic chemotherapy or radiation.
Prepubertal children have not had any options available to them to preserve fertility prior to receiving gonadotoxic therapy until recently. Due to recent advances in reproductive endocrinology, both prepubertal boys and girls have the ability to cryopreserve testicular or ovarian tissue, respectively, before beginning cancer therapy known to impact fertility adversely.
Ovarian tissue cryopreservation can also impact fertility preservation options for those postpubertal girls who are unable to undergo egg cryopreservation due to an inability to delay treatment for up to two weeks or for whom there is a contraindication to the hormonal stimulation required to harvest oocytes. "Although still considered experimental, for prepubertal girls and the subset of postpubertal girls unable to cryopreserve eggs, the prospect of ovarian tissue cryopreservation holds promise as a technique to salvage future reproductive potential," says Asma Javed, M.B.B.S., with Pediatric and Adolescent Medicine at Mayo Clinic's campus in Rochester, Minnesota.
Mayo researchers are offering ovarian and testicular tissue cryopreservation under a research protocol to prepubertal girls and boys, respectively. The research protocols are also open to pubertal children if they are unable to cryopreserve sperm or oocytes prior to cancer therapy initiation.
Science of ovarian tissue cryopreservation: Current research
The typical approach for ovarian tissue cryopreservation utilizes removal of ovarian cortical tissue through laparoscopy. A section of ovarian cortex, which houses the primordial follicles, is surgically excised. Depending on the volume of the ovary, less than half of the ovary is removed. The minimum cortical tissue equivalent to at least six strips measuring 2 cm by 0.5 cm with a thickness of 1 mm each is needed under Mayo's research protocol to optimize volume of storage for both patient and research use.
The tissue removed can be processed using slow cooling or rapid cooling (vitrification). "Evidence is accumulating regarding the superiority of the latter technique due to decreased formation of ice crystals, which may be destructive to cells when thawing takes place as well as superior preservation of the ovarian stroma and vasculature," says Jani R. Jensen, M.D., director of the in vitro fertilization program at Mayo Clinic in Rochester, Minnesota. Vitrification involves rapid rate cooling and rewarming of tissue by placing tissue immediately in liquid nitrogen after the use of cryoprotectant.
After completion of cancer therapy, once the patient expresses a desire to pursue fertility, the ovarian tissue can be thawed and implanted. At the present time, two surgical approaches exist for the purpose of autotransplantation. The tissue can be placed in an orthotopic location, such as the pelvic cavity, peritoneal window or ovarian fossa, or heterotopically in sites such as the forearm.
"Due to the potential risk of reintroduction of malignancy to the patient in cases of ovarian tissue autotransplantation, current research is concentrating on identifying techniques to allow live births from the tissue without implantation, such as with in vitro maturation of oocytes," says Zaraq Khan, M.B.B.S., with Obstetrics and Gynecology at Mayo Clinic in Rochester, Minnesota.
Two possible concepts are informing this line of research. The first is to isolate primordial follicles and proceed with in vitro follicle growth to obtain mature oocytes capable of fertilization utilizing growth factors such as vascular endothelial growth factor (VEGF). An alternative method employs the placement of primordial follicles on alginate matrices with isolated ovarian cells to ultimately lead to oocyte development.
Outcomes of ovarian tissue cryopreservation
Autologous ovarian tissue transplant has been successful in restoring endogenous hormone replacement and achieving pregnancy as well as live births. Over 70 live births have been reported from ovarian tissue transplantation, and endogenous hormone production has been restored in almost all cases of ovarian transplantation. The latter is significant for females after gonadotoxic cancer therapy since it obviates the need for lifelong hormone therapy.
"A particularly exciting recent development is that of a report of a live birth from ovarian tissue harvested prepubertally, in an almost-14-year-old premenarchal female with sickle cell anemia undergoing stem cell transplant," says Dr. Javed.
All of the live births reported thus far from ovarian tissue transplantation have been after orthotopic locations. Although successful in primates and for resumption of hormone function in humans, there are to date, no live births reported from heterotopic transplantation of ovarian tissue.
Despite overall encouraging results, the procedure is considered experimental and offered only under a research protocol, therefore is not covered by insurance.
Science of testicular tissue cryopreservation: Current research
Testicular tissue cryopreservation involves testicular biopsy and can be offered to boys of all ages starting from birth if they are facing gonadotoxic therapy and unable to bank sperm due to pubertal status or other impediment. In contrast to ovarian tissue cryopreservation, testicular cryopreservation has been attempted in fewer numbers of patients and no spermatogenic recovery or pregnancy has resulted thus far from this procedure.
Assessment of the gonadotoxic potential of a planned treatment regimen is the first step in determining eligibility of a patient for the testicular tissue protocol. In boys, treatment with some chemotherapeutic agents and regimens induce prolonged azoospermia, such as alkylating chemotherapeutic agents and cisplatin.
Although semen cryopreservation is available as a fertility-preserving option for postpubertal boys and adult men, there is a lack of fertility-preserving options available to prepubescent boys who are not yet producing sperm. Therefore, experimental techniques to preserve testicular tissue before gonadotoxic therapy are offered under research protocols at leading children's hospitals and clinics in the United States, including Mayo Clinic Children's Center.
Spermatogonial stem cells are the adult tissue stem cells of the testes that give rise to sperm through the process of spermatogenesis.
"This technique has shown promise in several animal models, including rhesus monkeys, where after freezing testicular tissue, scientists were able to thaw the samples and transplant the cells back into the testes where they produced sperm," states Candace F. Granberg, M.D., with Pediatric Urology at Mayo Clinic in Rochester, Minnesota.
The science behind testicular tissue cryopreservation holds promise as a viable future technique, and current research is keenly focused on optimizing techniques for cryopreservation of testicular cells, including spermatogonial stem cells as well as advancing the science on the ability to detect malignant cell contamination in harvested patient testicular tissue. Therefore, the eventual goal of the testicular tissue protocol is to develop strategies to isolate or enrich spermatogonial stem cells in addition to eliminating malignant contamination testicular cells.
The current research protocol at Mayo Clinic is designed to provide a pool of research tissue that will be used to develop and test methods for manipulation and cryopreservation of testicular tissue. Progress in these investigations may open up a range of new fertility preservation techniques to patients for whom currently there are no options. At the same time, as is the case for ovarian tissue cryopreservation, a substantial portion of the patient's tissue will be cryopreserved and reserved for future autologous use.
Future directions
"Although traditionally considered strictly within the realm of cancer therapy, the protocol at Mayo Clinic is unique in its inclusion of diagnoses other than cancer to extend fertility preservation options," says Dr. Javed. Genetic causes of premature loss of gonadal function, such as Turner's or Klinefelter's syndromes; nonmalignant hematologic, neurological or renal diseases requiring use of gonadotoxic therapies; as well as, potentially in the future, extension to transgender youth prior to undergoing reassignment surgeries can all be offered tissue cryopreservation. Further benefit and guaranteed preservation of fertility relies heavily on the outcome of ongoing research.