To the best of our knowledge, this is the first study in a cohort of leukemia patients where MFC is used to detect MRD in ovarian cortical tissue. This technology has previously been validated by our team, demonstrating its effectiveness for the detection of MRD in ovarian cortical tissue [42, 43, 49].
Ovarian cortical tissue cryopreservation is currently the only available method to preserve fertility for prepubertal children or women who cannot delay chemo- and/or radiotherapy [8, 24]. Indeed, ovarian cortex transplantation is the only established technique for re-use of ovarian cortex, with a high success rate [15, 17, 18, 20, 50–52]. Our team has set up a study in France called DATOR (Development of Ovarian Tissue Autograft in Order to Restore Ovarian Function) (NCT02846064) with the aim of assessing the safety and efficacy of ovarian cortex transplantation [19]. However, in leukemic patients, this technique incurs a risk, with the possibility of cancer reseeding. It is therefore important to develop techniques for MRD detection in ovarian cortical tissue.
Among 15 leukemia patients included in this study, ovarian cortical tissue was positive for MRD by MFC in 5 of them (30%). Results obtained by MFC were confirmed where possible by RT-qPCR (in 3 patients). Molecular markers were available for other patients, but analysis was not performed due to lack of ovarian cortex. However, molecular analysis could be performed just before autotransplantation of ovarian cortex to confirm MFC results, as was done in patient 2, for example. The findings presented in this study are congruent with previous reports from other teams [34, 35, 38, 40].
Xenograft studies failed to amplify leukemic cells identified by MFC. These results corroborate those reported by a Danish team [35], where no MRD amplification was observed, but contrast with those from a Belgian team [34], who observed clinical disease. Another study, published by Diaz-Garcia in 2019, also showed MRD amplification by a xenograft model [53]. However, this model for MRD detection is time consuming, and depends on multiple factors, such as the mouse model [54] (SCID, Nude, NSG for example), or the graft site [55], which can explain the difficulty of reproducing results. The major conclusion of all these studies is that they confirm the potential for leukemic cell contamination in ovarian cortical tissue [34–36, 53], but that positive MFC results do not necessarily translate into disease recurrence.
Currently, we do not know the level of MRD that can induce relapse after ovarian cortical tissue transplantation. Results differ between studies. Injection of 200 leukemic cells into nude mice was shown to induce leukemia in one study [56], whereas 1000 cells were unable to induce relapse in another [57]. In one recent study, malignant cells were found in mice injected with 1000 cells, and clinical disease was only caused by injection of 5x106 leukemic cells [53], in line with the findings of a previous study [57]. Host species and grafting site, as well as the heterogeneity of leukemic contamination in ovarian cortical tissue, may explain these discrepancies between studies [53, 58]. It is also important to bear in mind that a patient who receives ovarian cortical tissue transplantation has an immune system to fight leukemic cells, contrary to immunodeficient mice. Further research is needed to identify the threshold of leukemic cells that could induce cancer relapse in patients.
Despite the risk related to ovarian cortical tissue transplantation in case of leukemia, six live births have been reported in the literature after ovarian cortex transplantation in leukemia patients [18, 41, 59]. In each case, the search for MRD in ovarian cortical tissue was done with reliable techniques like histology, molecular techniques, next generation sequencing or xenograft into immunodeficient mice. However, these tests are time-consuming, expensive and hard to achieve for most laboratories (need for experience and facilities for animal experimentation).
MFC can also potentially be adapted to all leukemia patients with LAIP, contrary to PCR, which is potentially applicable in 28–89% of patients [41]. MFC has been used with success by other teams on ovarian cortical tissue [60, 61]. Many hospitals have a hematology laboratory, where the leukemia diagnosis is made. It is also easy to obtain the diagnostic information to constitute LAIP for MRD investigation in ovarian cortical tissue. When using MFC, the ideal method is to use leukemic blasts frozen at diagnosis to test the antibody panel on these cells, and reference ovarian tissue with no leukemic cells. The MFC technique can be implemented rapidly, contrary to xenografting into immunodeficient mice, for example. Nevertheless, it is important to perform reliable techniques to assess and confirm MRD results obtained by other methods. MRD evaluation in the residual medulla, when available, is recommended by ESHRE [22]. In our study, MRD results for the cortex and medulla were concordant in 3 patients, and in line with a recent study where 20/24 MRD results were concordant [40].
Whether chemotherapy is received before OCT or not does not seem to have any impact on ovarian cortical tissue MRD results. Indeed, in our study, positive MRD was observed in patients who received treatment, and in one patient without chemotherapy. The treatment received by the patients before OTC was at low risk in terms of gonadotoxicity, with a Cyclophosphamide Equivalent Dose < 4 000 mg/m² (0–2 500 mg/m²) and a Doxorubicin Isotoxic Equivalent > 250 mg/m² for 4/15 patients (cardiac toxicity) [62]. Some studies have reported that exposure to chemotherapy before OTC does not alter the future result of ovarian cortical tissue transplantation [50, 63, 64]. However, prior chemotherapy may decrease MRD in ovarian cortical tissue in leukemia patients [36]. As previously suggested by other groups, we recommend performing OTC after the first round of chemotherapy, or before hematopoietic stem cell transplantation, to reduce the risk of leukemic cells in ovarian cortical tissue.
In conclusion, cryopreserved ovarian cortex was positive for MRD by MFC in 5 out of 15 leukemia patients (4 AML and 1 T-ALL), even though RT-qPCR and/or xenograft MRD was negative for these patients, when performed. This study demonstrates that MFC is a reliable and easy-to-use technique to detect MRD in ovarian cortical tissue. This adds to the wide variety of techniques available to test ovarian MRD prior to transplantation in leukemia patients. This represents an important step to controlling oncological risk of ovarian cortex transplantation in leukemia patients.