Does Mixed Chimerism after Allogeneic Hematopoietic Cell Transplantation in Patients with Fanconi Anemia Impact on Outcome?

Fanconi anemia (FA) cells are characterized by genomic instability which places FA patients (pts) at risk for malignancies; leukemia and oropharyngeal/urogenital cancers. The risk of development of leukemia is theoretically eliminated after hematopoietic cell transplantation (HCT).

Simultaneous presence of both host- and donor-derived cells in the recipient often referred to as mixed chimerism (MC) is observed in a large proportion of patients after HCT with non-malignant disorders. In FA patients however, MC might have a unique implication and the persistent existence of FA cells represents a management dilemma as the lingering FA cells may theoretically evolve into a malignant clone. We have studied a large population of FA patients who underwent allogeneic HCT at our institution and report here the outcome of those with MC.

From January 1995 until December 2017, 163 FA pts underwent allogeneic HCT at our center; chimerism data at last contact were available on 100 pts who are the subject of this analysis. Females (51) had a mean age at HCT of 8.7 (SD:3.0, Min-Max: 2.0-14.1) years whereas in males (49) it was 8.1 (SD:3.4; Min-Max: 1.4-17.4) years (P-Value: 0.415). Donor was HLA matched family member in 78 pts, Haplo-identical family member in 17 (92 of them were bone marrow and 3 peripheral stem cells); 3 had unrelated cord blood. Total body irradiation was used in 24 pts.

Median time to ANC and platelets recovery was 14 and 22 days respectively. Cumulative incidence of overall aGVHD Grade III or above was 4%. Median follow up time was 67.2±14.6 months (95% CI: 38.7-95.7) from HCT date (Min: 7.3, Max:188.1). Chimerism analysis at last follow up showed full chimerism (100%; Myeloid/lymphoid) in 46 pts; 54 had MC defined as the presence of any residual recipient cells. No statistically significant association was noted between full and MC pts in the incidence of aGvHD (P-Value: 0.331). The 10-year cumulative probability of Overall Survival (OS) was 0.904±0.042. No significant difference was observed in OS between full and MC pts (0.954±0.032 vs. 0.883±0.059, P-value: 0.943) with 4 deaths in each group. New malignancy occurred in 4 pts; 2 in each group. In full Chimerism pts: Acute Mixed Lineage Leukemia and carcinoma and in MC pts: acute myeloid leukemia and acute lymphocytic leukemia, (P-Value=1.00). Graft failure occurred in 2 pts in the mixed chimerism group vs none in the full chimerism group (P-Value=1.0).

We conclude that mixed chimerism in FA pts does not appear to have an adverse effect on outcome in our follow-up period; longer follow-up time however is needed to confirm the validity of this statement.