Association of transplant outcome and clonal hematopoiesis with baseline bone marrow cellularity and fibrosis in patients with sickle cell disease undergoing non-myeloablative allogeneic hematopoietic cell transplantation

Background

Sickle cell disease (SCD) is an inherited hemoglobinopathy in which erythrocytes sickle during periods of hypoxia and stress, resulting in damage to body organs. Repeated insults to the bone marrow promote a hyperinflammatory state characterized by altered bone marrow cellularity and fibrosis. Individuals with SCD also have a higher prevalence of clonal hematopoiesis (CH), yet this relationship is not fully understood. Hematopoietic cell transplantation (HCT) offers curative therapy for those with SCD, yet the impact of baseline cellularity and fibrosis on HCT outcomes and CH remains unknown. We hypothesize that graft failure and declining donor chimerism are more frequent in patients with higher baseline bone marrow cellularity and fibrosis. Further, we hypothesize that patients with increased cellularity, a potential indicator of increased erythropoietic stress, will have a higher prevalence of CH.

Methods

We conducted a retrospective study to evaluate pre-HCT bone marrow samples from patients with SCD who received non-myeloablative haploidentical HCT at the National Institutes of Health (NIH) between 2010 and 2023. All patients received alemtuzumab and TBI with post- HCT cyclophosphamide; some received additional pre-conditioning with pentostatin and oral cyclophosphamide. All core and aspirate slides were reviewed by an expert hematopathologist and assigned a numerical cellularity value (0-100%) and a biopsy score (0-4). Cellularity values were adjusted for age, and primary variables were compared against metrics of HCT outcome, including graft rejection, donor chimerism, and the presence and development of CH (variant allele frequency >0.05%). Patients were categorized as having sustained engraftment or graft failure; the latter included those with low donor myeloid chimerism (< 20%) at last follow-up (Figure 1).

Results

A total of 39 patients had pre-HCT marrows; 34 had cellularity data, and 26 had fibrosis data. 22 (56.4%) patients demonstrated sustained engraftment. Median age, African American race, SCD genotype, and baseline laboratory values, including hemoglobin, reticulocyte count, absolute neutrophil count, and %HbS, did not differ significantly between these two groups (Table 1). Neither baseline cellularity nor fibrosis score impacted engraftment status (p = 0.33 and p = 0.81, respectively). Samples for CH evaluation were collected from 33 patients, both before and after transplant, to assess for variant mutations. We observed a significant difference in CH development between our two clinical trials following HCT (p = 0.01), but not before HCT (p = 0.41). 9 (33%) patients had a detectable clone before HCT, while 19 (59%) had a detectable clone following HCT. While these differences were not associated with baseline cellularity and/or fibrosis, baseline cellularity was associated with the presence of DNA-damage response (DDR) mutations (TP53, PPM1D, CHEK2, ATM) before HCT (p = 0.04) (Figure 2).

Conclusions

Our initial data suggest that neither baseline cellularity nor fibrosis significantly impacted HCT outcomes. Further, we observe that baseline cellularity is associated with the presence of DDR mutations before HCT. We will expand to our non-myeloablative matched-sibling cohort, increasing our overall study sample size and more definitively assessing the impact of baseline cellularity and fibrosis values on HCT outcomes and CH. Further studies are indicated to evaluate whether increased erythropoietic stress is associated with an increased prevalence of baseline DDR-CH.

Methods

Baseline lab values

Cellularity and pre-HCT DDR variants