Successful Engraftment In Children With Severe Aplastic Anemia After Transplantation Of HLA-Identical Hematopoietic Donor Cells With Shorter Telomere Length

Telomere length measurement is recommended in donors of patients with severe aplastic anemia (SAA) to screen for silent inherited telomeropathy. This was based on prior reports of delayed engraftment and graft failure after hematopoietic stem cell transplantation (HSCT) when donors had short telomeres or were silent carriers of dyskeratosis congenita (DC)-causative mutations (Fogarty et al. Lancet 2003 and Awaya et al. Biol Blood Marrow Transplant. 2002). The challenging question is, in the absence of a known causative mutation, how short the donor’s telomere length could be to be labeled as not suitable. Here, we summarize our patients with SAA who have been transplanted with HLA-matched related hematopoietic donor cells that were of variable telomere length. A total of eight patients were transplanted using the following conditioning in 6 patients (cases 2, 4, 5, 6 in table, and two with undetermined telomere length) that included fludarabine (35mg/m²) on day -6 to -2, cyclophosphamide (40mg/kg) on day -6 and -5, and rabbit antithymocyte globulin (thymoglobulin) (2.5mg/kg) on day -5 to -2. GVHD prophylaxis was mycophenolate mofetil and cyclosporine (CSA). Two patients (cases 1 and 3) had cyclophosphamide (50mg/kg) on day -5 to -2 and thymoglobulin (2.5mg/kg) on day -4 to -2 with CSA and methotrexate or steroids as GVHD prophylaxis.

All eight patients (5 females and 3 males) with an average age of 7 (range 0.8-13.3 years) had bone marrow failure as a predominate feature and except for one patient (case 4) who had short stature, mental retardation and a brain MRI with bilateral encephalomalacia there were no abnormal physical findings, specifically no classical features of DC i.e. nail dystrophy, leukoplakia, and skin pigmentation. All donors had normal physical examination and normal CBC. History of consanguinity was reported in all patients except one. Patients had normal chromosomal fragility test, but the telomere length values for six of the patients and their donors were in the lower reference range of healthy Caucasian individuals (Table). Three patients had very short telomeres in practically all lymphocytes subsets (cases 3, 4, and 6) similar to the pattern observed in patients with DC. The telomere length results of the six donors were variable (table). One donor had very short telomeres (case 3) in all lymphocyte subsets. The telomere length result was not available for this donor prior to commencing conditioning, however, CD34 cell dose infused was adequate (9.29 X10⁶/kg recipient weight). Genetic tests are ongoing to identify causative mutations. One patient with undetermined telomere length had a heterozygous mutation in the TINF2 gene that was not present in his donor.

All eight patients engrafted successfully, median time to neutrophil engraftment was 22 (range, 11-29 days) and platelet engraftment 30 (range, 15-62 days). Median infused nucleated cell dose was 2.8 (range, 1.6-6.5 X10⁸/kg) and CD34 cell dose was 5.8 (range, 1.1-11.2 X10⁶/kg). Median donor chimerism was 100% for myeloid cells and 62% for T cells around 100 days post HSCT. None of our patients had acute GVHD and one patient had mild classic chronic GVHD of the skin. All patients are alive with median follow-up duration of 377 days (range, 152-907 days).

In conclusion, fludarabine-based conditioning using reduced dose cyclophosphamide (80mg/kg) seems safe and feasible in our SAA patients with shorter telomeres. Hematopoietic stem cells obtained from related donors with shorter telomeres successfully rescued patients with SAA without signs of graft failure over a median follow-up of 377 days. It is not clear yet, whether these patients and donors have an inherited telomeropathy or whether the reference range of telomere lengths established from Caucasians is also applicable for Arabs or other ethnic groups as this could possibly exclude donors unnecessarily. These studies are ongoing and an update will be presented.

No relevant conflicts of interest to declare.