T-cell replete haploidentical transplantation with reduced post-transplant cyclophosphamide in six children with infantile osteopetrosis

Hematopoietic stem cell transplantation, a curative approach in infantile osteopetrosis

Most patients with osteopetrosis (OPT) can be causally and curatively treated with allogeneic hematopoietic stem cell transplantation (HSCT) because osteoclasts are derived from the HSC. However, HSCT is contraindicated in some forms of OPT, namely OPT with neurodegeneration (in all patients with OSTM1 and about half of patients with CLCN7 mutations) and OPT caused by an osteoblast defect (patients with RANKL mutations). HSCT for OPT risks serious side effects, such as transplant failure, venous occlusive disease, pulmonary hypertension, and hypercalcemic crises. Nevertheless, the success rate of HSCT has improved significantly in recent decades. This applies, in particular, to HSCT from non-HLA compatible (haploidentical) donors. Therefore, nowadays an HSCT can be discussed for intermediate OPT forms. After a successful HSCT, most patients have very good quality of life, but about two-thirds are visually impaired, and in rarer cases show motor and neurological disabilities. Early diagnosis, further improvements in transplantation procedures, and advances to improve quality-of-life after transplantation are challenges for the future.

Assessing the Efficacy of Alkylating Agent Regimens in the Treatment of Infantile Malignant Osteopetrosis: Cyclophosphamide, Busulfan, or Thiotepa

Infantile malignant osteopetrosis is a debilitating disease that requires total bone marrow irradiation and transplant procedures for patients to survive. The major complication of this procedure is graft vs host disease (GVHD), followed by infections and end organ toxicity. Therefore, current research efforts into treatment mainly aim to reduce GVHD while limiting infections and organ toxicity. Different regimens of alkylating agents have been used to try to reduce GVHD. The most common regimen is cyclophosphamide (Cy) with busulfan (Bu), followed by Cy with Bu and thiotepa (Thio). This meta-analysis aimed to evaluate the efficacy of different treatments by comparing mortality and morbidity causes and rates across groups. The mean one-year survival rate for the Cy, Bu, Thio regimen studies in the human leukocyte antigen (HLA) unmatched group (45.01%) was statistically lower than the one-year survival rate for the studies using just a Cy, Bu regimen (70.8%) in the HLA unmatched studies (p<0.00142). The one-year survival in the studies which had HLA-matched donors was 80.56%, which is statistically higher (p<0.001) than the one-year survival in the HLA-unmatched studies (53.96%), indicating a benefit of finding HLA-matched donors. It seems that price and availability could be a factor in the widespread use of Cy.

PTCy-based Haploidentical vs Matched Unrelated Donor Peripheral Blood HSCT Using Myeloablative Targeted Busulfan-based Conditioning for Pediatric Acute Leukemia

Haploidentical related donor (HRD) is a common alternative donor strategy used when matched sibling or unrelated donors are not available for hematopoietic stem cell transplantation (HSCT). However, there have been no studies comparing HRD HSCT with post-transplant cyclophosphamide (PTCy) and matched unrelated donor (MUD) HSCT with antithymocyte globulin, using similar busulfan-based myeloablative conditioning regimen in pediatric acute leukemia. Here, we compared the outcomes in children and adolescents with high-risk acute leukemia after HRD HSCT with PTCy (n=35) and MUD HSCT (n=45) after targeted busulfan-based myeloablative conditioning using intensive pharmacokinetic monitoring. The median follow-up times of the HRD and MUD groups were 3.7 and 4.6 years, respectively. No engraftment failure was observed in both groups. The cumulative incidence of acute graft-versus-host disease (GVHD) grades II-IV (34.3% versus 48.9%, p=0.142), grades III-IV (2.9% vs. 8.9%, p=0.272), moderate to severe chronic GVHD (11.4% vs. 18.3%, p=0.417), relapse (25.6% vs. 28.0%, p=0.832), and non-relapse mortality (0% vs. 2.2%, p=0.420) were not significantly different between the two groups. The 3-year severe chronic GVHD-free/relapse-free (GRFS), leukemia-free (LFS) and overall survival (OS) rates in the HRD and MUD groups were 62.9% (95% confidence intervals [CI], 45.8%, 80.0%) versus 49.8% (95% CI, 34.9%, 64.7%; p=0.318), 74.4% (95% CI, 58.7%, 90.1%) versus 67.5% (95% CI, 53.4%, 81.6%; p=0.585), and 88.6% (95% CI, 78.0%, 99.2%) versus 83.7% (95% CI, 72.5%, 94.9%; p=0.968), respectively. In a subgroup analysis of acute lymphoblastic leukemia patients (HRD, n=17; MUD, n=26), the 3-year GRFS, LFS, and OS rates of the HRD and MUD groups were 49.4% (95% CI, 24.3%, 74.5%) versus 39.5% (95% CI, 19.7%, 59.3%; p=0.601), 61.8% (95% CI, 37.5%, 86.1%) versus 63.6% (95% CI, 44.4%, 82.8%; p=0.872), and 82.4% (95% CI, 64.4%, 100%) versus 84.2% (95% CI, 70.1%, 98.3%; p=0.445), respectively. In acute myeloid leukemia patients (HRD, n=16; MUD, n=16), the 3-year GRFS, LFS, and OS rates of the HRD and MUD groups were 80.8% (95% CI, 61.2%, 100%) versus 61.9% (95% CI, 37.8%, 86.0%; p=0.326), 87.1% (95% CI, 70.2%, 100%) versus 73.9% (95% CI, 51.8%, 96.0%; p=0.478), and 93.8% (95% CI, 81.8%, 100%) versus 85.6% (95% CI, 67.0%, 100%; p=0.628), respectively. Although the difference was not statistically significant and the number of patients was small, the promising outcomes of HRD HSCT in AML patients were encouraging. Our results demonstrated that HRD HSCT with PTCy using a targeted busulfan-based myeloablative conditioning shows outcomes similar to those of MUD HSCT with antithymocyte globulin. HRD HSCT with PTCy could be a feasible option for pediatric high-risk acute leukemia patients who lack an HLA-matched related or unrelated donor.

The V-ATPase a3 Subunit: Structure, Function and Therapeutic Potential of an Essential Biomolecule in Osteoclastic Bone Resorption

This review focuses on one of the 16 proteins composing the V-ATPase complex responsible for resorbing bone: the a3 subunit. The rationale for focusing on this biomolecule is that mutations in this one protein account for over 50% of osteopetrosis cases, highlighting its critical role in bone physiology. Despite its essential role in bone remodeling and its involvement in bone diseases, little is known about the way in which this subunit is targeted and regulated within osteoclasts. To this end, this review is broadened to include the three other mammalian paralogues (a1, a2 and a4) and the two yeast orthologs (Vph1p and Stv1p). By examining the literature on all of the paralogues/orthologs of the V-ATPase a subunit, we hope to provide insight into the molecular mechanisms and future research directions specific to a3. This review starts with an overview on bone, highlighting the role of V-ATPases in osteoclastic bone resorption. We then cover V-ATPases in other location/functions, highlighting the roles which the four mammalian a subunit paralogues might play in differential targeting and/or regulation. We review the ways in which the energy of ATP hydrolysis is converted into proton translocation, and go in depth into the diverse role of the a subunit, not only in proton translocation but also in lipid binding, cell signaling and human diseases. Finally, the therapeutic implication of targeting a3 specifically for bone diseases and cancer is discussed, with concluding remarks on future directions.