Osteopetrosis is a disease caused by the differentiation, development, or dysfunction of osteoclasts. The role of osteoclasts is to absorb bone minerals and matrix components, creating conditions for osteoblasts in the process of bone remodeling. The balance between osteoclasts and osteoblasts is very important for maintaining bone homeostasis. When osteoclasts were dysfunction, a series of clinical manifestations may arise, such as short stature, fractures, osteomyelitis, hypocalcemia, convulsions, neurological lesions and so on. When the increased bone invades the bone marrow space, bone marrow failure will appear and progress gradually12. Defective osteoclast activity can be caused by mutations in genes affecting osteoclast development (such as RANK, RANKL), and function (such as TCIRG1, SNX10, CLCN7, OSTM1). Biallelic mutations of this gene will lead to malignant infantile osteopetrosis (MIOP). This subtype has severe clinical manifestations and poor prognosis. And the onset age is early, mostly in the first year after birth. Autosomal dominant osteopetrosis is caused by monoallelic mutations in the CLCN7 gene, which is called IOP. The onset age of this type is relatively late, and the progress of the disease is slow. Most of them show abnormal bone morphology, and some of them may show bone marrow failure and nerve involvement13. In this study, 22 cases (81.5%) were less than 1 year old at the time of onset, the age of onset of MIOP was 2 (0.04-29) months, and the age of onset of IOP was 33 (21–72) months.
MIOP is prone to death without proper treatment. Due to osteoclasts are myeloid cells and allogeneic hematopoietic stem cell transplantation can provide osteoclasts for patients, so allogeneic HSCT is the only effective treatment for MIOP at present14. The most reports of MIOP/IOP were accepted matched HSCT. Driessen et al. 5 reported 122 cases, who accepted allogeneic HSCT treatment of MIOP and found that the overall successfully engraftment rate was 77%. The higher failure rate of transplantation was mainly related to the dysfunction of bone hematopoietic capacity, spleen retention or destruction of stem cells and HLA-mismatch. This article was also found that the 5-year EFS of HLA-matched was significantly higher than that of HLA-mismatched (73% vs. 24%). Natsheh et al15. reported 38 cases of HSCT for MIOP, including 36 cases of HLA-matched, 1 case of haploidentical and 1 case of T cell-depleted (TCD) in vitro. The 5-year OS was 84%. Orchard et al16. reported long-term survival after transplantation for 193 patients with MIOP. Eighty-nine patients of transplants used grafts from HLA-matched siblings, 104 patients from HLA-mismatched donor (including 25 patients from mismatched sibling donor and 79 patients from mismatched unrelated donor). The conditioning regimen was Bu and Cy. The 5- and 10-year probabilities of survival were 62% and 62% after HLA-matched sibling and 42% and 39% after mismatched donor transplantation. Graft failure was the most common cause of death. Bahr et al17. reported 3 cases of MIOP treated with Haplo-HSCT. With post transplantation cyclophosphamide regimen, only 1 case survived. Stepensky et al13. reported that 6 of 7 children with IOP survived after receiving HSCT, and 1 died of CMV infection and pulmonary hypertension after Haplo-HSCT. In summary, previous research indicated that the first choice of MIOP treatment is an HLA-matched sibling donor for HSCT and Haplo-HSCT was not suitable for the treatment of MIOP and IOP. Graft failure was the main factor affecting the treatment of MIOP and IOP with Haplo-HSCT.
However, in our country- China, it is difficult to seek HLA-matched sibling donor for HSCT. And the time for seeking matched unrelated donor is long and the successful rate is not high. Parents as carriers can become potential donor, which can reduce the waiting time and complications. Torres et al10. reported 2 cases who were treated with vitro TCD (CD34 purified stem cells) and Bu, Flu combined with ATG as conditioning regimen. One case survived and 1 case died of graft failure. Driessen et al5. reported 37 patients with MIOP treated with TCD in vitro, with an overall survival rate of 34%, and the main cause of death was severe infection. Different from previous research, in this study, 27 patients received Haplo-HSCT treatment. Twenty patients were survived and 5-year OS was 73.9%. This retrospective observational study showed that vivo T removal regimen could achieve a more successful engraftment rate than vitro T removal regimen, which approximate or even higher than previous reports of HLA-matched HSCT. In term of conditioning regimen, previous research found that the 5-year OS in the fludarabine group was higher than that without fludarabine(96% vs. 58%). Our research indicated that the myeloablative conditioning regimen was beneficial to the stem cell engraftment. In this study, the 30-day neutrophil engraftment rate was more than 95%. Although the platelet engraftment time was late, the 120-day engraftment rate was more than 60%. Faster stem cell engraftment was beneficial to hematopoietic recovery, and it was more convenient for donor lymphocyte transfusion of poor engraftment. In addition, compared with umbilical cord blood HSCT, Haplo-HSCT had another advantage. Previous report showed 51 patients who received umbilical cord blood HSCT for MIOP, and the 6-year OS was 43%. Most of the patients died of engraftment failure18. Compared with cord blood HSCT, mobilized bone marrow and peripheral blood stem cells could provide more CD34+ cells for patients, which would benefit for engrafted.
According to the previous research, there were many complications related to Haplo-HSCT, especially GVHD, pulmonary hemorrhage and HVOD. Our study adopted the “Beijing Protocol”, which was including the myeloablative conditioning regimen (Flu + Bu + Cy), the vivo TCD regimen (ATG/ALG), and combined with stem cells collected through mobilized bone marrow and peripheral blood19. All these measures worked together to reduce the occurrence of severe GVHD. Acute GVHD degree I-II was observed in 20 patients, degree III in 1 patient and without degree IV. Chronic GVHD was observed in 11 patients, and ten were skin involved, including 7 were local limited. And the condition of GVHD was improved after using hormone and other anti-GVHD drugs, such as tacrolimus, CD25 monoclonal antibody and ruxolitinib. The date indicated that the incidence of GVHD was high but mild, and could be effectively controlled after appropriate treatment. And GVHD would not affect the survival of patients. Kapelushnik et al20. found that patients who died of acute respiratory distress syndrome or pulmonary hemorrhage after transplantation, may due to pulmonary arterial hypertension. In another study, the article pointed out that ATP6i (TCIRG1) mutations could lead to pulmonary arterial hypertension21. In our study, 3 children who died of pulmonary hemorrhage all had TCIRG1 mutation, but pulmonary hemorrhage and pulmonary arterial hypertension may also be related to the aggravation of pulmonary symptoms caused by high-dose chemotherapy and massive fluid infusion. Patients who experienced these severe toxicities were all got pneumonia before transplantation. So, the poor condition before HSCT might related to the severe toxicities. The myeloablative conditioning regimen led to a higher engraftment rate, but the toxicity was also relatively high, especially the incidence of HVOD22. Our study found that early addition of ursodeoxycholic acid, low molecular weight heparin calcium and defibrinylate could significantly reduce the death rate caused by HVOD. Hypercalcemia was also a serious complication after transplantation. It is mainly because the short-term release of large amounts of intraosseous calcium from bone into the blood23. In this study, only 4 cases had hypercalcemia, and the elevated serum calcium levels were not significant. This might be because the donor was also a carrier, the function of osteoclasts was lower than normal. It was difficult for a large amount of calcium to enter the blood in a short period, and be compensated by the body.
This retrospective observational study also found that the bone modeling was significantly improved after Haplo-HSCT, and the curative effect was lasting and stable. However, the other complication, especially blindness and deafness caused by nervous system involvement, could not be significantly improved after transplantation. Previous study showed that only 7% of the eyesight could be restored, and 25% of the patients’ eyesight would still progress. In our study, only one patient’s visual acuity returned to normal, and the visual acuity of 13 patients was improved, which may relate to the older age at the time of diagnosis and the involvement of the nervous system before HSCT. Although, our study didn’t find the relationship between diagnosis age, transplantation age and prognosis, early diagnosis transplantation were of great significance to improve the quality of life and reduced the rate of disability.
Inheritance can be divided into autosomal recessive, autosomal dominant and X-Link. The most common abnormality was TCIRG1 gene mutation, followed by CLCN7 gene mutation24. In this study, 66.7% of the patients had TCIRG1 gene mutation (recessive inheritance), and 22.2% had CLCN7 gene mutation (recessive and dominant inheritance). At present, there is still controversy about which type is suitable for HSCT. As for TCIRG1 mutation, once diagnosed, they should accept HSCT as soon as possible. However, for some patients with CLCN7 and OSTM1 gene mutation, previous articles showed that the central involvement was irreversible and might be further aggravated after HSCT, so transplantation was not recommended25–26. However, there were 6 cases of CLCN7 gene mutation in this group, including 2 cases of compound heterozygosity and 4 cases of heterozygous mutation (without neurodegeneration). In this study, it was found that after transplantation, the clinical symptoms were significantly improved, and there was no aggravation of nervous system involvement. Therefore, we thought that for such patients, HSCT should still be performed if the disease occurs early with typical clinical manifestations13,27. Furthermore, if patient had RANKL gene mutation, HSCT was not recommended because the abnormality of the gene can impede osteoclast maturation28.
The number of patients in this study was small and the follow-up time needed further prolong
to expand the prognosis of MIOP and IOP, especially the growth and intellectual development of children. The rate of patients dying from pulmonary hemorrhage and severe pneumonia was high, which might be related to the intensity of conditioning regimen, so it was necessary to further adjust the dose of conditioning regimen. The occurrence of aGVHD might be related to the high number of infused cells, which also need further adjust.