Osteopetrosis

Definition A heterogeneous group of heritable conditions in which there is a defect in bone resorption by osteoclasts. The disease is associated with an increased skeletal mass due to abnormally dense bone. Generalized osteosclerosis is apparent radiographically, often with a “bone within a bone” appearance.

Definition Transverse radiolucent bands may be observed, and it may be difficult to discern the marrow cavity. The decrease in osteoclast activity also affects the shape and structure of bone by altering its capacity to remodel during growth.

Definition In severely affected patients, the medullary cavity is filled with endochondral new bone, with little space remaining for hematopoietic cells. This abnormality contributes to the brittleness of bone in osteopetrosis. (The abnormal skeletal radiographs and microscopical appearance of bone can be reversed by hematopoietic stem- cell transplantation.)

Normalization of bone after doing HSCT The characteristic changes of increased bone density with no recognizable bone marrow cavity are apparent in a radiograph of the forearm of an Infant with autosomal recessive osteopetrosis . Two years after hematopoietic cell transplantation, normalization of the bone is observed.

Normalization of bone after doing HSCT

Pathogenesis The attachment of the osteoclast to bone is facilitated by podosomes containing filamentous actin and the αvβ3 integrin. To achieve acidification of the resorption lacunae and begin the process of bone demineralization, carbonic anhydrase II (CAII) generates a proton and bicarbonate from carbon dioxide and water. The proton is actively transported across the membrane of the ruffled border through the action of the osteoclast- specific vacuolar-type H+ –ATPase “proton pump.” A chloride channel coupled to the proton pump facilitates balancing the charge of ions across the membrane. Finally, excess bicarbonate is removed through the basolateral membrane by passive exchange with chloride. The organic matrix of the bone is removed through enzymatic activity, with cathepsin K playing a large role in this process.

Classification could be categorized only on the basis of the clinical aspects: infantile, or “malignant,” osteopetrosis, inherited in an autosomal recessive inheritance pattern; “intermediate” autosomal recessive osteopetrosis; and autosomal dominant osteopetrosis

Genotyping & Clinical Manifestations

Management and Therapy: BMT 1975: Transplantation of splenocytes from unaffected littermates shown to cure bony manifestations of disease. Reverse also true, osteopetrosis developed in unaffected animals after transplantation of splenocytes from osteopetrotic animals.

HLA-haploidentical blood progenitor cell transplantation in osteopetrosis University of Ulm, Ulm, Germany. 1996-99

Clinical findings and patient histories Sex Genotype Positive FH Clinical Abnormalities Age at diagnosis (mo) Age at HSCT F V-ATPase Yes Hepatomegaly, anemia 1 5.5 No Visual impairment, hypocalcemic Convulsions, hepatosplenomegaly, anemia, thrombocytopenia 2 6 Visual impairment, hypocalcemic Convulsions, hepatosplenomegaly, Anemia 3.5 Visual impairment, hepatosplenomegaly, hypocalcemia, anemia 0.5 ND Hypocalcemic convulsions At birth 1.5 M Hypocalcemic convulsions, Splenomegaly 5 Visual impairment, anemia, thrombocytopenia

Transplant Characteristics Donor/HLA mismatch 1. Mother / 1 Ag (A) 2. Father / 3 Ag (A, B, DR-B1) 3. Father / 3 Ag (A, B, DR-B1) 4. Father /1 Ag (DR-B1) 5. Father / 3 Ag (A, B, DR-B1) 6. Father / 3 Ag (A, B, DR-B1) 7. Aunt /1 Ag (DR-B1)

Transplant Characteristics Conditioning: TCD Myeloablative BU/CY/THIO/ATG  5 Pt BU/THIO/FLU/ATG  1 Pt BU/THIO/FLU/OKT3  1 Pt CD34+ :12 to 56 x 106/kg (median 30 x 106/kg) CD3: 0.8 to 11.1 x 104/kg (median 1.7 x 104/kg) Boost CD34 cells in 6 Pt, early (between day 7 & 11) in 5 cases and late (day 42) in 2 cases, ranging in numbers from 10 x 106/kg to 43 x 106/kg (median 15 x 106/kg) Recipients received rHuG-CSF until stable engraftment was achieved. Besides T-cell depletion, no further GVHD prophylaxis was administered.

Outcome Full donor Chimerism: 6 Rejection: 1 Alive: 6 Death: 1 (d +84) Five of 7 patients survive for 25 to 60 months (median, 47) Major complications: Respiratory insufficiency  3 VOD  3 CMV disease  1

Long-term outcome of haematopoietic stem cell transplantation in autosomal recessive osteopetrosis:an EBMT report GJA Driessen, et al 1980-2001

Engraftment after first HSCT and DFS at last follow-up Group Donora N Engraftment total Engraftment after TCD DFS A HLA= 40 37/39 (95%) 0/1 29/40 (73%) B MUD 20 15/19 (79%) 3/5 (60%) 8/20 (40%) C HLA 0-1# 21 11/21 (52%) 4/11 (36%) 9/21 (43%) D Haplo # 41 22/36(61%) 20/34(59%) 10/41(24%) Total 122 85/111(77%) 27/51(53%) 56/122(46%)

DFS after first or subsequent HSCT in relation to HLA compatibility with the HSC donor

Main causes of death in patients after HSCT for AR-OP Group Compatibility with the donor HLA= MUD HLA 0-1 Haplo Total A (n=40) B (n=20) C (n=21) D (n=41) (n=122) Cause of death Septicemia 1 2 2 9 14 Pneumonia/pneumonitis - 3 2 8 13 Veno - occlusive disease 1 3 2 2 8 Aplasia/haemorrhage 2 — 1 4 7 Neurological 1 1 1 1 4 GVHD 2 1 — — 3 Pulmonary hypertension — — 1 1 2 Hypercalcaemia 1 — — — 1 Other 1 1 3 2 7 Unknown 2 — — — 2 Total (n=61) 11(28%) 11(55%) 12(57%) 27(66%) 61(50%)

Eurocord Study Unrelated cord blood is an alternative option for patients lacking a HLA matched related or unrelated donor. However no data has been published describing outcomes of UCBT for children with OP.

Demographic Data 41 patients who received UCBT for AR-OP from July 1992 to October 2009 are reported to Eurocord registry. Twelve patients were transplanted outside EBMT centers.

Demographic Data Median age at UCBT was 7.83 months (0.80-90). Female to male ratio was 2/3. Median interval between diagnosis and doing transplant was 4.2 months (1.2- 56.1). Donor-recipient pairs were matched at HLA-A and -B (antigen level) and DRB1 (allelic level) in 14 or HLA mismatched at 1 (n= 23) or 2 (n= 10) loci.

Transplant Data Conditioning regimen (was available for 35 patients); myeloablative (mostly busulfan-based) in 29 children reduced intensity in 6 patients. GvHD prophylaxis consisted mainly of cyclosporine combined prednisolone (n= 20), methotrexate (n=6), mycophenolate mofetil (n=3). Anti thymocyte globulin (ATG) or alemtuzumab was given to 37/40 patients Median TNC  10.5x107 /kg (3.3-48.5) CD34+  3.35x105 /kg (1.23-43.8)

Results Median follow-up for survivors was 44 (range 4-144) months. Neutrophil recovery with donor chimerism was documented in 25/41 patients; 19/25 evaluable patients presented full donor engraftment, while 6 children presented mixed donor chimerism. Median time to neutrophil recovery was 20 (range 10-60) days. Eighteen patients experienced graft failure 3/18 are alive at last follow up. Information on treatment post-graft failure was available 7/18 children: 6 patients underwent a second HSCT and 3 of them survived.

Results Stem cell dose was associated with a trend for a better probability of donor engraftment: the cumulative incidence of donor engraftment was 46% in patients who received a CD34+ cell dose<2 x 105/kg, versus 71% in children receiving a CD34+ cell dose ≥2x105/kg (p = 0.09). Eleven patients developed grade II-IV aGvHD: n=6 grade II, n=4 grade III, n=1 grade IV) and 5 patients cGvHD: n=3 limited, n=2 extensive)

Results Overall survival (OS) at 3 years was 45±8 %. Twenty-two patients died after UCBT due to: infections (n=8), acute respiratory distress syndrome (n=2), veno-occlusive disease (VOD) (n=2) hemorrhage (n=2), GVHD (n=2) or other causes (n=6).

Results Stem cell dose and HLA disparity were the only predictors of superior outcome in univariate analysis. The 3-year probability of OS was: 50% in patients who received grafts with a CD34+ cell dose >2x105/kg versus 0% in children receiving grafts with a CD34+ cell dose < 2x105/kg (p=0.001). According to HLA disparities, 3-year probability OS was 54% versus 58% versus 0% in patients receiving a 6/6, 5/6 and 4/6 HLA- mismatched graft, respectively (p=0.01). Interestingly, 4/4 children receiving a treosulfan-based myeloablative regimen achieved donor engraftment and 3 children are alive at last follow up.

Conclusion These data suggest that transplantation of unrelated UCB is a valid alternative for children with OP without a matched sibling or a suitable matched unrelated adult donor. The use of CB units mismatched at >one HLA locus should be avoided. The incidence of primary graft failure was high and therefore the optimization of the conditioning regimen and/or the use of CB units containing a high TNC and CD34+ cell dose must be considered.