Mobilization of CD34+ Peripheral Blood Stem Cells in Patients With Diamond Blackfan Anemia Syndrome (DBAS)

Diamond Blackfan anemia syndrome (DBAS) is a rare, inherited bone marrow failure syndrome, characterized by severe anemia, birth defects and a predisposition to cancer. The majority of patients with DBAS have a mutation in a gene encoding either a small (S) or large (L) subunit-associated ribosomal protein (RP). DBAS typically presents with severe anemia within the first two months of life. Approximately 80% of patients are initially responsive to corticosteroids with an improvement of their anemia, whereas 20% will not respond and will require red blood cell transfusions. Of the 80% of patients that initially respond to corticosteroid treatment, only about half of them will sustain a therapeutic response at a tolerable dose and the remainder will have to resort to transfusions for life. Overall ~20% of the patients may ultimately be able to discontinue corticosteroid or transfusion therapy and enter a treatment-independent phase. The only curative therapy for the anemia associated with DBAS at this time is allogeneic hematopoietic stem cell transplantation, however, this is limited by the availability of matched donors as well as the potentially life-threatening complications associated with transplantation.

There is a critical need for novel therapeutic options for these patients. Of the 28 known DBAS genes, RPS19 mutations are noted in 25% of patients. Gene therapy represents a potential curative option for the anemia of DBAS. This therapy involves inserting the corrected vector (in this case containing the RPS19 gene) into hematopoietic stem cells (HSCs), leading to correction of the anemia in animal models and, potentially, in humans. The application of gene therapy requires sufficient numbers of HSCs on which the correction can be performed. It has not been determined if patients with DBAS can mobilize enough HSCs into the peripheral blood to allow for the harvesting of sufficient numbers to permit genetic manipulation. It is important to demonstrate the ability to harvest an adequate number of HSCs before gene therapy can be contemplated for this rare population.

Patients with DBAS are known to have a reduced number of cells in the bone marrow, especially as the patients age, which raises the concern that HSCs may not be able to be mobilized into the peripheral blood. The goal of this project is to determine the feasibility of peripheral blood HSC collection in patients with DBAS to obtain the numbers necessary for effective gene therapy. Participants in this study will undergo bone marrow aspiration and biopsy to assess the cellularity and CD34+ cell count of the bone marrow, then begin a standard stem cell mobilization protocol consisting of filgrastim (granulocyte-colony stimulating factor, G-CSF; a white blood cell stimulating factor) and plerixafor. Participants will then undergo bloodwork by venipuncture to quantify the stem cell count in the peripheral blood to determine if enough stem cells can be collected. There will NOT be an actual stem cell collection. The study will also assess factors that may be predictive of successful peripheral blood HSC mobilization, including peripheral blood stem cell count, initial bone marrow cell number, and initial bone marrow CD34 count. The target population for this study will include individuals with a known DBAS mutation who are red blood cell transfusion dependent. The study aims to recruit a total of 10 participants between the ages of 3 and 30 years. At least 4 participants will be recruited with an RPS19 mutation and at least 6 participants will be under 18 years of age.

Inclusion criteria

• Diamond Blackfan anemia syndrome as defined by the known criteria with a known gene mutation
• Male or female patients of all ethnic background, greater than or equal to 3 years of age and weighing at least 10 kg, and less than or equal to 30 years of age
• Enrolled in Diamond Blackfan Anemia Registry of North America (DBAR)
• Chronically red blood cell transfusion dependent for at least 6 months
• Performance scale (Lansky Play-performance Scale for Pediatric Functional Status for age <16 years; Karnofsky Performance Scale for age ≥16 years) ≥ 70
• Must sign informed consent

Exclusion criteria

• Receiving prednisone therapy for treatment of DBAS (this does not include patients receiving physiologic steroid replacement for adrenal insufficiency)
• Known history of myelodysplasia or presence of a hematopoietic clone
• Current malignancy or previous treatment for malignancy
• Pregnancy or breast-feeding mother
• Known history of severe iron overload as defined by a liver iron concentration (LIC) > 15 mg Fe/ g dry liver weight
• Significant cytopenias, defined as:

• Platelet count <100,000/mcL
• Absolute neutrophil count <750/mCL
• Any GCSF use in the 3 months prior to enrollment
• Liver dysfunction: aspartate aminotransferase (AST), alanine aminotransferase (ALT), or direct bilirubin values >3 x the upper limit of normal (ULN)
• Kidney dysfunction: baseline estimated glomerular filtration rate (GFR) <70 mL/min/1.73 m2