Evaluation of clonal hematopoiesis in pediatric ADA-SCID gene therapy participants

Clonal hematopoiesis in children with predisposing conditions

Clonal hematopoiesis in children and young adults differs from that occuring in the older adult population. A variety of stressors drive this phenomenon, sometimes independent of age-related processes. For the purposes of this review, we adopt the term clonal hematopoiesis in predisposed individuals (CHIPI) to differentiate it from classical, age-related clonal hematopoiesis of indeterminate potential (CHIP). Stress-induced CHIPI selection can be extrinsic, such as following immunologic, infectious, pharmacologic, or genotoxic exposures, or intrinsic, involving germline predisposition from inherited bone marrow failure syndromes. In these conditions, clonal advantage relates to adaptations allowing improved cell fitness despite intrinsic defects affecting proliferation and differentiation. In certain contexts, CHIPI can improve competitive fitness by compensating for germline defects; however, the downstream effects of clonal expansion are often unpredictable - they may either counteract the underlying pathology or worsen disease outcomes. A more complete understanding of how CHIPI arises in young people can lead to the definition of preleukemic states and strategies to assess risk, surveillance, and prevention to leukemic transformation. Our review summarizes current research on stress-induced clonal dynamics in individuals with germline predisposition syndromes.

Busulfan and subsequent malignancy: An evidence-based risk assessment

BACKGROUND

The incidence of secondary malignancies associated with busulfan exposure is considered low, but has been poorly characterized. Because this alkylating agent is increasingly utilized as conditioning prior to gene therapy in nonmalignant hematologic and related disorders, more precise characterization of busulfan's potential contribution to subsequent malignant risk is warranted.

PROCEDURE

We conducted a literature-based assessment of busulfan and subsequent late effects, with emphasis on secondary malignancies, identifying publications via PubMed searches, and selecting those reporting at least 3 years of follow-up.

RESULTS

We identified eight pediatric and 13 adult publications describing long-term follow-up in 570 pediatric and 2076 adult hematopoietic cell transplant (HCT) recipients. Secondary malignancies were reported in 0.5% of pediatric HCT recipients, with no cases of myelodysplastic syndrome (MDS) or acute myelocytic leukemia (AML). Fatal secondary malignancies were reported in 0.8% of 1887 evaluable adult HCT recipients, and an overall incidence of secondary malignancies of 4.8% was reported in a subset of 389 evaluable adult patients. We also reviewed long-term results from eight publications evaluating lentiviral- and human promotor-based HSC-targeted gene therapy in 215 patients with nonmalignant conditions, in which busulfan/treosulfan monotherapy or busulfan/fludarabine was the only conditioning. Two malignancies were reported in patients with sickle cell disease (SCD), one of which was potentially busulfan-related. No additional malignancies were reported in 173 patients with follow-up of 5-12 years.

CONCLUSION

The incidence of busulfan-related secondary malignancies is low, and likely to be substantially less than 1% in pediatric transplant recipients, especially those receiving busulfan monotherapy for nonmalignant conditions other than SCD.

Successes and challenges in clinical gene therapy

Despite the ups and downs in the field over three decades, the science of gene therapy has continued to advance and provide enduring treatments for increasing number of diseases. There are active clinical trials approaching a variety of inherited and acquired disorders of different organ systems. Approaches include ex vivo modification of hematologic stem cells (HSC), T lymphocytes and other immune cells, as well as in vivo delivery of genes or gene editing reagents to the relevant target cells by either local or systemic administration. In this article, we highlight success and ongoing challenges in three areas of high activity in gene therapy: inherited blood cell diseases by targeting hematopoietic stem cells, malignant disorders using immune effector cells genetically modified with chimeric antigen receptors, and ophthalmologic, neurologic, and coagulation disorders using in vivo administration of adeno-associated virus (AAV) vectors. In recent years, there have been true cures for many of these diseases, with sustained clinical benefit that exceed those from other medical approaches. Each of these treatments faces ongoing challenges, namely their high one-time costs and the complexity of manufacturing the therapeutic agents, which are biological viruses and cell products, at pharmacologic standards of quality and consistency. New models of reimbursement are needed to make these innovative treatments widely available to patients in need.