Tetraspanins set the stage for bone marrow microenvironment-induced chemoprotection in hematologic malignancies.

Despite recent advances in the treatment of hematologic malignancies, relapse still remains a consistent issue. One of the primary contributors to relapse is the bone marrow microenvironment providing a sanctuary to malignant cells. These cells interact with bone marrow components such as osteoblasts and stromal cells, extracellular matrix proteins, and soluble factors. These interactions, mediated by the cell surface proteins like cellular adhesion molecules (CAMs), induce intracellular signaling that leads to the development of bone marrow microenvironment-induced chemoprotection (BMC). Although extensive study has gone into these CAMs, including the development of targeted therapies, very little focus in hematologic malignancies has been put on a family of cell surface proteins that are just as important for mediating bone marrow interactions: the transmembrane 4 superfamily (tetraspanins; TSPANs). TSPANs are known to be important mediators of microenvironmental interactions and metastasis based on numerous studies in solid tumors. Recently, evidence of their possible role in hematologic malignancies, specifically in the regulation of cellular adhesion, bone marrow homing, intracellular signaling, and stem cell dynamics in malignant hematologic cells has come to light. Many of these effects are facilitated by associations with CAMs and other receptors on the cell surface in TSPAN-enriched microdomains. This could suggest that TSPANs play an important role in mediating BMC in hematologic malignancies and could be used as therapeutic targets. In this review, we discuss TSPAN structure and function in hematologic cells, their interactions with different cell surface and signaling proteins, and possible ways to target/inhibit their effects.

Using Clustered Regularly Interspaced Short Palindromic Repeats gene editing to induce permanent expression of fetal hemoglobin in ß-thalassemia and sickle cell disease: A comparative meta-analysis.

ß-hemoglobinopathies like sickle cell disease (SCD) and ß-thalassemia are characterized by differing mutations in the hemoglobin subunit beta gene (HBB). These disorders vary in phenotypic presentation and severity, with more severe manifestations leading to transfusion dependence along with associated complications such as infection and iron overload. ß-hemoglobinopathies symptoms rapidly worsen after birth as the levels of fetal hemoglobin (HbF) begin to decline. To reverse this decline, current treatment plans typically involve the use of pharmacological agents such as hydroxyurea to raise expression levels of HbF. However, these treatments only result in transient effects and must be consistently administered. Gene editing technologies such as CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats- CRISPR associated protein) offer the opportunity to create novel treatments which can raise HbF expression with potential permanent effects. Two gene targets, B-cell lymphoma/leukemia 11A gene (BCL11A) and the promoter regions of gamma globin genes (HBG1/2), have been identified to significantly increase HbF protein expression. In order to differentiate the effectiveness of BCL11A and HBG1/2 editing, a meta-analysis was performed by first identifying 119 studies for inclusion based on the search terms terms "ß-Thalassemia," "beta-thal" "sickle cell disease," "SCD," and "CRISPR." Following application of exclusion and inclusion criteria, we performed analysis on 8 peer-reviewed published studies from 2018 to 2021 were included in the study. Forest plots were generated using R (version 4.1.2). Primary comparative analysis shows HBG1/2 had a significantly (p < 0.01)greater impact on induction of HbF expression compared to BCL11A. This analysis leads us to conclude that HBG1/2 merits further investigation as a possible gene editing target for treatment of SCD and ß-thalassemia.

Understanding the Mechanisms by Which Epigenetic Modifiers Avert Therapy Resistance in Cancer.

The development of resistance to anti-cancer therapeutics remains one of the core issues preventing the improvement of survival rates in cancer. Therapy resistance can arise in a multitude of ways, including the accumulation of epigenetic alterations in cancer cells. By remodeling DNA methylation patterns or modifying histone proteins during oncogenesis, cancer cells reorient their epigenomic landscapes in order to aggressively resist anti-cancer therapy. To combat these chemoresistant effects, epigenetic modifiers such as DNA hypomethylating agents, histone deacetylase inhibitors, histone demethylase inhibitors, along with others have been used. While these modifiers have achieved moderate success when used either alone or in combination with one another, the most positive outcomes were achieved when they were used in conjunction with conventional anti-cancer therapies. Epigenome modifying drugs have succeeded in sensitizing cancer cells to anti-cancer therapy via a variety of mechanisms: disrupting pro-survival/anti-apoptotic signaling, restoring cell cycle control and preventing DNA damage repair, suppressing immune system evasion, regulating altered metabolism, disengaging pro-survival microenvironmental interactions and increasing protein expression for targeted therapies. In this review, we explore different mechanisms by which epigenetic modifiers induce sensitivity to anti-cancer therapies and encourage the further identification of the specific genes involved with sensitization to facilitate development of clinical trials.

CD81 knockout promotes chemosensitivity and disrupts in vivo homing and engraftment in acute lymphoblastic leukemia.

Relapse remains a major obstacle to achieving 100% overall survival rate in pediatric hematologic malignancies like acute lymphoblastic leukemia (ALL). Relapse often results from the development of chemoresistance. One of the mechanisms of chemoresistance involves ALL cell interactions with the bone marrow (BM) microenvironment, providing a sanctuary. This phenomenon is known as BM microenvironment-induced chemoprotection. Members of the transmembrane 4 superfamily (tetraspanins; TSPANs) are known to mediate microenvironmental interactions and have been extensively studied in solid tumors. Although the TSPAN family member CD81 is a minimal residual disease marker, its biological role in ALL is not well characterized. We show for the first time that CD81 knockout induces chemosensitivity, reduces cellular adhesion, and disrupts in vivo BM homing and engraftment in B-ALL. This chemosensitization is mediated through control of Bruton tyrosine kinase signaling and induction of p53-mediated cell death. We then show how CD81-related signaling can be disrupted by treatment with the epigenetic drug combination of DNA hypomethylating agent azacitidine (aza) and histone deacetylase inhibitor panobinostat (pano), which we previously used to sensitize ALL cells to chemotherapy under conditions that promote BM microenvironment-induced chemoprotection. Aza/pano-mediated modulation of CD81 surface expression is involved in decreasing BM load by promoting ALL cell mobilization from BM to peripheral blood and increasing response to chemotherapy in disseminated patient-derived xenograft models. This study identifies the novel role of CD81 in BM microenvironment-induced chemoprotection and delineates the mechanism by which aza/pano successfully sensitizes ALL cells via modulation of CD81.

Epigenetic drug combination overcomes osteoblast-induced chemoprotection in pediatric acute lymphoid leukemia.

Although there has been much progress in the treatment of acute lymphoblastic leukemia (ALL), decreased sensitivity to chemotherapy remains a significant issue. Recent studies have shown how interactions with the bone marrow microenvironment can protect ALL cells from chemotherapy and allow for the persistence of the disease. Epigenetic drugs have been used for the treatment of ALL, but there are no reports on whether these drugs can overcome bone marrow-induced chemoprotection. Our study investigates the ability of the DNA methyltransferase inhibitor azacitidine and the histone deacetylase inhibitor panobinostat to overcome chemoprotective effects mediated by osteoblasts. We show that the combination of azacitidine and panobinostat has a synergistic killing effect and that this combination is more effective than cytarabine in inducing ALL cell death in co-culture with osteoblasts. We also show that this combination can be used to sensitize ALL cells to chemotherapeutics in the presence of osteoblasts. Finally, we demonstrate that these effects can be replicated ex vivo in a number of mouse passaged xenograft lines from both B-ALL and T-ALL patients with varying cytogenetics. Thus, our data provides evidence that azacitidine and panobinostat can successfully overcome osteoblast-induced chemoprotection in vitro and ex vivo in both B-ALL and T-ALL cells.

Eviction from the sanctuary: Development of targeted therapy against cell adhesion molecules in acute lymphoblastic leukemia.

Acute lymphoblastic leukemia (ALL) is a malignant hematological disease afflicting hematopoiesis in the bone marrow. While 80%-90% of patients diagnosed with ALL will achieve complete remission at some point during treatment, ALL is associated with high relapse rate, with a 5-year overall survival rate of 68%. The initial remission failure and the high rate of relapse can be attributed to intrinsic chemoprotective mechanisms that allow persistence of ALL cells despite therapy. These mechanisms are mediated, at least in part, through the engagement of cell adhesion molecules (CAMs) within the bone marrow microenvironment. This review assembles CAMs implicated in protection of leukemic cells from chemotherapy. Such studies are limited in ALL. Therefore, CAMs that are associated with poor outcomes or are overexpressed in ALL and have been shown to be involved in chemoprotection in other hematological cancers are also included. It is likely that these molecules play parallel roles in ALL because the CAMs identified to be a factor in ALL chemoresistance also work similarly in other hematological malignancies. We review the signaling mechanisms activated by the engagement of CAMs that provide protection from chemotherapy. Development of targeted therapies against CAMs could improve outcome and raise the overall cure rate in ALL.