The High Mobility Group A1 (HMGA1) Chromatin Regulator Induces Inflammatory Signals and Promotes Atherogenesis in Tet2 Mutant Clonal Hematopoiesis

Introduction: Clonal hematopoiesis (CH) is common with aging and associated with an increased risk for atherosclerosis (AS) and other cardiovascular diseases, although the molecular mechanisms underlying atherogenesis in CH are only beginning to emerge. CH occurs when hematopoietic stem cells (HSCs) acquire a mutation that confers a fitness mutation, leading to expansion of mutant clones. TET2 is among the most commonly mutated genes in CH and prior studies show that TET2 mutant CH increases AS in mouse models (Jaiswal et al, NEJM, 2017). While Tet2 mutant macrophages induce inflammatory cytokine genes and secrete inflammatory cytokines linked to atherogenesis, how this occurs remains poorly understood. We discovered that the High Mobility Group A1 (HMGA1) chromatin regulator up-regulates transcriptional networks involved in clonal expansion and inflammatory signals in diverse settings, including myeloproliferative neoplasms, acute leukemia (Li et al, Blood, 2022, Resar et al, Cancer Res, 2018), and solid tumors (Chia et al, JCI 2023). Moreover, variants in the HMGA1 locus increase the risk for CH. We therefore sought to test the hypothesis that HMGA1 is required for clonal expansion, inflammatory signals, and atheroma formation in TET2 mutant CH (TET2 CH).

Methods: To investigate HMGA1 in AS arising in the setting of TET2 CH, we used an established mouse model of AS with biallelic deletion in the low-density lipoprotein receptor gene (Ldlr-/-). Ldlr-/- mice develop robust AS on a high fat diet (HFD). To model TET2 CH, Ldlr-/- mice underwent bone marrow transplantation (BMT) from donors with hematopoietic-specific, biallelic Tet2 mutation. To define HMGA1 function in TET2 CH and AS, we generated mice with Tet2 CH and intact Hmga1 or genetic deletion of Hmga1 (heterozygous or homozygous) for BMT into Ldlr-/- recipients. Following BMT and a HFD for 18 weeks, Ldlr-/- mice with Tet2 CH and varied levels of Hmga1 underwent necropsy to assess atherogenesis. Serum lipid profiles were assessed on all mice. We compared atheroma lesion area expressed as percentage involvement of the aortic root using in silico analyses. Macrophage infiltration was assessed by immunohistochemistry (IHC) and inflammatory serum cytokines were detected using cytokine arrays.

Results: We discovered a striking decrease in atheromatous plaque area by ~50% within the proximal aorta of Ldlr-/- mice with Tet2 CH and Hmga1 heterozygous deficiency compared to those with Tet2 CH and intact Hmga1 (P<0.005). Decreases were more pronounced in mice with Tet2 CH and Hmga1 homozygous deficiency (P<0.001). Surprisingly, loss of just a single Hmga1 allele in the context of Tet2 CH was sufficient to reduce atheromatous plaque formation to levels observed in Ldlr-/- mice lacking CH. Next, we assessed macrophage infiltration, which is associated with end-organ tissue inflammation and atherogenesis. By IHC, we found a marked decrease in macrophage infiltration in the kidney, spleen, and lungs of mice with Tet2 CH and Hmga1 haploinsufficiency. Similarly, serum cytokine analysis revealed a significant decrease in inflammatory cytokines from mice with Tet2 CH and Hmga1 haploinsufficiency, including decreases in CXCL1, CXCL2, IL-1α, IL-6, and TNFa, all of which are cytokines associated with inflammation and atherogenesis in humans. Serum lipid levels were similar across all genotypes. Single cell transcriptomics are underway to dissect gene networks regulated by Hmga1 in Tet2 CH. Preliminary genetic association studies of human cohorts suggest that HMGA1 variants are associated with an increased risk for AS and other cardiovascular diseases.

Conclusions: Together, our studies reveal HMGA1 as a novel driver of atherogenesis in Tet2-mediated CH. HMGA1 increases macrophage infiltration, circulating inflammatory cytokines, and atheroma formation in an established mouse model of AS and Tet2 CH. Strikingly, loss of just a single Hmga1 allele within the hematopoietic compartment was sufficient to decrease atherogenesis and circulating levels of inflammatory cytokines. Preliminary analyses suggest that variants in the HMGA1 locus associate with an increased risk for cardiovascular disease. Studies with single cell transcriptomics to further dissect HMGA1 function in AS and CH have the potential to reveal actionable mechanisms for therapeutic interventions to prevent AS and associated cardiovascular diseases.