HOXB6

HOXB6 is a homeodomain transcription factor that controls anteroposterior axial patterning during development and regulates differentiation decisions in hematopoietic and progenitor cell compartments (PMID:7828847, PMID:15522959). In the embryo it specifies the cervicothoracic vertebral region and promotes rib formation, with loss of function producing missing first ribs, bifid second ribs, and anteriorizing homeotic transformations (PMID:7828847); the linker region connecting the homeodomain and hexapeptide is essential for rib-promoting activity and recruits additional factors (e.g., Pax3) to target enhancers, while HOXB6 separately governs somitogenesis through dysregulation of Lfng (PMID:26718008). As a transcriptional repressor, HOXB6 binds DNA through its homeodomain and acts independently of cooperative DNA binding with the PBX1 co-factor (PMID:15269212, PMID:15522959); it interacts via its homeodomain with CREB-binding protein (CBP) to repress alpha- and gamma-globin genes, and directly binds the Sox9 promoter to repress Sox9 in liver progenitor cells (PMID:15269212, PMID:32763157). In hematopoiesis, HOXB6 expands hematopoietic stem cells and myeloid precursors while blocking erythroid, lymphoid, and granulocytic/monocytic differentiation, and its enforced expression causes AML in vivo with latency dramatically shortened by MEIS1 co-expression (PMID:15522959, PMID:12094253). HOXB6 is phosphorylated in vivo at Ser-214 by casein kinase II, though this site is dispensable for globin repression (PMID:10327653, PMID:15269212).

1. 1993 Medium

   Before any functional role was assigned, it was unknown how Hoxb-6 expression is spatially restricted; dissecting its regulatory landscape established the tissue-specific control elements driving its developmental expression.

   Evidence LacZ reporter transgenic mouse embryos with systematic deletion across 13.2 kb of genomic DNA

   PMID:8104549

   Open questions at the time

   • The trans-acting factors binding each element were not identified
   • Does not address HOXB6 protein function, only its regulation
2. 1995 High

   The in vivo developmental requirement was unknown; knockout and noncomplementation analysis established Hoxb-6 as a specifier of cervicothoracic axial identity acting together with Hoxb-5.

   Evidence Targeted gene disruption in mice and transheterozygote genetic complementation test

   PMID:7828847

   Open questions at the time

   • Direct transcriptional targets driving the skeletal phenotype not identified
   • Molecular basis of Hoxb-5/Hoxb-6 cooperativity not resolved
3. 1996 Low

   Following enhancer mapping, it remained unknown what nuclear proteins act through the limb/LPM enhancer; EMSA detected tissue-specific DNA-protein complexes implicating sequence-specific factors.

   Evidence EMSA with embryonic nuclear extracts plus LacZ reporter transgenics

   PMID:8786996

   Open questions at the time

   • Single EMSA-based pulldown without identification of the bound proteins
   • No functional validation of the specific binding partners
4. 1999 High

   Whether HOXB6 is post-translationally modified was unknown; phosphopeptide mapping identified a conserved CK-II phosphorylation site at Ser-214 modified in vivo.

   Evidence Baculovirus expression, 2D tryptic phosphopeptide mapping, in vitro kinase assays, and IP from mouse embryonic spinal cords

   PMID:10327653

   Open questions at the time

   • Functional consequence of Ser-214 phosphorylation not defined
   • Whether PKA phosphorylation occurs in vivo not established
5. 2000 Medium

   The hematopoietic role was unknown; knockout analysis revealed Hoxb-6 controls the generation, proliferation, or survival of early erythroid progenitors.

   Evidence Knockout mice with clonogenic progenitor assays in bone marrow and fetal liver

   PMID:10996827

   Open questions at the time

   • Whether the effect is on generation versus proliferation versus survival not distinguished
   • Molecular targets in erythroid progenitors not identified
6. 2000 Medium

   It was unclear whether HOXB6 requires PBX co-factors and how its localization is controlled; isoform and GFP-fusion experiments showed homeodomain-dependent nuclear localization that does not co-localize with PBX in epidermis.

   Evidence Immunohistochemistry, GFP-fusion live imaging, subcellular fractionation, and Western blot of protein isoforms

   PMID:10906782

   Open questions at the time

   • Functional consequence of cytoplasmic-to-nuclear shift inferred, not directly tested
   • Regulator of the developmental localization switch unknown
7. 2002 Medium

   Whether HOXB6 affects myeloid maturation was unknown; enforced expression in promyelocytic and myeloblastic lines established that it blocks granulocytic and monocytic differentiation.

   Evidence Forced overexpression in NB4 and HL60 cells with morphology and surface-marker differentiation assays

   PMID:12094253

   Open questions at the time

   • Target genes mediating the differentiation block not identified
   • Based on cell lines rather than primary cells
8. 2004 High

   The transcriptional mechanism and co-factor dependence were unresolved; structure-function and reciprocal co-IP showed HOXB6 represses globin genes via a DNA-binding-dependent, PBX1-independent mechanism using a homeodomain interaction with CBP.

   Evidence Stable K562 transfection, RT-PCR, structure-function mutagenesis, and reciprocal endogenous co-IP including fetal liver

   PMID:15269212

   Open questions at the time

   • How CBP recruitment yields repression rather than activation not mechanistically explained
   • N-terminal region, polyGlu C-terminus, and Ser-214 shown dispensable but their functions elsewhere unknown
9. 2004 High

   Whether HOXB6 dysregulation is oncogenic was unknown; bone marrow overexpression established that it expands HSCs and myeloid precursors and causes AML, with MEIS1 as a cooperating accelerator.

   Evidence Retroviral transduction of murine bone marrow, transplantation, immortalization assay, and DNA-binding/PBX1-interaction mutants

   PMID:15522959

   Open questions at the time

   • Direct transcriptional targets driving leukemogenesis not defined
   • Mechanism of MEIS1 cooperativity not resolved
10. 2015 Medium

    The domain requirements for axial function were unknown; an LR-deletion knock-in showed the linker region is essential for rib promotion by recruiting co-factors, and that somitogenesis control via Lfng is a separable activity.

    Evidence Knock-in mice with linker-region deletion, enhancer/ChIP binding assays with Pax3, and Lfng expression analysis

    PMID:26718008

    Open questions at the time

    • Identity of the additional factors recruited by the LR (beyond Pax3) not fully resolved
    • How Lfng dysregulation is mechanistically linked to HOXB6 not detailed
11. 2020 Medium

    A direct transcriptional target in tissue regeneration was unknown; promoter-binding and rescue experiments established that HOXB6 directly represses Sox9 and is itself suppressed by miR-126 to de-repress liver progenitor expansion.

    Evidence Sox9 luciferase reporter, ChIP/promoter binding, miR-126 overexpression/knockdown, and CCl4-induced liver injury model

    PMID:32763157

    Open questions at the time

    • Whether co-factors are required for Sox9 promoter repression not addressed
    • Generality of the miR-126/HOXB6/Sox9 axis beyond liver injury unknown

• The genome-wide direct target repertoire of HOXB6 and the structural basis by which its homeodomain and linker region select repressive co-factors (CBP versus Pax3) across different tissues remain unresolved.
• No unbiased genome-wide binding map across developmental and hematopoietic contexts
• No structural model of the homeodomain-CBP or LR-Pax3 interfaces
• Functional role of Ser-214 phosphorylation still undefined