MAML3

MAML3 is a transcriptional coactivator that participates in multiple developmental and oncogenic signaling pathways, most critically Notch signaling, where it forms ternary complexes with Notch intracellular domains and CSL/RBPJ DNA-binding proteins to drive target gene expression. Genetic studies in mice demonstrate that MAML3 functions redundantly with MAML1 to support Notch signaling during embryonic organogenesis, as double-null embryos exhibit lethal pan-Notch defects including loss of Notch target gene expression in presomitic mesoderm (PMID:22069191). Beyond Notch, MAML3 physically interacts with β-catenin to activate TCF/LEF-dependent WNT transcription (PMID:33986121), cooperates with RBPJ to upregulate Smoothened transcription under hypoxia thereby linking it to Hedgehog pathway activation (PMID:27466498, PMID:37351966), and promotes epithelial-to-mesenchymal transition through Slug (PMID:28512191). The PAX3-MAML3 fusion protein, generated by a recurrent chromosomal translocation, functions as a potent transcriptional activator of PAX3 response elements and drives the neuroectodermal and myogenic gene programs characteristic of biphenotypic sinonasal sarcoma (PMID:24859338).

1. 2011 High

   The question of whether MAML3 is essential for Notch signaling in vivo was resolved: MamL1/MamL3 double-null mice die with pan-Notch phenotypes and abolished Notch target gene expression, establishing that Mastermind-like coactivators are required components of the Notch transcriptional activation complex during mammalian development.

   Evidence Single and double genetic knockout mice with phenotypic and gene expression analysis of Notch targets (lunatic fringe) in presomitic mesoderm

   PMID:22069191

   Open questions at the time

   • Individual contribution of MAML3 versus MAML1 to specific Notch target loci is unresolved
   • Structural basis for MAML3 preference or selectivity among Notch paralog complexes unknown
   • Post-translational regulation of MAML3 in the Notch activation complex not addressed
2. 2014 High

   The oncogenic mechanism of PAX3-MAML3 was established: the fusion protein generated by t(2;4)(q35;q31.1) translocation hijacks the MAML3 transactivation domain to constitutively activate PAX3 response elements, driving aberrant neuroectodermal and myogenic programs that define biphenotypic sinonasal sarcoma.

   Evidence RNA sequencing identifying the fusion transcript, transcriptional activator assays on PAX3 response elements, gene expression profiling of tumor specimens

   PMID:24859338

   Open questions at the time

   • Which specific coactivator domains of MAML3 are required for fusion-driven transactivation is not mapped
   • Whether the fusion disrupts normal MAML3 Notch coactivator function in cis is untested
   • Downstream chromatin remodeling recruited by the fusion is uncharacterized
3. 2016 Medium

   MAML3 was shown to contribute to Hedgehog pathway activation under hypoxia by cooperating with RBPJ to transcriptionally upregulate Smoothened, expanding its coactivator role beyond canonical Notch signaling.

   Evidence siRNA knockdown of MAML3 in pancreatic cancer cells under hypoxia, with western blot for SMO, HES1, and MMPs; replicated in small cell lung cancer (2018) and gallbladder cancer (2023) systems

   PMID:27466498 PMID:30061220 PMID:37351966

   Open questions at the time

   • Whether MAML3 directly binds the SMO promoter or acts indirectly through RBPJ-dependent intermediates is not resolved
   • Findings from single labs using siRNA; genetic loss-of-function validation lacking
   • Hypoxia-dependent regulation of MAML3 itself (transcriptional versus post-translational) is not elucidated
4. 2017 Medium

   A role for MAML3 in promoting epithelial-to-mesenchymal transition was demonstrated through its regulation of Slug expression, establishing a miR-2392–MAML3–Slug axis in gastric cancer metastasis.

   Evidence miRNA target validation by luciferase reporter, siRNA knockdown of MAML3 with assessment of Slug expression and invasion/metastasis in vitro and in vivo

   PMID:28512191

   Open questions at the time

   • Whether MAML3 directly transactivates the Slug promoter or acts through Notch/WNT intermediates is unknown
   • Generalizability of the miR-2392–MAML3 axis beyond gastric cancer untested
   • No rescue experiment with MAML3 re-expression reported
5. 2021 Medium

   The scope of MAML3 coactivator function was extended to WNT signaling: MAML3 physically interacts with β-catenin and activates TCF/LEF-dependent transcription, and its overexpression promotes invasion and anchorage-independent growth in neuroendocrine tumor cells.

   Evidence Co-immunoprecipitation of MAML3 with β-catenin, TCF/LEF luciferase reporter assay, invasion and soft-agar assays with full-length and exon-1-deleted MAML3

   PMID:33986121

   Open questions at the time

   • Whether MAML3 bridges Notch and WNT signaling simultaneously or functions in them independently is unresolved
   • Endogenous stoichiometry and competition between MAML3–Notch and MAML3–β-catenin complexes not measured
   • Single lab; independent replication awaited
6. 2025 Medium

   MAML3 was placed upstream of eNOS expression in endothelial cells, with its silencing reducing eNOS protein levels and impairing angiogenesis, revealing a role in vascular biology distinct from its known developmental functions.

   Evidence Biotinylated miRNA pulldown and RNA-seq identifying MAML3 as miR-486-5p target; siRNA knockdown of MAML3 in HUVECs with eNOS western blot and tube formation assay

   PMID:40432288

   Open questions at the time

   • Whether MAML3 regulates eNOS transcription directly or through Notch/other pathways is not determined
   • In vivo vascular phenotype of MAML3 loss not assessed
   • Single lab finding without independent confirmation

• Key open questions include how MAML3 is recruited to and coordinates among Notch, WNT, and Hedgehog transcriptional complexes, what post-translational modifications regulate its pathway selectivity, and whether its endothelial function operates through canonical Notch-dependent or independent mechanisms.
• No structural model of MAML3 in complex with β-catenin or RBPJ–SMO promoter complex
• Genome-wide chromatin occupancy (ChIP-seq) of endogenous MAML3 not reported
• Post-translational modifications and their functional consequences uncharacterized