The C-type lectin domain family 12 member A (CLEC12A) belonging to the C-type lectin superfamily of proteins, is a type II transmembrane protein. CLEC12A is alternatively called myeloid inhibitory C-type lectin-like receptor (MICL), C-type lectin-like molecule 1 (CLL-1), dendritic cell-associated lectin 2 (DCAL-2), or KLRL1. This 265 amino acid protein, with a theoretical molecular mass of 30.8 kDa, consists of an extracellular region that forms a single C-type lectin-like domain with the six canonical cysteines typically present in C-type lectins and has 6 putative N-glycosylation and one predicted O-glycosylation sites. The rest of the protein comprises a putative hydrophobic single transmembrane domain and a cytoplasmic tail. Detailed characterization of CLEC12A demonstrated that it contains a cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM). Its ITIM domain recruits the tyrosine phosphatases SHP-1 and SHP-2 to counteract activating positive regulatory signals. CLEC12A can inhibit cellular activation through its cytoplasmic ITIM and is a negative regulator of granulocyte and monocyte function. CLEC12A is predominantly expressed by innate immune cells such as granulocytes, monocytes, macrophages, and dendritic cells (DCs). CLEC12A is a relevant marker of leukemic blasts and has been proposed as a specific marker of leukemic stem cells in acute myeloid leukemia (AML).
Excessive or deregulated cell death results in the release of damage-associated molecular patterns (DAMPs). Among them, uric acid forms monosodium urate (MSU) crystals by contacting extracellular sodium ions. CLEC12A is a sensor of cell death that recognizes MSU crystals suggesting that CLEC12A could potentially inhibit cell death-induced immune cell activation by ITAM-coupled pattern recognition receptors (PRRs). In addition, CLEC12A limits MSU crystal-induced respiratory burst (reactive oxygen species (ROS) production) and interleukin-8 (IL-8) but not IL-1β secretion. MSU crystals induce activation of the complement system, which leads to cleavage of C5 to C5a and C5b. C5a induces priming of the inflammasome by upregulating the transcription of pro-IL-1β. Direct or indirect interaction of the crystals with the plasma membrane induces ITAM-dependent activation of Syk, which is required for NLRP3 inflammasome activation and IL-1β secretion. Syk also activates a signaling cascade leading to NADPH oxidase activation that produces reactive oxygen species and to the production of the chemotactic cytokine IL-8. Upon cross-linking to the activated ITAMs, CLEC12A is phosphorylated and recruits via its ITIM protein phosphatase SHP-1 or SHP-2, which counteract Syk-induced NADPH oxidase activation and IL-8 production.
Fig.1 Representation of the negative regulation of immune responses against damaged self by CLEC12A. (Yamasaki, 2014)
CLEC12A was independently identified in a screen for novel antibody targets of AML. CLEC12A is expressed in the malignant CD34+CD38- stem cell compartment in the majority of CD34+ AML patients, while it is absent on normal CD34+CD38- resting bone marrow cells. The presence of measurable (‘minimal’) residual disease and detection of persistent putative leukemic stem cells (LSCs) is associated with poor outcomes and prognosis in AML. Since CLEC12A seems to be applicable for the detection of leukemic blasts as well as LSCs, it results be a suitable diagnostic marker for follow-up and risk prognostication in the treatment of AML and is included in the one tube approach to quantify CD34+CD38− leukemic stem cells.
Since the discovery of CLEC12A as an expression marker on the surface of multiple myeloid hematopoietic cells, its potential in diagnosis, follow-up, risk stratification, and treatment modalities in AML, has been explored. Creative Biolabs provides a full set of CLEC12A assay portfolio services, including but not limited to
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Reference
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