SAMSN1 (SAM Domain, SH3 Domain, and Nuclear Localization Signal)
The gene encoding SAMSN1 maps to human chromosome 21q11.2 in a region that is frequently disrupted by translocation events in hematopoietic malignancies (Mitelman et al. 1997). The full-length cDNA of human SAMSN1 was first identified and cloned in 2001 by two groups from myeloma cells and human cord blood–derived mast cells, named as HACS1 and Nash1, respectively (Claudio et al. 2001; Uchida et al. 2001). The predicted protein showed unique primary structure with a nuclear localization signal (NLS), a sterile alpha motif (SAM), and a Src homology 3 domain (SH3). SAMSN1 belongs to a novel family of scaffolding and adaptor proteins that includes SASH3/SLY/HACS2 (Beer et al. 2001) and SASH1/KIAA0790 (Zeller et al. 2003). The function of this novel family is not well characterized. The mouse, rat, and chimp orthologs have been identified, and unnamed protein homologs exist in chicken, zebrafish, and fly.
Expression of SAMSN1
SAMSN1 is expressed in hematopoietic tissues, particularly in B cells, macrophages, mast cells, and dendritic cells (Claudio et al. 2001). In embryonic mouse (Gitton et al. 2002; Reymond et al. 2002), it is highly expressed in blood vessels at 9.5–10.5 days post coitum (dpc). It is also expressed in the brain, future spinal cord, dorsal root ganglia, otocyst, eye, limb, heart, surface ectoderm, and bronchial arch, albeit at lower levels. At 14.5 dpc, weak and regionally restricted expression in the skin and homogenous weak expression in mesenchyme can be detected by in situ hybridization, but expression can no longer be detected in the brain or arterial and venous systems. In human, it is also expressed in the adult heart, kidney, placenta, and lung, and major expression can be detected in the bone marrow, peripheral blood, and immune tissues, including lymph node, spleen, and thymus. The Gene Expression Atlas database (http://www.ebi.ac.uk/gxa) shows human SAMSN1 is differentially expressed in 95 experiments, 40 organism parts, 100 disease states, 37 cell types, 27 cell lines, 13 compound treatments, and 20 other conditions.
Protein and Splice Variants
SAMSN1 encodes a 441 amino acid protein containing two domains frequently associated with signaling molecules. An SH3 motif is in the middle half of the protein and a SAM domain is located toward the C-terminal end. The SH3 motif shares 39% homology with the well-characterized adaptor protein Crk. There are three predicted consensus nuclear localization signals. A tyrosine kinase phosphorylation motif is predicted at amino acids 221–228. The orthologous mouse Hacs1 gene is located on murine chromosome 16, which encodes a protein of 364 aa with an overall 87% homology to human SAMSN1 at protein level.
Two major splice variants have been detected in both human and mouse. The difference between these major variants is the N terminus, but both contain the SAM and SH3 domains. These two variants suggest that there are two different promoters. In addition to truncation of the N terminus, six additional transcripts appear to differ by truncation of the 3′ end and the presence or absence of, or differential use of, boundaries of eight cassette exons (http://www.ncbi.nlm.nih.gov/IEB/Research/Acembly).
Function of SAMSN1 in Immune Responses
An in vitro study and immunoblotting analysis showed SAMSN1 expression is upregulated in activated human B cells treated with interleukin (IL)-4, IL-13, CD40L, and anti-immunoglobulin (Ig) M. In murine spleen B cells, Samsn1 can also be upregulated by IL-4, lipopolysaccharide, but not IL-13 (Zhu et al. 2004). SAMSN1 associates with tyrosine-phosphorylated proteins after B cell activation and binds in vitro to the inhibitory molecule paired Ig-like receptor B (PIR-B). Overexpression of Samsn1 in murine spleen B cells resulted in a downregulation of the activation marker CD23 and enhancement of CD138 expression, IgM secretion, and Xbp-1 expression. Silencing SAMSN1 in a human B lymphoma cell line by small interfering ribonucleic acid did not significantly change IL-4-stimulated B cell proliferation.
In vivo study was performed by generating Samsn1 gene knockout mice (Samsn1 −/− ) (Wang et al. 2010). Samsn1 knockout mice were viable and fertile and had normal bone marrow B cell development and normal splenic T cell and B cell populations. However, similar to PIR-B-deficient mice, Samsn1 −/− mice have an increased peritoneal B1 cell population, increased phosphorylation of Lyn kinase in B cells, and augmented adaptive immune responses, suggesting that Samsn1 plays an important role in suppression of adaptive immunity. Interestingly, SASH3/Sly, another member in the same family of SAMSN1, which has a 50% amino acid identity with SAMSN1 and also similarly expressed in lymphocytes, was found to play an opposite role to Samsn1 (Beer et al. 2005). Mice expressing a defective SLY (SLY1d) protein exhibit impaired immune responses and prolonged allograft survival.
SAMSN1 and Cancer
SAMSN1 protein was detected from different hematopoietic malignant cells, including myeloid leukemia, lymphoma, and multiple myeloma. The function of SAMSN1 in those cells remains unknown. SAMSN1 was suggested as a candidate tumor suppressor in a recent extensive screening of a large panel of lung cancer cell lines (Yamada et al. 2008). In another study, SASH1, another member in this family, has been described as a candidate tumor suppressor gene downregulated in breast cancer (Zeller et al. 2003). SASH1 was also reported to serve as a candidate tumor suppressor in colon cancer and associated with patient prognosis (Rimkus et al. 2006).
SAMSN1 is a member of a novel gene family of putative adaptors and scaffold proteins containing SH3 and SAM domains. Recent studies demonstrated SAMSN1 is a component of immune response and is upregulated and functions in activation of adaptive immunity. SAMSN1 was suggested as a candidate tumor repressor. Future studies will continue to investigate the functions of SAMSN1 in different normal cells and tumor cells, to identify the pathways, partners and mechanisms involved in SAMSN1-mediated biological or pathologic consequences.
- Uchida T, Nakao A, Nakano N, Kuramasu A, Saito H, Okumura K, et al. Identification of nash1, a novel protein containing a nuclear localization signal, a sterile alpha motif, and an SH3 domain preferentially expressed in mast cells. Biochem Biophys Res Commun. 2001;288(1):137–41.PubMedCrossRefGoogle Scholar