Structural and functional characterization of a novel Polycomb group gene SCML1.

Polycomb group （PcG）基因编码转录抑制因子，调控同源性（HOX）基因和其他基因，并参与所谓的细胞记忆机制，这是维持细胞身份所必需的。我们分离并鉴定了一个新的属于PcG基因的哺乳动物基因。从小鼠、大鼠和人脑中分离的cdna的核苷酸序列分析表明，新基因编码的蛋白质（863个氨基酸）与果蝇PcG基因Scm（中腿性梳）的产物在氨基酸序列上有显著的相似性。此外，该蛋白含有4个mbt结构域的串联拷贝，这些结构域最初是在果蝇肿瘤抑制基因1 (3)mbt（致命(3)恶性脑肿瘤）中报道的。我们最初将该新基因命名为SCML1 (Scm like-1)，但后来因含有四个mbt结构域的Scm相关基因而将其重新命名为Sfmbt。小鼠Sfmbt基因的克隆和鉴定表明，该编码序列由20个外显子组成，分布在约40kb的染色体上，位于第14染色体的近端。人类Sfmbt基因定位于染色体3p21.1。Northern blot分析显示，Sfmbt mrna在成年睾丸中表达最丰富，但在所有小鼠组织和人细胞系中表达较少。利用腺病毒载体进行的瞬时过表达实验表明，Sfmbt基因参与调控HOXA1基因在U251人胶质瘤细胞系中的表达，并对细胞增殖速率进行修饰。这些功能数据与属于PcG基因的Sfmbt基因的结构数据很好地对应，Sfmbt基因类似于果蝇肿瘤抑制基因l(3)mbt。为了研究Sfmbt基因的突变表型，目前正在对Sfmbt缺陷小鼠进行靶向诱变实验。

Polycomb group (PcG) genes encode transcriptional repressors which regulate homeotic (HOX) genes and others, and are involved in so-called cellular memory mechanisms which is essential in the maintenance of cell identity. We isolated and characterized a novel mammalian gene belonging to the PcG genes.The nucleotide sequence analysis of the cDNAs isolated from the mouse, rat, and human brain revealed that the novel gene encoded the protein (863 amino acids) that showed significant similarity in amino acid sequence to the product of Drosophila PcG gene Scm (Sex comb on midleg). In addition, this protein contained four tandem copies of the mbt domains, which were originally reported in the Drosophila tumor suppressor gene l(3)mbt (lethal(3) malignant brain tumor). We initially named this novel gene SCML1 (Scm like-1), but thereafter renamed it Sfmbt for Scm-related gene containing four mbt domains.Cloning and characterization of the mouse Sfmbt gene revealed that the coding sequence comprised 20 exons, dispersed along approximately 40 kb, and was mapped to the proximal part of Chromosome 14. The human Sfmbt gene was mapped to the chromosome 3p21.1.Northern blot analysis showed that the Sfmbt mRNAs were expressed most abundantly in the adult testis, but less intensively in all the mouse tissues and human cell lines examined.Transient over-expression experiments using adenovirus vector indicated that the Sfmbt gene was involved in the regulation of HOXA1 gene expression, and the modification of the cell proliferation rate in U251 human glioma cell line. These functional data well correspond to the structural data of the Sfmbt gene that belongs to the PcG genes and resembles the Drosophila tumor suppressor gene l(3)mbt. To investigate the mutated phenotype of the Sfmbt gene, the experiment on Sfmbt-deficient mice, generated by targeted mutagenesis, is now in progres.