Kamholz J, Toffenetti J, Lazzarini R A
Laboratory of Molecular Genetics, National Institute of Neurological and Communicative Disorders and Stroke, Bethesda, Maryland 20892.
J Neurosci Res. 1988 Sep;21(1):62-70. doi: 10.1002/jnr.490210110.
The human brain contains four isoforms of myelin basic protein (MBP), previously identified by cDNA cloning. We have now isolated and characterized genomic clones encoding the human MBP gene. The gene is 45 kb in extent and consists of seven exons. Alternative splicing of the primary MBP transcript can account for all four human MBP isoforms. The intron-exon boundaries of the gene have also been determined, and all conform to the known consensus splice sequences. These sequences, however, do not explain the alternative splicing pattern found in human brain. Transcription of the human MBP gene begins at a single site within the MBP promoter, and all four MBP isoforms are transcribed from this same site. The promoter region does not contain any known sequence elements, but does have a 12-bp sequence also found in the JC virus 98-bp tandem repeat. A relative gradient of MBP transcription is found from caudal to rostral within the developing human brain, which parallels the known sequence of myelination found in these areas. RNase protection of brain RNA demonstrates more of the 21.5-kD and 20.5-kD MBP mRNAs in neonatal brain than in the adult frontal cortex, which suggests that alternative splicing of the primary MBP transcript is also regulated temporally during myelin development. These data show that regulation of myelination is complex, involving regional cellular interactions and trans activation of transcription, as well as modulation of alternative splicing. Comparison of the human and mouse data also suggests that alternative splicing plays an important role in myelin biogenesis.
人类大脑含有四种髓鞘碱性蛋白（MBP）异构体，此前已通过cDNA克隆鉴定出来。我们现在已经分离并鉴定了编码人类MBP基因的基因组克隆。该基因长度为45 kb，由七个外显子组成。初级MBP转录本的可变剪接可以解释所有四种人类MBP异构体。该基因的内含子-外显子边界也已确定，并且都符合已知的共有剪接序列。然而，这些序列并不能解释在人类大脑中发现的可变剪接模式。人类MBP基因的转录起始于MBP启动子内的一个单一位置，所有四种MBP异构体都从这个相同的位置转录。启动子区域不包含任何已知的序列元件，但确实有一个在JC病毒98 bp串联重复序列中也发现的12 bp序列。在发育中的人类大脑中，从尾侧到吻侧发现了MBP转录的相对梯度，这与这些区域中已知的髓鞘形成序列平行。对脑RNA的核糖核酸酶保护实验表明，新生儿脑中21.5-kD和20.5-kD MBP mRNA的含量比成人额叶皮质中的多，这表明初级MBP转录本的可变剪接在髓鞘发育过程中也受到时间调控。这些数据表明，髓鞘形成的调控是复杂的，涉及区域细胞相互作用和转录的反式激活，以及可变剪接的调节。对人类和小鼠数据的比较还表明，可变剪接在髓鞘生物发生中起重要作用。
