Identification of a human achaete-scute homolog highly expressed in neuroendocrine tumors.

Basic helix-loop-helix transcription factors of the achaete-scute family are instrumental in Drosophila neurosensory development and are candidate regulators of development in the mammalian central nervous system and neural crest. We report the isolation and initial characterization of a human achaete-scute homolog that is highly expressed in two neuroendocrine cancers, medullary thyroid cancer (MTC) and small cell lung cancer (SCLC). The human gene, which we have termed human achaete-scute homology 1 (hASH1), was cloned from a human MTC cDNA library. It encodes a predicted protein of 238 aa that is 95% homologous to mammalian achaete-scute homolog (MASH) 1, a rodent basic helix-loop-helix factor. The 57-residue basic helix-loop-helix domain is identical to that in the rodent gene, and the basic and helical regions, excluding the loop, are 72-80% identical to Drosophila achaete-scute family members. The proximal coding region of the hASH1 cDNA contains a striking 14-copy repeat of the triplet CAG that exhibits polymorphism in human genomic DNA. Thus, hASH1 is a candidate locus for disease-causing mutations via triplet repeat amplification. Analysis of rodent-human somatic cell hybrids permitted assignment of hASH1 to human chromosome 12. Northern blots revealed hASH1 transcripts in RNA from a human MTC cell line, two fresh MTC tumors, fetal brain, and three lines of human SCLC. In contrast, cultured lines of non-SCLC lung cancers and a panel of normal adult human tissues showed no detectable hASH1 transcripts. Expression of hASH1 may provide a useful marker for cancers with neuroendocrine features and may contribute to the differentiation and growth regulation of these cells. IMAGES:

无刚毛-翅瓣（achaete-scute）家族的碱性螺旋-环-螺旋（basic helix-loop-helix）转录因子，在果蝇（Drosophila）神经感觉发育过程中发挥关键调控作用，同时也是哺乳动物中枢神经系统与神经嵴发育的候选调控因子。本研究报道了一种人类无刚毛-翅瓣同源基因的克隆与初步表征，该基因在两种神经内分泌癌——甲状腺髓样癌（medullary thyroid cancer, MTC）与小细胞肺癌（small cell lung cancer, SCLC）中呈高表达。
我们将该人类基因命名为人类无刚毛-翅瓣同源基因1（human achaete-scute homology 1, hASH1），其从人类甲状腺髓样癌cDNA文库中克隆得到。该基因编码一个含238个氨基酸残基的预测蛋白，与啮齿类碱性螺旋-环-螺旋因子哺乳动物无刚毛-翅瓣同源基因1（mammalian achaete-scute homolog 1, MASH1）的同源性高达95%。其由57个氨基酸残基构成的碱性螺旋-环-螺旋结构域与啮齿类基因完全一致，且其碱性区域与螺旋区域（不包含环区）与果蝇无刚毛-翅瓣家族成员的同源性达72%~80%。
hASH1的cDNA编码区近端存在一段引人注目的14次重复的CAG三核苷酸序列，该序列在人类基因组DNA中呈现多态性。因此，hASH1是一个可通过三核苷酸重复扩增引发致病突变的候选基因座。通过啮齿类-人类体细胞杂种分析，我们将hASH1定位于人类12号染色体。
Northern印迹（Northern blots）检测结果显示，在人类甲状腺髓样癌细胞系、两例新鲜甲状腺髓样癌肿瘤组织、胎儿脑组织以及三株人类小细胞肺癌细胞系的RNA样本中，均可检测到hASH1的转录本。与之相反，非小细胞肺癌细胞系以及一组正常成人组织样本中均未检测到可检出水平的hASH1转录本。hASH1的表达可作为具有神经内分泌特征的癌症的有效标志物，同时可能参与这类细胞的分化与生长调控。
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