Mochmann Liliana H, Wells Robert D
Institute of Biosciences and Technology, Center for Genome Research, Texas A&M University System Health Science Center, Texas Medical Center, 2121 W. Holcombe Blvd, Houston, TX 77030, USA.
Nucleic Acids Res. 2004 Aug 18;32(15):4469-79. doi: 10.1093/nar/gkh787. Print 2004.
The genetic instability of (GACGTC)n (where n = 6-74) was investigated in an Escherichia coli-based plasmid system. Prior work implicated the instability of a (GACGTC)5 tract in the cartilage oligomeric matrix protein (COMP) gene to the 4, 6 or 7mers in the etiology of pseudoachondroplasia and multiple epiphyseal dysplasia. The effects of triplet repeat length and orientation were studied after multiple replication cycles in vivo. A transcribed plasmid containing (GACGTC)49 repeats led to large deletions (>3 repeats) after propagation in E.coli; however, if transcription was silenced by the LacI(Q) repressor, small expansions and deletions (<3 repeats) predominated the mutation spectra. In contrast, propagation of similar length but opposing orientation (GTCGAC)53 containing plasmid led to small instabilities that were unaffected by the repression of transcription. Thus, by inhibiting transcription, the genetic instability of (GACGTC)49 repeats did not significantly differ from the opposing orientation, (GTCGAC)53. We postulate that small instabilities of GACGTC repeats are achieved through replicative slippage, whereas large deletion events are found when GACGTC repeats are transcribed. Herein, we report the first genetic study on GAC*GTC repeat instability describing two types of mutational patterns that can be partitioned by transcription modulation. Along with prior biophysical data, these results lay the initial groundwork for understanding the genetic processes responsible for triplet repeat mutations in the COMP gene.
在基于大肠杆菌的质粒系统中研究了(GACGTC)n（其中n = 6 - 74）的遗传不稳定性。先前的研究表明，软骨寡聚基质蛋白（COMP）基因中(GACGTC)5序列的不稳定性与假性软骨发育不全和多发性骨骺发育异常病因中的4、6或7聚体有关。在体内多次复制循环后，研究了三联体重复长度和方向的影响。含有(GACGTC)49重复序列的转录质粒在大肠杆菌中繁殖后会导致大的缺失（>3个重复）；然而，如果转录被LacI(Q)阻遏物沉默，小的扩增和缺失（<3个重复）在突变谱中占主导。相比之下，含有相似长度但方向相反的(GTCGAC)53质粒的繁殖导致小的不稳定性，且不受转录抑制的影响。因此，通过抑制转录，(GACGTC)49重复序列的遗传不稳定性与相反方向的(GTCGAC)53没有显著差异。我们推测，GACGTC重复序列的小不稳定性是通过复制滑移实现的，而当GACGTC重复序列被转录时会出现大的缺失事件。在此，我们报告了关于GAC*GTC重复序列不稳定性的首次遗传学研究，描述了两种可通过转录调控区分的突变模式。连同先前的生物物理数据，这些结果为理解COMP基因中三联体重复突变的遗传过程奠定了初步基础。
