Ching A K, Li P S, Chan W Y, Ma C H, Lee S S, Lim P L, Chui Y L
Clinical Immunology Unit and Sir Y. K. Pao Centre for Cancer, Prince of Wales Hospital, Shatin, NT.
Int Immunol. 2000 Sep;12(9):1245-53. doi: 10.1093/intimm/12.9.1245.
Ig genes undergo hypermutation with a nucleotide preference of A over T for mutation on the coding strand. As only with concomitant strand bias can such nucleotide bias be observed, Ig gene hypermutation is generally accepted as a strand-specific process, for which the mechanistic basis remains unknown. It has previously been shown that different non-Ig sequences replacing the LVJ region of an Ig transgene to various extents are targeted for hypermutation with similar mutation frequencies. However, the nucleotide bias characteristic of Ig hypermutation was not found in two of the three such sequences studied. To test whether it is the DNA sequences of the non-Ig substrates that determine the pattern of nucleotide bias in hypermutation or whether the LVJ sequence may contain element(s) that confer strand bias, we have added back all the replaced LVJ sequences to one of the transgenes, L(kappa)-Vgpt*, that expresses no strand bias in hypermutation and studied the outcome. The results show that the gpt sequence in the presence of the complete LVJ sequence hypermutates differently from the same sequence in L(kappa)-Vgpt* where 84% of the LVJ was replaced. The main difference is the resumption of strand bias characteristic of Ig hypermutation. Thus, whether or not a substrate sequence manifests strand bias in hypermutation is not inherently determined by the substrate DNA sequence. This indicates the presence of special element(s) within the LVJ that confer strand bias.
免疫球蛋白（Ig）基因发生高频突变，在编码链上，突变的核苷酸偏好为A而非T。由于只有伴随链偏向才能观察到这种核苷酸偏向，Ig基因高频突变通常被认为是一个链特异性过程，其机制基础尚不清楚。先前的研究表明，不同程度取代Ig转基因LVJ区域的不同非Ig序列，以相似的突变频率成为高频突变的靶点。然而，在所研究的三个此类序列中的两个中，未发现Ig高频突变的核苷酸偏向特征。为了测试是由非Ig底物的DNA序列决定高频突变中核苷酸偏向的模式，还是LVJ序列可能包含赋予链偏向的元件，我们将所有被取代的LVJ序列添加回其中一个转基因L（κ）-Vgpt中，该转基因在高频突变中不表现出链偏向，并研究了结果。结果表明，在完整LVJ序列存在的情况下，gpt序列的高频突变与L（κ）-Vgpt中84%的LVJ被取代时的相同序列不同。主要区别在于Ig高频突变的链偏向特征得以恢复。因此，底物序列在高频突变中是否表现出链偏向并非由底物DNA序列固有决定。这表明LVJ内存在赋予链偏向的特殊元件。
