Kozu Tomoko, Fukuyama Tomofusa, Yamami Tohru, Akagi Kiwamu, Kaneko Yasuhiko
Research Institute for Clinical Oncology, Saitama Cancer Center, Ina, Saitama, Japan.
Genes Chromosomes Cancer. 2005 May;43(1):45-53. doi: 10.1002/gcc.20165.
The AML1-MTG8 fusion gene is generated by chromosome translocation t(8;21), which is frequently observed in acute myeloid leukemia. The fusion gene produces a chimeric transcription factor that suppresses the expression of AML1-target genes via the MTG8 part of the chimeric protein, which is thought to be the primary cause of leukemia. The C-terminal region of MTG8 contains the MYND domain, represented by highly conserved zinc-finger-like protein motifs, and is known to interact with corepressor proteins. We found that, instead of the MYND domain, an alternative last exon of MTG8 encoding 27 amino acids in-frame is expressed naturally in human adult testis and in several leukemia cell lines. This type of alternative splicing also occurred in the AML1-MTG8 fusion gene at high levels in leukemia cell lines with t(8;21), as well as in blast cells of leukemia patients with t(8;21). The variant proteins of both MTG8 and AML1-MTG8 reduced transcriptional repressor activity in a mammalian two-hybrid assay. However, mixed expression of these variants with wild-type MTG8 recovered their repressor activity, suggesting that these variants also act as repressors in vivo where wild-type MTG8 and other family members exist in abundance. On the other hand, the MYND-less variants acquired a higher affinity for binding to MTG8 and formed a multimer, whereas the wild-type protein forms a dimer. Thus, expression of the MYND-less variants by the dysregulation of splicing machinery, which stimulates the oligomerization of fusion proteins in leukemia cells, may enhance malignant conversion of hematopoietic cells.
AML1-MTG8融合基因由染色体易位t(8;21)产生，这在急性髓系白血病中经常可见。该融合基因产生一种嵌合转录因子，其通过嵌合蛋白的MTG8部分抑制AML1靶基因的表达，这被认为是白血病的主要病因。MTG8的C末端区域包含MYND结构域，由高度保守的锌指样蛋白基序代表，并且已知与共抑制蛋白相互作用。我们发现，在人类成年睾丸和几种白血病细胞系中天然表达的是MTG8的一个替代的最后外显子，其编码27个框内氨基酸，而不是MYND结构域。这种类型的可变剪接在具有t(8;21)的白血病细胞系以及具有t(8;21)的白血病患者的原始细胞中，在AML1-MTG8融合基因中也高水平发生。在哺乳动物双杂交试验中，MTG8和AML1-MTG8的变体蛋白均降低了转录抑制活性。然而，这些变体与野生型MTG8的混合表达恢复了它们的抑制活性，这表明这些变体在体内野生型MTG8和其他家族成员大量存在的情况下也作为抑制因子起作用。另一方面，无MYND结构域的变体对与MTG8结合具有更高的亲和力并形成多聚体，而野生型蛋白形成二聚体。因此，通过剪接机制失调导致的无MYND结构域变体的表达，会刺激白血病细胞中融合蛋白的寡聚化，这可能会增强造血细胞的恶性转化。
