BACKGROUND Members of the Paired-like homeodomain transcription factor (PITX) gene family, particularly PITX1 and PITX2, play important roles in normal development and in differentiated cell functions. Three major isoforms of PITX2 were previously reported to be produced through both alternative mRNA splicing (PITX2A and PITX2B) and alternative promoter usage (PITX2C). The proteins derived from these mRNAs contain identical homeodomain and carboxyl termini. Differences in the amino-termini of the proteins may confer functional differences in some contexts. RESULTS Here, we report the identification of two novel PITX2 isoforms. First, we demonstrate that the Pitx2c mRNA generates two protein products, PITX2Calpha and PITX2Cbeta, via alternative translation initiation. Second, we identified a novel mRNA splice variant, Pitx2b2, which uses the same 5' splice donor in intron 2 as Pitx2b (hereafter referred to as Pitx2b1), but employs an alternative 3' splice acceptor, leading to an in-frame deletion of 39 base pairs relative to Pitx2b1. Pitx2b2 mRNA is expressed in both murine and human pituitary. The data show that in a murine gonadotrope cell line and adult murine pituitary what was previously thought to be PITX2B1 is actually PITX2Cbeta, or perhaps PITX2B2. PITX2B1 is expressed at lower levels than previously thought. PITX2Cbeta and PITX2B2 activate gonadotrope-specific gene promoter-reporters similarly to known PITX2 isoforms. CONCLUSION We have identified and characterized two novel isoforms of PITX2, generated by alternative translation initiation (PITX2Cbeta) and alternative mRNA splicing (PITX2B2). These proteins show similar DNA binding and trans-activation functions as other PITX2 isoforms in vitro, though their conservation across species suggests that they may play distinct, as yet unidentified, roles in vivo.

中文翻译：

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配对样同源域转录因子 2 (PITX2) 的新形式：通过替代翻译起始和 mRNA 剪接产生。

背景配对样同源域转录因子(PITX) 基因家族的成员，特别是PITX1 和PITX2，在正常发育和分化的细胞功能中起重要作用。先前有报道称 PITX2 的三种主要同工型是通过替代 mRNA 剪接（PITX2A 和 PITX2B）和替代启动子使用（PITX2C）产生的。源自这些 mRNA 的蛋白质包含相同的同源域和羧基末端。在某些情况下，蛋白质氨基末端的差异可能会导致功能差异。结果 在这里，我们报告了两种新型 PITX2 同种型的鉴定。首先，我们证明 Pitx2c mRNA 通过替代翻译起始生成两种蛋白质产品，PITX2Calpha 和 PITX2Cbeta。其次，我们鉴定了一种新的 mRNA 剪接变体 Pitx2b2，它在内含子 2 中使用与 Pitx2b（以下称为 Pitx2b1）相同的 5' 剪接供体，但使用替代的 3' 剪接受体，导致相对于 Pitx2b1 的 39 个碱基对的框内缺失。Pitx2b2 mRNA 在小鼠和人类垂体中均有表达。数据显示，在小鼠促性腺激素细胞系和成年小鼠垂体中，以前认为是 PITX2B1 的实际上是 PITX2Cbeta，或者可能是 PITX2B2。PITX2B1 的表达水平比以前认为的要低。PITX2Cbeta 和 PITX2B2 激活促性腺激素特异性基因启动子报告基因类似于已知的 PITX2 异构体。结论我们已经确定并表征了由替代翻译起始（PITX2Cbeta）和替代mRNA剪接（PITX2B2）生成的两种新型PITX2亚型。