Cardiomyopathy in Irx4-Deficient Mice Is Preceded by Abnormal Ventricular Gene Expression

To define the role of Irx4, a member of the Iroquois family of homeobox transcription factors in mammalian heart development and function, we disrupted the murine Irx4 gene. Cardiac morphology in Irx4-deficient mice (designated Irx4(Δex2/Δex2)) was normal during embryogenesis and in early postnatal life. Adult Irx4(Δex2/Δex2) mice developed a cardiomyopathy characterized by cardiac hypertrophy and impaired contractile function. Prior to the development of cardiomyopathy, Irx4(Δex2/Δex2) hearts had abnormal ventricular gene expression: Irx4-deficient embryos exhibited reduced ventricular expression of the basic helix-loop-helix transcription factor eHand (Hand1), increased Irx2 expression, and ventricular induction of an atrial chamber-specific transgene. In neonatal hearts, ventricular expression of atrial natriuretic factor and α-skeletal actin was markedly increased. Several weeks subsequent to these changes in embryonic and neonatal gene expression, increased expression of hypertrophic markers BNP and β-myosin heavy chain accompanied adult-onset cardiac hypertrophy. Cardiac expression of Irx1, Irx2, and Irx5 may partially compensate for loss of Irx4 function. We conclude that Irx4 is not sufficient for ventricular chamber formation but is required for the establishment of some components of a ventricle-specific gene expression program. In the absence of genes under the control of Irx4, ventricular function deteriorates and cardiomyopathy ensues.

为阐明同源盒转录因子Iroquois家族（Iroquois family of homeobox transcription factors）成员Irx4在哺乳动物心脏发育与功能中的作用，我们敲除了小鼠Irx4基因。Irx4缺陷小鼠（命名为Irx4(Δex2/Δex2)）的心脏形态在胚胎发生期及出生后早期均处于正常状态，成年个体则出现以心肌肥厚与收缩功能受损为特征的心肌病。在心肌病发生前，Irx4(Δex2/Δex2)小鼠心脏已出现心室基因表达异常：Irx4缺陷胚胎的碱性螺旋-环-螺旋（basic helix-loop-helix）转录因子eHand（Hand1）心室表达水平降低，Irx2表达水平升高，且心房腔特异性转基因在心室中出现异位诱导表达；在新生小鼠心脏中，心房利钠因子（atrial natriuretic factor）与α-骨骼肌肌动蛋白的心室表达水平显著升高。在胚胎及新生期的基因表达改变发生数周后，肥厚标志物脑钠肽（BNP）与β-肌球蛋白重链（β-myosin heavy chain）的表达上调伴随成年迟发性心肌肥厚。Irx1、Irx2与Irx5的心脏表达可部分代偿Irx4功能缺失的影响。本研究表明，Irx4并非心室腔形成的充分条件，但却是心室特异性基因表达程序部分组分建立所必需的；当Irx4调控的基因缺失时，心室功能会发生衰退并最终引发心肌病。