Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada.
Mol Cell Biol. 2012 Oct;32(19):3872-90. doi: 10.1128/MCB.00751-12. Epub 2012 Jul 23.
Noonan syndrome (NS) is a relatively common autosomal dominant disorder characterized by congenital heart defects, short stature, and facial dysmorphia. NS is caused by germ line mutations in several components of the RAS-RAF-MEK-extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway, including both kinase-activating and kinase-impaired alleles of RAF1 (∼3 to 5%), which encodes a serine-threonine kinase for MEK1/2. To investigate how kinase-impaired RAF1 mutants cause NS, we generated knock-in mice expressing Raf1(D486N). Raf1(D486N/+) (here D486N/+) female mice exhibited a mild growth defect. Male and female D486N/D486N mice developed concentric cardiac hypertrophy and incompletely penetrant, but severe, growth defects. Remarkably, Mek/Erk activation was enhanced in Raf1(D486N)-expressing cells compared with controls. RAF1(D486N), as well as other kinase-impaired RAF1 mutants, showed increased heterodimerization with BRAF, which was necessary and sufficient to promote increased MEK/ERK activation. Furthermore, kinase-activating RAF1 mutants also required heterodimerization to enhance MEK/ERK activation. Our results suggest that an increased heterodimerization ability is the common pathogenic mechanism for NS-associated RAF1 mutations.
努南综合征（Noonan syndrome，NS）是一种相对常见的常染色体显性遗传病，其特征为先天性心脏缺陷、身材矮小和面部畸形。NS 由 RAS-RAF-MEK-细胞外信号调节激酶（extracellular signal-regulated kinase，ERK）丝裂原活化蛋白激酶（mitogen-activated protein kinase，MAPK）通路中几个成分的种系突变引起，包括 RAF1 的激酶激活和激酶失活等位基因（约 3%~5%），后者编码 MEK1/2 的丝氨酸-苏氨酸激酶。为了研究激酶失活 RAF1 突变体如何引起 NS，我们生成了表达 Raf1(D486N)的基因敲入小鼠。Raf1(D486N/+)（这里表示 D486N/+）雌性小鼠表现出轻度生长缺陷。雄性和雌性 D486N/D486N 小鼠出现同心性心肌肥厚和不完全穿透性但严重的生长缺陷。值得注意的是，与对照相比，Raf1(D486N)表达细胞中的 Mek/Erk 激活增强。与 RAF1(D486N)一样，其他激酶失活 RAF1 突变体与 BRAF 的异二聚化增加，这对于促进 Mek/ERK 激活是必要且充分的。此外，激酶激活 RAF1 突变体也需要异二聚化来增强 Mek/ERK 激活。我们的结果表明，增加的异二聚化能力是 NS 相关 RAF1 突变的共同致病机制。
