Digenic inheritance of mutations in EPHA2 and SLC26A4 in Pendred syndrome

Enlarged vestibular aqueducts (EVA) is one of the most commonly identified inner ear malformations in hearing loss patients including Pendred syndrome. While biallelic mutations of the SLC26A4 gene, encoding pendrin, causes non-syndromic hearing loss with EVA or Pendred syndrome, a considerable number of patients appear to carry mono-allelic mutation. This suggests faulty pendrin regulatory machinery results in hearing loss. Here we identify EPHA2 as another causative gene of Pendred syndrome with SLC26A4. EphA2 forms a protein complex with pendrin controlling pendrin localization, which is disrupted in some pathogenic forms of pendrin. Moreover, point mutations leading to amino acid substitution in the EPHA2 gene are identified from patients bearing mono-allelic mutation of SLC26A4. Ephrin-B2 binds to EphA2 triggering internalization with pendrin inducing EphA2 autophosphorylation weakly. The identified EphA2 mutants attenuate ephrin-B2- but not ephrin-A1-induced EphA2 internalization with pendrin. Our results uncover an unexpected role of the Eph/ephrin system in epithelial function.

扩大前庭导水管（Enlarged vestibular aqueducts, EVA）是听力损失患者中最常被检出的内耳畸形之一，此类患者可伴发彭德莱综合征（Pendred syndrome）。编码潘定蛋白（pendrin）的SLC26A4基因发生双等位基因突变时，可引发伴EVA的非综合征性听力损失或彭德莱综合征；但相当比例的患者仅携带单等位基因突变，这提示潘定蛋白的调控机制异常可导致听力损失。本研究鉴定出EPHA2是与SLC26A4并列的彭德莱综合征另一致病基因。EphA2可与潘定蛋白形成蛋白质复合物，调控潘定蛋白的细胞定位，而部分致病型潘定蛋白会破坏这一相互作用。此外，在携带SLC26A4单等位基因突变的患者中，我们检出了EPHA2基因中导致氨基酸替换的点突变。Ephrin-B2与EphA2结合后，可触发其与潘定蛋白一同发生内吞，并微弱诱导EphA2的自磷酸化。本研究鉴定出的EphA2突变体可减弱ephrin-B2介导的EphA2与潘定蛋白共同内吞过程，但对ephrin-A1介导的该过程无影响。本研究结果揭示了Eph/ephrin信号系统在上皮功能中此前未被发现的作用。