The pathophysiology of SBP2 abnormalities

人类硒蛋白组包含至少25种硒蛋白。大多数是硒酶，需要硒半胱氨酸（Sec）形式的硒（Se）来维持适当的酶活性。一些作为抗氧化剂或氧化还原酶[谷胱甘肽过氧化物酶（GPx）和硫氧还蛋白还原酶]，参与甲状腺激素代谢（去碘酶），硒运输、储存和输送到大脑（SePP）和精子活力（PHGPx）。Sec结合的机制编码UGA密码子，需要在所有硒蛋白的mRNA中存在顺式作用序列，帧内UGA和Sec插入序列（SECIS），而Sec特异性tRNASec和SECIS结合蛋白（SECISBP2或SBP2）是一些必需的反式作用因子。SBP2被认为是Sec结合的主要决定因素，因为它的免疫缺失消除了Sec的结合。硒蛋白的表达受硒缺乏的不同影响，在硒蛋白的合成过程中存在一个尚不清楚但明显的层次结构，并且SBP2优先识别SECIS。随着SBP2基因突变的报道，硒蛋白合成受损的后果的重要医学线索变得明显，SBP2基因突变导致几个家庭的儿童部分缺乏SBP2。受影响的受试者表现为短暂的生长迟缓和甲状腺功能异常，这是由于脱碘酶缺乏导致甲状腺激素代谢改变所致。SBP2基因突变的新报道描述了其他特征，其中一例为先天性肌病和发育迟缓的复杂表型，另一例为无精子症和感音神经性听力损失，因此反映了多重硒蛋白缺乏。这种新的遗传缺陷需要动物模型来回答由人类表型引起的许多问题。为了满足这一需求，我设计了一个研究计划，利用重组技术生成Sbp2缺乏的小鼠模型。由于在体外没有发现SBP2功能的冗余，并且SBP2缺乏被认为是致命的，因此我将针对在严重表型患者中报道的C端突变设计Sbp2KI小鼠，并设计可诱导的Sbp2KO小鼠。无限制地进入组织将有助于区分体内不同的调节层和硒蛋白层次。对这些小鼠的初步研究将揭示甲状腺表型、不育、生长迟缓的潜在机制，体内c端功能域的重要性，以及肌病的具体原因。对衰老动物的研究将允许密切监测生命后期的表现，以及其他假定的表型，包括癌症、神经退行性疾病和免疫功能障碍。这些研究涉及多种生理功能和途径，并将有助于阐明硒蛋白介导的病理机制。最终，这些动物模型将使体内测试各种具有潜在治疗特性的化合物成为可能，从而使该模型成为必要的工具。

DESCRIPTION (provided by applicant): Project Summary The human selenoproteome comprises at least 25 selenoproteins. The majority are selenoenzymes that require selenium (Se) in the form of Selenocystein (Sec) for proper enzymatic activity. Some serve as antioxidants or oxido-reductases [glutathione peroxidases (GPx) and thioredoxin reductases], in thyroid hormone metabolism (deiodinases), in Se transport, storage, and delivery to the brain (SePP) and in sperm viability (PHGPx). The machinery for Sec incorporation recodes the UGA codon and requires cis-acting sequences present in the mRNA of all selenoproteins, the in frame UGA and the Sec insertion sequence (SECIS), while Sec-specific tRNASec, and SECIS-binding protein (SECISBP2 or SBP2) are some of the required trans-acting factors. SBP2 is believed to be the major determinant of Sec incorporation as its immunodepletion eliminates Sec incorporation. A not well understood but distinct hierarchy exists in the synthesis of selenoproteins as their expression is differentially affected by Se deficiency, and preferential SECIS recognition by SBP2 was demonstrated. Important medical clues to the consequences of impaired selenoprotein synthesis became apparent with the report of mutations in the SBP2 gene, causing partial SBP2 deficiency in children of several families. Affected subjects presented with transient growth delay and abnormal thyroid function tests resulting from altered thyroid hormone metabolism due to deficiency in the deiodinases. New reports of SBP2 gene mutations describe additional features, a complex phenotype with congenital myopathy and developmental delay in one case, and azoospermia, sensorineural hearing loss in another, thus reflecting multiple selenoprotein deficiencies. Animal models for this new genetic defect are required to answer the many questions raised by the human phenotype. To address this need, I designed a research plan to generate mouse models with Sbp2 deficiency, using recombineering techniques. As redundancy in SBP2 function was not found in-vitro and lack of SBP2 is putatively lethal, I will engineer a Sbp2KI mouse for a C- terminus mutation reported in a patient with a severe phenotype, and an inducible Sbp2KO mouse. The unlimited access to tissues will help distinguish in-vivo the different layers of regulation and selenoprotein hierarchy. Initial investigations of these mice will uncover the underlying mechanisms for the thyroid phenotype, infertility, growth delay, the importance of the putative C-terminal functional domain in-vivo, and the specific cause for the myopathy. The study of the ageing animals will allow close monitoring for manifestations later in life, and other presumed phenotypes, including cancer, neurodegenerative disorders and immune dysfunction. These investigations are relevant to multiple physiological functions and pathways, and will help elucidate the mechanisms underlying selenoprotein-mediated pathology. Ultimately, these animal models will make possible in-vivo testing of various compounds with potential therapeutic properties applicable to humans, thus making this model a necessary tool. PUBLIC HEALTH RELEVANCE: A unique insight into selenoprotein biology was provided by the recent identification of SBP2 gene mutations in humans. The different layers of selenoprotein regulation and hierarchy will be defined by studying the phenotype of an Sbp2 gene knock-in mouse model and that of temporally controlled and tissue specific Sbp2 gene knock-out mouse. These investigations are relevant to multiple physiological functions and pathways, and will help elucidate the mechanisms underlying selenoprotein-mediated pathology