Defining the Mechanism of Altered Very Long Chain Fatty Acid Metabolism in Neuro-Ichthyotic Disorders.

极长链脂肪酸-4（VLVL 4）基因的延长突变引起以干燥鳞状皮肤（鱼鳞病）和认知缺陷、癫痫发作和年龄相关的小脑变性为特征的神经鱼鳞病病症。VL 4是介导超长链（≥C28）饱和脂肪酸（VLC-SFA）和多不饱和脂肪酸（VLC-PUFA）（统称为超长链脂肪酸（VLC-FA））生物合成的必需酶。VL 4在特定组织中表达，其中它对VLC-SFA和/或VLC-PUFA生物合成表现出组织特异性选择性。在皮肤、脑和睑板腺中，VLC 4主要产生VLC-SFA，而在视网膜和睾丸中，它产生VLC-PUFA。然而，一种组织产生VLC-SFA和另一种VLC-PUFA的机制，以及这些VLC-FA如何在每种组织中发挥重要的保护作用，仍然是未知的。考虑到VLC 4中的突变影响皮肤和神经元组织的事实，答案必须是不同突变蛋白的固有酶活性或突变蛋白对WT酶合成的VLC-FA的质量和数量的显性负效应。该提议的目的是检查哪些因素决定VLC-SFA生物合成相对于VLC-PUFA的组织特异性选择性，以及突变体VL 4如何改变这些因素以引起不同的组织特异性病症。为了实现他的目标，我们产生了导致脊髓小脑共济失调34（SCA 34）和变异性红斑角皮病（EKV）的第一个Long Evans大鼠模型，该模型具有EKV VL 4突变（c.736T>G，p.W246G）。与野生型对照大鼠相比，杂合和纯合SCA 34大鼠从出生起就出现皮肤损伤，后来显示出人类疾病典型的小脑功能障碍体征19。皮肤VLC-SFA的分析显示，与WT对照相比，纯合KI组织仅具有约33%的总VLC-SFA。这些发现表明，该突变体SCA 34模型中的神经鱼鳞病病理学可能是由皮肤和神经元VLC-FA水平降低引起的，这是VLC 4突变的影响。我们假设，组织特异性因素决定了哪种类型的VLC-FA（饱和与多不饱和），并在SCA 34患者中发现的皮肤和小脑病理缺陷的影响，突变体VL 4酶对野生型VL 4的合成VLC-FA的能力。我们提出了两个具体的目标来检验这一假设。在成功完成所提出的实验后，我们将回答未解决的问题，即不同的突变体VL 2 VL 4蛋白如何影响VLC-FA的生物合成，从而导致神经性鱼鳞病。我们预计，这将使我们能够在未来的研究中寻求实验性治疗方案，这些研究可能会迅速进展到人类临床试验中，用于治疗一些人类突变型VL 4疾病。

Mutations in the Elongation of Very Long Chain Fatty Acids-4 (ELOVL4) gene cause neuro-ichthyotic disorder characterized by dry scaly skin (ichthyosis) and cognitive deficits, seizures and age-related cerebellar degeneration. ELOVL4 is an essential enzyme that mediates biosynthesis of very long chain (≥C28) saturated (VLC-SFA) and polyunsaturated fatty acids (VLC-PUFA) that are collectively called very long chain fatty acids (VLC-FA). ELOVL4 is expressed in specific tissues in which it exhibits tissue-specific selectivity towards VLC- SFA and/or VLC-PUFA biosynthesis. In the skin, brain and Meibomian glands, ELOVL4 makes mainly VLC- SFA, while in the retina and testes it makes VLC-PUFA. However, the mechanisms by which one tissue makes VLC-SFA and the other VLC-PUFA, and how these VLC-FA exert their important protective roles in each tissue, remain unknown. Considering the fact that mutations in ELOVL4 affect skin and neuronal tissues, the answer must be in either the innate enzyme activity of the different mutant proteins or in a dominant negative effect of the mutant proteins on the quality and quantity of VLC-FA synthesized by the WT enzyme. The objective of this proposal is to examine what factors determine the tissue-specific selectivity of VLC-SFA biosynthesis relative to VLC-PUFA and how mutant ELOVL4 alters such factors to cause the different tissue- specific disorders. To achieve his goal, we generated the first Long Evans rat model of ELOVL4 mutation (c.736T>G, p.W246G) that causes Spinocerebellar ataxia 34 (SCA34) with Erythrokeratodermia variabilis (EKV). Compared to wild type control rats, heterozygous and homozygous SCA34 rats developed skin lesions from birth and later showed signs of cerebellar dysfunction typical of the human disease19. Analysis of skin VLC-SFA showed that the homozygous KI tissue had only about 33% of total VLC-SFA compared to WT controls. These findings indicate that the neuro-ichthyotic pathology in this mutant SCA34 model is likely caused by decreased skin and neuronal VLC-FA levels, an effect of the ELOVL4 mutation. We hypothesize that tissue-specific factors determine which type of VLC-FA (saturated versus polyunsaturated) is made and that the skin and cerebellar pathological defects found in SCA34 patients result from effects of the mutant ELOVL4 enzyme on the wild type ELOVL4’s ability to synthesize VLC-FA. We propose two specific aims to test this hypothesis. Upon successful completion of the proposed experiments, we will answer unresolved questions of how the different mutant ELOVL4 proteins affect VLC-FA biosynthesis to cause neuro-ichthyotic disorders. We anticipate this will allow us to pursue experimental treatment options in future studies which could progress rapidly into human clinical trials for treating some of the human mutant ELOVL4 disorders.