Department of Chemistry and Biomolecular Sciences and ARC center of excellence in Bioinformatics, Macquarie University, NSW 2109, Australia.
BMC Genomics. 2009 Dec 3;10 Suppl 3(Suppl 3):S33. doi: 10.1186/1471-2164-10-S3-S33.
Lysosomal alpha-mannosidase is an enzyme that acts to degrade N-linked oligosaccharides and hence plays an important role in mannose metabolism in humans and other mammalian species, especially livestock. Mutations in the gene (MAN2B1) encoding lysosomal alpha-D-mannosidase cause improper coding, resulting in dysfunctional or non-functional protein, causing the disease alpha-mannosidosis. Mapping disease mutations to the structure of the protein can help in understanding the functional consequences of these mutations and thus indirectly, the finer aspects of the pathology and clinical manifestations of the disease, including phenotypic severity as a function of the genotype.
A comprehensive homology modeling study of all the wild-type and inherited mutations of lysosomal alpha-mannosidase in four different species, human, cow, cat and guinea pig, reveals a significant correlation between the severity of the genotype and the phenotype in alpha-mannosidosis. We used the X-ray crystallographic structure of bovine lysosomal alpha-mannosidase as template, containing only two disulphide bonds and some ligands, to build structural models of wild-type structures with four disulfide linkages and all bound ligands. These wild-type models were then used as templates for disease mutations. All the truncations and substitutions involving the residues in and around the active site and those that destabilize the fold led to severe genotypes resulting in lethal phenotypes, whereas the mutations lying away from the active site were milder in both their genotypic and phenotypic expression.
Based on the co-location of mutations from different organisms and their proximity to the enzyme active site, we have extrapolated observed mutations from one species to homologous positions in other organisms, as a predictive approach for detecting likely alpha-mannosidosis. Besides predicting new disease mutations, this approach also provides a way for detecting mutation hotspots in the gene, where novel mutations could be implicated in disease. The current study has identified five mutational hot-spot regions along the MAN2B1 gene. Structural mapping can thus provide a rational approach for predicting the phenotype of a disease, based on observed genotypic variations.
溶酶体α-甘露糖苷酶是一种能够降解 N-连接寡糖的酶，因此在人类和其他哺乳动物物种（尤其是家畜）的甘露糖代谢中起着重要作用。编码溶酶体α-D-甘露糖苷酶的基因 (MAN2B1) 中的突变导致编码错误，导致功能失调或无功能的蛋白质，从而引起α-甘露糖苷贮积症。将疾病突变映射到蛋白质结构上可以帮助理解这些突变的功能后果，从而间接地了解疾病的病理学和临床表现的更细微方面，包括表型严重程度与基因型的关系。
对来自 4 种不同物种（人、牛、猫和豚鼠）的溶酶体α-甘露糖苷酶的所有野生型和遗传性突变进行全面同源建模研究，揭示了α-甘露糖苷贮积症中基因型和表型严重程度之间存在显著相关性。我们使用含有仅两个二硫键和一些配体的牛溶酶体α-甘露糖苷酶的 X 射线晶体结构作为模板，构建了具有四个二硫键和所有结合配体的野生型结构的结构模型。然后，将这些野生型模型用作疾病突变的模板。所有涉及活性部位及其周围残基的截断和取代以及那些使折叠不稳定的突变都会导致严重的基因型，从而导致致命的表型，而远离活性部位的突变在基因型和表型表达上都较为温和。
基于不同生物体的突变的共定位及其与酶活性部位的接近程度，我们从一种生物体推断出同源位置的观察到的突变，作为一种预测方法来检测可能的α-甘露糖苷贮积症。除了预测新的疾病突变外，这种方法还为检测基因中的突变热点提供了一种方法，其中新的突变可能与疾病有关。目前的研究已经确定了 MAN2B1 基因中的五个突变热点区域。因此，结构映射可以为基于观察到的基因型变化预测疾病的表型提供一种合理的方法。
