High-Throughput Glycome Analysis Is Set To Join High-Throughput Genomics
G
lycomics involves the comprehensive study of the entire complement of sugars attached to glycoconjugates within an organism or its components. Detailed structural analysis of the glycome complements information derived from genomics and proteomics analyses. This analysis provides a holistic view of molecules and organisms and brings glycobiology into mainstream systems biology. It would be a mistake to continue to view glycosylation as an independent parameter because, clearly, glycosylation processing pathways are integrated into complex, multifactorial systems. Glycosylation is cell-, protein-, and site-specific, and hundreds of genes and proteins are involved in the processing of complex glycans. This argues in favor of strong genetic regulation. A landmark paper1 contains the first comprehensive information regarding heritability of N-glycans released from plasma proteins from a human population of >1000 individuals with defined pedigrees. According to Knezˇevic´ et al., variability in glycan levels is determined to a significant extent by both inherited and environmental factors, as is the case with most complex biological traits. However, the median ratio of maximal to minimal observed values (6.17) indicates exceptional levels of diversity at the level of individuals compared with other quantitative human traits.2 Interestingly, the heritability of glycans and, therefore, the relative importance of their genetic and environmental determinants differed among individual glycan species. This suggests that the levels of some N-glycans in plasma may be more genetically determined than others. Possibly, the inherent variability of other plasma glycans may mask heritability. However, interpreting heritabilities based on familial resemblance involves making a number of assumptions,3 and care should be taken with data analysis. In general, significant heritability implies that the causal genes can be identified, but it does not indicate the number of genes or the sizes of their effects. Even plasma glycans with low heritability may have a substantial genetic component when the environmental influences are also large. Several previous studies have indicated an age-dependent decrease in fucosylation of glycans attached to individual proteins, and for some glycans, gender-age interaction is statistically significant. This is consistent with our understanding that the regulation of glycan levels in humans is very complex and may include effects that are difficult to measure precisely for statistical analysis. This argues for the analysis of large numbers of samples, now possible since the introduction of a 96-well-plate robotics platform with computerized data interpretation.4,5 Glycans display a much higher interspecies variability than proteins. Structural and conformational aspects of glycans are
10.1021/pr900040s
© 2009 American Chemical Society
important; a small change in structure can have important functional consequences, indicating that glycan variability might explain aspects of human phenotypic variability. The findings in Knezˇevic´ et al. have implications for glyco-diagnostic tests. Environmental variables (such as smoking) were associated with plasma glycome components in a complex relationship, and these variables should be taken into account in future studies investigating the relationship between plasma glycan levels and disease and also in gene-association studies. Although glycan analysis is inherently more complicated than protein or DNA sequencing, major improvements in sensitivity now enable sugars to be released and analyzed directly from complex protein mixtures immobilized in 2DE gels. Therefore, many glycoproteins can be analyzed simultaneously. This brings detailed glycan analysis more into line with other highthroughput technologies and opens the way to directly linking proteomic and glycan analytical data with genomics and metabolomics. A major challenge is to discover the mechanisms by which a glycome changes during development and in the disease process. Genome-wide association studies are promising tools for acquiring such an understanding.6 In these investigations, genetic variants underlying complex human diseases are identified, and quantitative traits can be measured with adequate precision. Current advances in glycan analysis and genomics coupled with advanced statistical analysis are paving the way for a deeper understanding of biology because almost every species depends on appropriate glycosylation. PAULINE M. RUDD National Institute for Bioprocessing Research and Training, Dublin-Oxford Glycobiology Laboratory, Conway Institute, University College Dublin IGOR RUDAN Department of Public Health Sciences, University of Edinburgh Medical School (U.K.) ALAN F. WRIGHT MRC Human Genetics Unit, Western General Hospital (U.K.)
References (1) (2) (3) (4) (5) (6)
Knezˇevic´, A.; et al. J. Proteome Res. 2008, DOI 10.1021/pr800737u. Wright, A.; et al. Trends Genet. 2003, 19, 97–106. Visscher, P. M.; et al. Nat. Rev. Genet. 2008, 9, 255–266. Royle, L.; et al. Anal. Biochem. 2008, 76, 1–12. Campbell, M. P.; et al. Bioinformatics 2008, 24, 1214–1216. Altshuler, D.; et al. Science 2008, 322, 881–888.
Journal of Proteome Research • Vol. 8, No. 3, 2009 1105
