Structural variation (also genomic structural variation) is the variation in structure of an organism's chromosome. It consists of many kinds of variation in the genome of one species, and usually includes microscopic and submicroscopic types, such as deletions, duplications, copy-number variants, insertions, inversions and translocations. Typically a structure variation affects a sequence length about 1Kb to 3Mb, which is larger than SNPs and smaller than chromosome abnormality (though the definitions have some overlapping). The definition of structural variation does not imply anything about frequency or phenotypical effects. Many structural variants are associated with genetic diseases, however more are not. Recent research about SVs indicates that SVs are more difficult to detect than SNPs. SNPs always occur in two alleles, while approximately 5% of the human genome are defined as structurally variant in the normal population, involving more than 800 independent genes. Rapidly accumulating evidence indicates that structural variations can comprise millions of nucleotides of heterogeneity within every genome, and are likely to make an important contribution to human diversity and disease susceptibility.
Microscopic structural variation
Microscopic means that it can be detected with optical microscopes, such as aneuploidies, marker chromosome, gross rearrangements and variation in chromosome size. The frequency in human population is thought to be underestimated due to the fact that some of these are not actually easy to identify. These structural abnormalities exist in 1 every 375 live births by putative information.
Copy-number variation (CNV) is a large category of structural variation, which includes insertions, deletions and duplications. In recent studies, copy-number variations are tested on people who do not have genetic diseases, using methods that are used for quantitative SNP genotyping. Results show that 28% of the suspected regions in the individuals actually do contain copy number variations. Also, CNVs in human genome affect more nucleotides than Single Nucleotide Polymorphism (SNP). It is also noteworthy that many of CNVs are not in coding regions. Because CNVs are usually caused by unequal recombination, widespread similar sequences such as LINEs and SINEs may be a common mechanism of CNV creation.
There are several inversions known which are related to human disease. For instance, recurrent 400kb inversion in factor VIII gene is a common cause of haemophilia A, and smaller inversions affecting idunorate 2-sulphatase (IDS) will cause Hunter syndrome. More examples include Angelman syndrome and Sotos syndrome. However, recent research shows that one person can have 56 putative inversions, thus the non-disease inversions are more common than previously supposed. Also in this study it's indicated that inversion breakpoints are commonly associated with segmental duplications. One 900 kb inversion in the chromosome 17 is under positive selection and are predicted to increase its frequency in European population.
Other structural variants
In addition to the most common kinds, there are also cryptic translocations and segmental uniparental disomy (UPD), among others. There are increasing reports of these variations, but are more difficult to detect than traditional variations because these variants are balanced and array-based or PCR-based methods are not able to locate them.
Structural variation and phenotypes
Some genetic diseases are suspected to be caused by structural variations, but the relation is not very certain. It is not plausible to divide these variants into two classes as "normal" or "disease", because the actual output of the same variant will also vary. Also, a few of the variants are actually positively selected for (mentioned above). In 2007, one study, new Copy-Number Variations are identified to play some role in Autism Spectrum Disorder and maybe other mental situations.
Structural variations also have its function in population genetics. Different frequency of a same variation can be used as a genetic mark to infer relationship between populations in different areas. A complete comparison between human and chimpanzee structural variation also suggested that some of these may be fixed in one species because of its adaptative function. There are also deletions related to resistance against malaria and AIDS. Also, some highly variable segments are thought to be caused by balancing selection, but there are also studies against this hypothesis.
Database of structural variation
Some of genome browsers and bioinformatic databases have a list of structural variations in human genome with an emphasis on CNVs, and can show them in the genome browsing page, for example, UCSC Genome Browser. Under the page viewing a part of the genome, there are "Common Cell CNVs" and "Structural Var" which can be enabled. On NCBI, there is a special page  for structural variation. In that system, both "inner" and "outer" coordinates are shown; they are both not actual breakpoints, but surmised minimal and maximum range of sequence affected by the structural variation. The types are classified as insertion, loss, gain, inversion, LOH, everted, transchr and UPD.
Software using Next-generation sequencing data to detect structural variations.
|GROM-RD||CNV||read depth||C||Smith & al.||http://grigoriev.rutgers.edu/software/|
|CNVnator||CNV||read depth||C||Abyzov & al.||http://sv.gersteinlab.org/cnvnator/|
|ForestSV||Structural variant discovery with random forests||aligned paired-end||R||Michaelson & al.||http://sebatlab.ucsd.edu/index.php/software-data|
|Delly||copy number variable deletion
|short insert paired-ends
|C++||Rausch & al.||http://www.korbel.embl.de/software.html|
|ERDS||CNV||Zhu & al.|
|BreakDancer||Chen & al.||http://breakdancer.sourceforge.net/|
|VariationHunter||Hormozdiari & al.||http://compbio.cs.sfu.ca/strvar.htm|
|inGAP-sv||Qi & al.||http://ingap.sourceforge.net/|
|Manta||Large SVs (deletions, duplications, inversions, translocations), large insertions, and medium-sized indels (8bp default minimum indel size)||Paired and split read alignments||C++, Python||https://github.com/Illumina/manta|
- Feuk, Lars; Carson, Andrew R.; Scherer, Stephen W. (2006). "Structural variation in the human genome". Nature Reviews Genetics 7 (2): 85–97. doi:10.1038/nrg1767. PMID 16418744.
- Reich, David E.; Schaffner, Stephen F.; Daly, Mark J.; McVean, Gil; Mullikin, James C.; Higgins, John M.; Richter, Daniel J.; Lander, Eric S.; Altshuler, David (2002). "Human genome sequence variation and the influence of gene history, mutation and recombination". Nature Genetics 32 (1): 135–42. doi:10.1038/ng947. PMID 12161752.
- Gripenberg, Ulla (1964). "Size variation and orientation of the human Y chromosome". Chromosoma 15 (5): 618–29. doi:10.1007/BF00319995. PMID 14333154.
- Wyandt, H. E.; Tonk, V. S. (2004). Atlas of Human Chromosome Heteromorphisms. Netherlands: Kluwer Academic. ISBN 978-90-481-6296-3.[page needed]
- Sebat, J.; Lakshmi, B; Troge, J; Alexander, J; Young, J; Lundin, P; Månér, S; Massa, H; et al. (2004). "Large-Scale Copy Number Polymorphism in the Human Genome". Science 305 (5683): 525–8. doi:10.1126/science.1098918. PMID 15273396.
- Iafrate, A John; Feuk, Lars; Rivera, Miguel N; Listewnik, Marc L; Donahoe, Patricia K; Qi, Ying; Scherer, Stephen W; Lee, Charles (2004). "Detection of large-scale variation in the human genome". Nature Genetics 36 (9): 949–51. doi:10.1038/ng1416. PMID 15286789.
- Lupski, James R. (2010). "Retrotransposition and Structural Variation in the Human Genome". Cell 141 (7): 1110–2. doi:10.1016/j.cell.2010.06.014. PMID 20602993.
- Lam, Hugo YK; Mu, Xinmeng Jasmine; Stutz, Adrian M; Tanzer, Andrea; Cayting, Philip D; Snyder, Michael; Kim, Philip M; Korbel, Jan O; Gerstein, Mark B (2010). "Nucleotide-resolution analysis of structural variants using BreakSeq and a breakpoint library". Nature Biotechnology 28 (1): 47–55. doi:10.1038/nbt.1600. PMC 2951730. PMID 20037582.
- Lakich, Delia; Kazazian, Haig H.; Antonarakis, Stylianos E.; Gitschier, Jane (1993). "Inversions disrupting the factor VIII gene are a common cause of severe haemophilia A". Nature Genetics 5 (3): 236–41. doi:10.1038/ng1193-236. PMID 8275087.
- Bondeson, Maire-Louise; Dahl, Niklas; Malmgren, Helena; Kleijer, Wim J.; Tönnesen, Tönne; Carlberg, Britt-Marie; Pettersson, Ulf (1995). "Inversion of the IDS gene resulting from recombination with IDS-related sequences in a common cause of the Hunter syndrome". Human Molecular Genetics 4 (4): 615–21. doi:10.1093/hmg/4.4.615. PMID 7633410.
- Tuzun, Eray; Sharp, Andrew J; Bailey, Jeffrey A; Kaul, Rajinder; Morrison, V Anne; Pertz, Lisa M; Haugen, Eric; Hayden, Hillary; et al. (2005). "Fine-scale structural variation of the human genome". Nature Genetics 37 (7): 727–32. doi:10.1038/ng1562. PMID 15895083.
- Stefansson, Hreinn; Helgason, Agnar; Thorleifsson, Gudmar; Steinthorsdottir, Valgerdur; Masson, Gisli; Barnard, John; Baker, Adam; Jonasdottir, Aslaug; et al. (2005). "A common inversion under selection in Europeans". Nature Genetics 37 (2): 129–37. doi:10.1038/ng1508. PMID 15654335.
- Marshall, Christian R.; Noor, Abdul; Vincent, John B.; Lionel, Anath C.; Feuk, Lars; Skaug, Jennifer; Shago, Mary; Moessner, Rainald; et al. (2008). "Structural Variation of Chromosomes in Autism Spectrum Disorder". The American Journal of Human Genetics 82 (2): 477–88. doi:10.1016/j.ajhg.2007.12.009. PMC 2426913. PMID 18252227.
- Johnson, Matthew E.; Viggiano, Luigi; Bailey, Jeffrey A.; Abdul-Rauf, Munah; Goodwin, Graham; Rocchi, Mariano; Eichler, Evan E. (2001). "Positive selection of a gene family during the emergence of humans and African apes". Nature 413 (6855): 514–9. doi:10.1038/35097067. PMID 11586358.
- Redon, Richard; Ishikawa, Shumpei; Fitch, Karen R.; Feuk, Lars; Perry, George H.; Andrews, T. Daniel; Fiegler, Heike; Shapero, Michael H.; et al. (2006). "Global variation in copy number in the human genome". Nature 444 (7118): 444–54. doi:10.1038/nature05329. PMC 2669898. PMID 17122850.
- Gonzalez, E.; Kulkarni, H; Bolivar, H; Mangano, A; Sanchez, R; Catano, G; Nibbs, RJ; Freedman, BI; et al. (2005). "The Influence of CCL3L1 Gene-Containing Segmental Duplications on HIV-1/AIDS Susceptibility". Science 307 (5714): 1434–40. doi:10.1126/science.1101160. PMID 15637236.
- Bubb, K. L.; Bovee, D; Buckley, D; Haugen, E; Kibukawa, M; Paddock, M; Palmieri, A; Subramanian, S; et al. (2006). "Scan of Human Genome Reveals No New Loci Under Ancient Balancing Selection". Genetics 173 (4): 2165–77. doi:10.1534/genetics.106.055715. PMC 1569689. PMID 16751668.
- Smith, Sean D; Kawash Joseph K; Grigoriev Andrey (March 2015). "GROM-RD: resolving genomic biases to improve read depth detection of copy number variants". PeerJ 3: e836. doi:10.7717/peerj.836.
- "CNVnator: an approach to discover, genotype, and characterize typical and atypical CNVs from family and population genome sequencing". PMID 21324876.
- Michaelson, Jacob J; Sebat Jonathan (July 2012). "forestSV: structural variant discovery through statistical learning". Nature Methods (9): i819–i821. doi:10.1038/nmeth.2085. PMC 3427657. PMID 22751202.
- Rausch, Tobias; Zichner Thomas; Schlattl Andreas; Stütz Adrian M; Benes Vladimir; Korbel Jan O (September 2012). "DELLY: structural variant discovery by integrated paired-end and split-read analysis". Bioinformatics (England) 28 (18): i333–i339. doi:10.1093/bioinformatics/bts378. PMC 3436805. PMID 22962449.
- Zhu, Mingfu; Need Anna C, Han Yujun, Ge Dongliang, Maia Jessica M, Zhu Qianqian, Heinzen Erin L, Cirulli Elizabeth T, Pelak Kimberly, He Min, Ruzzo Elizabeth K, Gumbs Curtis, Singh Abanish, Feng Sheng, Shianna Kevin V, Goldstein David B (September 2012). "Using ERDS to Infer Copy-Number Variants in High-Coverage Genomes". Am. J. Hum. Genet. (United States) 91 (3): 408–21. doi:10.1016/j.ajhg.2012.07.004. PMC 3511991. PMID 22939633.
- Chen, Ken; Wallis John W, McLellan Michael D, Larson David E, Kalicki Joelle M, Pohl Craig S, McGrath Sean D, Wendl Michael C, Zhang Qunyuan, Locke Devin P, Shi Xiaoqi, Fulton Robert S, Ley Timothy J, Wilson Richard K, Ding Li, Mardis Elaine R (September 2009). "BreakDancer: an algorithm for high-resolution mapping of genomic structural variation". Nat. Methods (United States) 6 (9): 677–81. doi:10.1038/nmeth.1363. PMID 19668202.
- Hormozdiari, Fereydoun; Hajirasouliha Iman; Dao Phuong; Hach Faraz; Yorukoglu Deniz; Alkan Can; Eichler Evan E; Sahinalp S Cenk (June 2010). "Next-generation VariationHunter: combinatorial algorithms for transposon insertion discovery". Bioinformatics (England) 26 (12): i350–7. doi:10.1093/bioinformatics/btq216. PMC 2881400. PMID 20529927.
- Qi, Ji; Fangqing Zhao (June 2011). "inGAP-sv: a novel scheme to identify and visualize structural variation from paired end mapping data". Nucleic Acids Research (England) 39 (Web Server issue): W567–W575. doi:10.1093/nar/gkr506. PMC 3125812. PMID 21715388.