Copy-number variation



Copy-number variations (CNVs)—a form of structural variation—are alterations of the DNA of a genome that results in the cell having an abnormal number of copies of one or more sections of the DNA. CNVs correspond to relatively large regions of the genome that have been deleted (fewer than the normal number) or duplicated (more than the normal number) on certain chromosomes. For example, the chromosome that normally has sections in order as  might instead have sections   (a duplication of "C") or   (a deletion of "C").

This variation accounts for roughly 12% of human genomic DNA and each variation may range from about one kilobase (1,000 nucleotide bases) to several megabases in size. CNVs contrast with single-nucleotide polymorphisms (SNPs), which affect only one single nucleotide base.

Identification
Copy number variation can be discovered by cytogenetic techniques such as fluorescent in situ hybridization, comparative genomic hybridization, array comparative genomic hybridization, and by virtual karyotyping with SNP arrays. Recent advances in DNA sequencing technology have further enabled the identification of CNVs by next-generation sequencing.

CNVs can be limited to a single gene or include a contiguous set of genes. CNVs can result in having either too many or too few of the dosage-sensitive genes, which may be responsible for a substantial amount of human phenotypic variability, complex behavioral traits and disease susceptibility.

In certain cases, such as rapidly growing Escherichia coli cells, the gene copy number can be 4-fold greater for genes located near the origin of DNA replication, rather than at the terminus of DNA replication. Elevating the gene copy number of a particular gene can increase the expression of the protein that it encodes.

Prevalence in humans
The fact that DNA copy number variation is a widespread and common phenomenon among humans was first uncovered following the completion of the human genome project. It is estimated that approximately 0.4% of the genomes of unrelated people typically differ with respect to copy number. De novo CNVs have been observed between identical twins who otherwise have identical genomes.

Role in disease
Like other types of genetic variation, some CNVs have been associated with susceptibility or resistance to disease. Gene copy number can be elevated in cancer cells. For instance, the EGFR copy number can be higher than normal in non-small cell lung cancer. In addition, a higher copy number of CCL3L1 has been associated with lower susceptibility to HIV infection, and a low copy number of FCGR3B (the CD16 cell surface immunoglobulin receptor) can increase susceptibility to systemic lupus erythematosus and similar inflammatory autoimmune disorders. Copy number variation has also been associated with autism,  schizophrenia, and idiopathic learning disability.

However, although once touted as the explanation for the elusive hereditary causes of complex diseases like rheumatoid arthritis, the most common CNVs have little or no role in causing disease.

Among common functional CNVs, gene gains outnumber losses, suggesting that many of them are favored in evolution and, therefore, beneficial in some way. One example of CNV is the human salivary amylase gene (AMY1). This gene is typically present as two diploid copies in chimpanzees. Humans average over 6 copies and may have as many as 15. This is thought to be an adaptation to a high-starch diet that improves the ability to digest starchy foods.