Characterising Structural Variation by Means of Next‐Generation Sequencing
Authors: Ku Chee Seng , Naidoo Nasheen , Teo Shu Mei , Pawitan Yudi 
Published: February 2011

Abstract

A new era of copy number variants (CNVs) discovery began when two separate studies, published concurrently in 2004, identified several hundred deletions and duplications in the human genome. Over the past several years, most of the CNV data were generated by microarrays. These methods have several shortcomings, such as the inability to detect copy‐neutral variants (e.g. inversions and translocations), limited sensitivity to detect smaller CNVs and poor resolution in determining CNV breakpoints especially with lower resolution microarrays. A paradigm shift in the discovery of copy‐neutral variants was attributed to the development of a sequencing‐based method known as paired‐end mapping. This method was first demonstrated to be powerful in detecting structural variants using next‐generation sequencing technologies in 2007. Further studies have also leveraged an important feature of sequencing data, where several hundred million short sequence reads are produced by next‐generation sequencers, to detect CNVs based on the abundance or density of the sequence reads aligned to a reference genome. This approach is known as depth‐of‐coverage. These emerging sequencing‐based methods will continue playing an important role in the discovery of structural variants until

de novo

genome assembly becomes more feasible.

Key Concepts:

A new era of copy number variants (CNVs) discovery began when two separate studies, published concurrently in 2004, identified several hundred deletions and duplications in the human genome.

Both the sample size and the resolution of microarray are critical factors in determining the discovery of less common and smaller CNVs.

‘Human Genetic Variation’ was recognised as the ‘Breakthrough of The Year’ in 2007 by the journal
Science
.

Other types of chromosomal rearrangements, particularly inversions and balanced translocations, have received relatively less attention.

Inversions and translocations are also known as ‘copy‐neutral variants’ or ‘balanced chromosomal rearrangements’ and do not involve changes in copies number.

Collectively these copy number and copy‐neutral variants are broadly classified as ‘structural variants’.

The paradigm shift in the discovery of copy‐neutral variants was attributed to the development of the paired‐end mapping (PEM) method and concurrent advances in next‐generation sequencing technologies.

Further studies have also leveraged on an important feature of next‐generation sequencing data where several hundred million short sequence reads are produced to detect CNVs based on the density of the sequence reads aligning to the reference genome, and this approach is known as depth‐of‐coverage (DOC).

Although the PEM and DOC methods have overcome the major shortcomings of microarrays in detecting structural variants, these methods have their own weaknesses.

The emerging sequencing‐based methods (PEM and DOC) will continue to play a role in the discovery of structural variants until

de novo

genome assembly is more feasible