The evolution of molecular ruler analysis for characterizing telomere imbalances: from fluorescence in situ hybridization to array comparative genomic hybridization

Molecular cytogenetic analysis of telomere rearrangements

Genomic imbalances involving the telomeric regions of human chromosomes, which contain the highest gene concentration in the genome, are proposed to have severe phenotypic consequences. For this reason, it is important to identify telomere rearrangements and assess their contribution to human pathology. This unit describes the structure and function of human telomeres and outlines several methodologies that can be employed to study these unique regions of human chromosomes. It is a revision of the original version of the unit published in 2000, now including an introductory section describing advances in the discipline that have taken place since the original publication.

Structural variation in subtelomeres

Subtelomeres are an incredibly dynamic part of the human genome located at the ends of chromosomes just proximal to telomere repeats. Although subtelomeric variation contributes to normal polymorphism in the human genome and is a by-product of rapid evolution in these regions, rearrangements in subtelomeres can also cause intellectual disabilities and birth defects, making robust methods of detecting copy number variation in chromosome ends a must for cytogenetics labs. In recent years, methods for detecting structural variation in subtelomeres have moved from fluorescence in situ hybridization (FISH) to array technology; however, FISH is still necessary to determine the chromosomal structure of subtelomeric gains and losses identified by arrays.

Diverse mutational mechanisms cause pathogenic subtelomeric rearrangements

Chromosome rearrangements are a significant cause of intellectual disability and birth defects. Subtelomeric rearrangements, including deletions, duplications and translocations of chromosome ends, were first discovered over 40 years ago and are now recognized as being responsible for several genetic syndromes. Unlike the deletions and duplications that cause some genomic disorders, subtelomeric rearrangements do not typically have recurrent breakpoints and involve many different chromosome ends. To capture the molecular mechanisms responsible for this heterogeneous class of chromosome abnormality, we coupled high-resolution array CGH with breakpoint junction sequencing of a diverse collection of subtelomeric rearrangements. We analyzed 102 breakpoints corresponding to 78 rearrangements involving 28 chromosome ends. Sequencing 21 breakpoint junctions revealed signatures of non-homologous end-joining, non-allelic homologous recombination between interspersed repeats and DNA replication processes. Thus, subtelomeric rearrangements arise from diverse mutational mechanisms. In addition, we find hotspots of subtelomeric breakage at the end of chromosomes 9q and 22q; these sites may correspond to genomic regions that are particularly susceptible to double-strand breaks. Finally, fine-mapping the smallest subtelomeric rearrangements has narrowed the critical regions for some chromosomal disorders.

Microarray-Based Genomic DNA Profiling Technologies in Clinical Molecular Diagnostics

Background: Microarray-based genomic DNA profiling (MGDP) technologies are rapidly moving from translational research to clinical diagnostics and have revolutionized medical practices. Such technologies have shown great advantages in detecting genomic imbalances associated with genomic disorders and single-gene diseases.

Content: We discuss the development and applications of the major array platforms that are being used in both academic and commercial laboratories. Although no standardized platform is expected to emerge soon, comprehensive oligonucleotide microarray platforms—both comparative genomic hybridization arrays and genotyping hybrid arrays—are rapidly becoming the methods of choice for their demonstrated analytical validity in detecting genomic imbalances, for their flexibility in incorporating customized designs and updates, and for the advantage of being easily manufactured. Copy number variants (CNVs), the form of genomic deletions/duplications detected through MGDP, are a common etiology for a variety of clinical phenotypes. The widespread distribution of CNVs poses great challenges in interpretation. A broad survey of CNVs in the healthy population, combined with the data accumulated from the patient population in clinical laboratories, will provide a better understanding of the nature of CNVs and enhance the power of identifying genetic risk factors for medical conditions.

Summary: MGDP technologies for molecular diagnostics are still at an early stage but are rapidly evolving. We are in the process of extensive clinical validation and utility evaluation of different array designs and technical platforms. CNVs of currently unknown importance will be a rich source of novel discoveries.

Identification of chromosome abnormalities in subtelomeric regions by microarray analysis: a study of 5,380 cases

Subtelomeric imbalances are a significant cause of congenital disorders. Screening for these abnormalities has traditionally utilized GTG-banding analysis, fluorescence in situ hybridization (FISH) assays, and multiplex ligation-dependent probe amplification. Microarray-based comparative genomic hybridization (array-CGH) is a relatively new technology that can identify microscopic and submicroscopic chromosomal imbalances. It has been proposed that an array with extended coverage at subtelomeric regions could characterize subtelomeric aberrations more efficiently in a single experiment. The targeted arrays for chromosome microarray analysis (CMA), developed by Baylor College of Medicine, have on average 12 BAC/PAC clones covering 10 Mb of each of the 41 subtelomeric regions. We screened 5,380 consecutive clinical patients using CMA. The most common reasons for referral included developmental delay (DD), and/or mental retardation (MR), dysmorphic features (DF), multiple congenital anomalies (MCA), seizure disorders (SD), and autistic, or other behavioral abnormalities. We found pathogenic rearrangements at subtelomeric regions in 236 patients (4.4%). Among these patients, 103 had a deletion, 58 had a duplication, 44 had an unbalanced translocation, and 31 had a complex rearrangement. The detection rates varied among patients with a normal karyotype analysis (2.98%), with an abnormal karyotype analysis (43.4%), and with an unavailable or no karyotype analysis (3.16%). Six patients out of 278 with a prior normal subtelomere-FISH analysis showed an abnormality including an interstitial deletion, two terminal deletions, two interstitial duplications, and a terminal duplication. In conclusion, genomic imbalances at subtelomeric regions contribute significantly to congenital disorders. Targeted array-CGH with extended coverage (up to 10 Mb) of subtelomeric regions will enhance the detection of subtelomeric imbalances, especially for submicroscopic imbalances.

Enhanced detection of clinically relevant genomic imbalances using a targeted plus whole genome oligonucleotide microarray

PURPOSE

Array comparative genomic hybridization is rapidly becoming an integral part of cytogenetic diagnostics. We report the design, validation, and clinical utility of an oligonucleotide array which combines genome-wide coverage with targeted enhancement at known clinically relevant regions.

METHODS

Probes were placed every 75 kb across the entire euchromatic genome to establish a chromosomal "backbone" with a resolution of approximately 500 kb, which is increased to approximately 50 kb in targeted regions.

RESULTS

For validation, 30 samples showed 100% concordance with previous G-banding and/or fluorescence in situ hybridization results. Prospective array analysis of 211 clinical samples identified 33 (15.6%) cases with clinically significant abnormalities. Of these, 23 (10.9%) were detected by the "targeted" coverage and 10 (4.7%) by the genome-wide coverage (average size of 3.7 Mb). All abnormalities were verified by fluorescence in situ hybridization, using commercially available or homebrew probes using the 32K bacterial artificial chromosome set. Four (1.9%) cases had previously reported imbalances of uncertain clinical significance. Five (2.4%) cases required parental studies for interpretation and all were benign familial variants.

CONCLUSIONS

Our results highlight the enhanced diagnostic utility of a genome-wide plus targeted array design, as the use of only a targeted array would have failed to detect 4.7% of the clinically relevant imbalances.