Design considerations for array CGH to oligonucleotide arrays

BACKGROUND

Representational oligonucleotide microarray analysis has been developed for detection of single nucleotide polymorphisms and/or for genome copy number changes. In this process, the intensity of hybridization to oligonucleotides arrays is increased by hybridizing a polymerase chain reaction (PCR)-amplified representation of reduced genomic complexity. However, hybridization to some oligonucleotides is not sufficiently high to allow precise analysis of that portion of the genome.

METHODS

In an effort to identify aspects of oligonucleotide hybridization affecting signal intensity, we explored the importance of the PCR product strand to which each oligonucleotide is homologous and the sequence of the array oligonucleotides. We accomplished this by hybridizing multiple PCR-amplified products to oligonucleotide arrays carrying two sense and two antisense 50-mer oligonucleotides for each PCR amplicon.

RESULTS

In some cases, hybridization intensity depended more strongly on the PCR amplicon strand (i.e., sense vs. antisense) than on the detection oligonucleotide sequence. In other cases, the oligonucleotide sequence seemed to dominate.

CONCLUSION

Oligonucleotide arrays for analysis of DNA copy number or for single nucleotide polymorphism content should be designed to carry probes to sense and antisense strands of each PCR amplicon to ensure sufficient hybridization and signal intensity.

Comprehensive genome sequence analysis of a breast cancer amplicon

Gene amplification occurs in most solid tumors and is associated with poor prognosis. Amplification of 20q13.2 is common to several tumor types including breast cancer. The 1 Mb of sequence spanning the 20q13.2 breast cancer amplicon is one of the most exhaustively studied segments of the human genome. These studies have included amplicon mapping by comparative genomic hybridization (CGH), fluorescent in-situ hybridization (FISH), array-CGH, quantitative microsatellite analysis (QUMA), and functional genomic studies. Together these studies revealed a complex amplicon structure suggesting the presence of at least two driver genes in some tumors. One of these, ZNF217, is capable of immortalizing human mammary epithelial cells (HMEC) when overexpressed. In addition, we now report the sequencing of this region in human and mouse, and on quantitative expression studies in tumors. Amplicon localization now is straightforward and the availability of human and mouse genomic sequence facilitates their functional analysis. However, comprehensive annotation of megabase-scale regions requires integration of vast amounts of information. We present a system for integrative analysis and demonstrate its utility on 1.2 Mb of sequence spanning the 20q13.2 breast cancer amplicon and 865 kb of syntenic murine sequence. We integrate tumor genome copy number measurements with exhaustive genome landscape mapping, showing that amplicon boundaries are associated with maxima in repetitive element density and a region of evolutionary instability. This integration of comprehensive sequence annotation, quantitative expression analysis, and tumor amplicon boundaries provide evidence for an additional driver gene prefoldin 4 (PFDN4), coregulated genes, conserved noncoding regions, and associate repetitive elements with regions of genomic instability at this locus.

Quantitative mapping of amplicon structure by array CGH identifies CYP24 as a candidate oncogene

We show here that quantitative measurement of DNA copy number across amplified regions using array comparative genomic hybridization (CGH) may facilitate oncogene identification by providing precise information on the locations of both amplicon boundaries and amplification maxima. Using this analytical capability, we resolved two regions of amplification within an approximately 2-Mb region of recurrent aberration at 20q13.2 in breast cancer. The putative oncogene ZNF217 (ref. 5) mapped to one peak, and CYP24 (encoding vitamin D 24 hydroxylase), whose overexpression is likely to lead to abrogation of growth control mediated by vitamin D, mapped to the other.

High resolution analysis of DNA copy number variation using comparative genomic hybridization to microarrays

Gene dosage variations occur in many diseases. In cancer, deletions and copy number increases contribute to alterations in the expression of tumour-suppressor genes and oncogenes, respectively. Developmental abnormalities, such as Down, Prader Willi, Angelman and Cri du Chat syndromes, result from gain or loss of one copy of a chromosome or chromosomal region. Thus, detection and mapping of copy number abnormalities provide an approach for associating aberrations with disease phenotype and for localizing critical genes. Comparative genomic hybridization (CGH) was developed for genome-wide analysis of DNA sequence copy number in a single experiment. In CGH, differentially labelled total genomic DNA from a 'test' and a 'reference' cell population are cohybridized to normal metaphase chromosomes, using blocking DNA to suppress signals from repetitive sequences. The resulting ratio of the fluorescence intensities at a location on the 'cytogenetic map', provided by the chromosomes, is approximately proportional to the ratio of the copy numbers of the corresponding DNA sequences in the test and reference genomes. CGH has been broadly applied to human and mouse malignancies. The use of metaphase chromosomes, however, limits detection of events involving small regions (of less than 20 Mb) of the genome, resolution of closely spaced aberrations and linking ratio changes to genomic/genetic markers. Therefore, more laborious locus-by-locus techniques have been required for higher resolution studies. Hybridization to an array of mapped sequences instead of metaphase chromosomes could overcome the limitations of conventional CGH (ref. 6) if adequate performance could be achieved. Copy number would be related to the test/reference fluorescence ratio on the array targets, and genomic resolution could be determined by the map distance between the targets, or by the length of the cloned DNA segments. We describe here our implementation of array CGH. We demonstrate its ability to measure copy number with high precision in the human genome, and to analyse clinical specimens by obtaining new information on chromosome 20 aberrations in breast cancer.

Positional cloning of ZNF217 and NABC1: genes amplified at 20q13.2 and overexpressed in breast carcinoma

We report here the molecular cloning of an approximately 1-Mb region of recurrent amplification at 20q13.2 in breast cancer and other tumors and the delineation of a 260-kb common region of amplification. Analysis of the 1-Mb region produced evidence for five genes, ZNF217, ZNF218, and NABC1, PIC1L (PIC1-like), CYP24, and a pseudogene CRP (Cyclophillin Related Pseudogene). ZNF217 and NABC1 emerged as strong candidate oncogenes and were characterized in detail. NABC1 is predicted to encode a 585-aa protein of unknown function and is overexpressed in most but not all breast cancer cell lines in which it was amplified. ZNF217 is centrally located in the 260-kb common region of amplification, transcribed in multiple normal tissues, and overexpressed in all cell lines and tumors in which it is amplified and in two in which it is not. ZNF217 is predicted to encode alternately spliced, Kruppel-like transcription factors of 1,062 and 1,108 aa, each having a DNA-binding domain (eight C2H2 zinc fingers) and a proline-rich transcription activation domain.

Independent amplification and frequent co-amplification of three nonsyntenic regions on the long arm of chromosome 20 in human breast cancer

DNA amplification at 20q13.2 is common in breast cancer, correlates with poor prognosis, and may reflect location of an important oncogene. Recently, other regions along 20q were also found to undergo amplification. Here, amplification levels and patterns of co-amplification were analyzed by interphase fluorescence in situ hybridization at 14 loci along 20q in 146 uncultured breast carcinomas and 14 cell lines. Three regions were independently amplified in uncultured tumors: RMC20C001 region at 20q13.2 (highly amplified in 9.6% of the cases), PTPN1 region 3 Mb proximal (6.2%), and AIB3 region at 20q11 (6.2%). Co-amplifications involving two or three of these regions were seen in 11 of the 19 highly amplified tumors. The results suggest that three distinct nonsyntenic regions along 20q may be important and that complex chromosomal rearrangements underlie their frequent co-amplification in breast cancer.

Amplification of chromosomal region 20q13 in invasive breast cancer: prognostic implications

Amplification of the chromosome 20q13 region was recently discovered in breast cancer by comparative genomic hybridization and subsequently further defined by fluorescence in situ hybridization with specific probes. The target gene of the amplification remains unknown. Here, fluorescence in situ hybridization with a cosmid probe for the minimal region of amplification (RMC20C001) was used to study 20q13 amplification in 132 primary breast carcinomas and 11 metastases. The size of the amplicon was studied with four flanking probes. Thirty-eight (29%) primary tumors and 3 (27%) metastases showed increased copy number of the RMC20C001 probe (>1.5-fold relative to the p-arm control). Nine (6.8%) of the primary tumors were highly (>3-fold) amplified. Although the size and location of the amplified region varied from one tumor to another, only the RMC20C001 probe was consistently amplified. 20q13 amplification was significantly associated with a high histological grade (P = 0.01), DNA aneuploidy (P = 0.01), and high S-phase fraction (P = 0.0085). High-level amplification was also associated with short disease-free survival of patients with node-negative breast cancer (P = 0.002). We conclude that high-level 20q13 amplification may be an indicator of poor clinical outcome in node-negative breast cancer and that this chromosomal region is likely to contain a gene with an important role in breast cancer progression. A large definitive study is warranted to assess the independent prognostic value of 20q13 amplification.

A physical map of chromosome 20 established using fluorescence in situ hybridization and digital image analysis

The physical locations of 46 cosmid clones and 21 P1 clones were determined along the chromosome 20 axis relative to the p terminus (FLpter) using fluorescence in situ hybridization (FISH) and digital image microscopy. The cosmid clones were selected from the chromosomally enriched library LA20NC01. Nine P1 clones were selected from a pooled DuPont genomic library using PCR with primer pairs selected to amplify genetically mapped sequence-tagged sites. This information was used to relate the physical map to the genetic map. Twelve P1 clones were selected from the same library using PCR primer pairs that amplified known genes. Two of these, E2F and BCLX, had not been mapped previously.

Increased copy number at 20q13 in breast cancer: defining the critical region and exclusion of candidate genes

Studies by comparative genomic hybridization have indicated that a major new locus for DNA amplification in breast cancer is 20q13 and suggested that this genetic event is associated with aggressive clinical behavior. We used interphase fluorescence in situ hybridization with anonymous cosmid probes and gene-specific P1 clones to determine the minimal common region of increased copy number and to study involvement of known genes at 20q13. Based on high-level copy number increases (3 to 10-fold) found with one or more probes in 5 of 14 (35%) breast cancer cell lines and in 3 of 36 (8%) primary tumors, the critical region was narrowed to approximately 1.5 megabases at 20q13.2 defined by fractional length pter values 0.81-0.84. Previously known genes were excluded as candidates, implying that this chromosomal region harbors a novel oncogene that contributes to the malignant progression of breast cancer.

The human beta-subunit of rod photoreceptor cGMP phosphodiesterase: complete retinal cDNA sequence and evidence for expression in brain

We have identified and sequenced cDNA clones that encode for the human beta-subunit of rod cGMP phosphodiesterase (PDEB). A single 2565-bp open reading frame that codes for an 854-amino-acid protein was identified. The human beta-subunit protein is 90% identical to the bovine beta-subunit and 91% identical to the mouse protein. Northern blot analysis indicates that the gene is expressed as an abundant 3.5-kb transcript in retina and as a rare 2.9-kb transcript in brain. The isolation of cDNAs from human brain cDNA libraries confirms the brain as a site of expression for this gene. The molecular defect underlying retinal degeneration in the rd mouse has been found to be a nonsense mutation in the beta-subunit of the mouse cGMP PDE, resulting in a truncated protein (Pittler et al., 1991b, Proc. Natl. Acad. Sci. USA. 88: 8322-8326). The molecular cloning of the cDNA encoding for the PDEB represents the first step in establishing whether this gene plays a causative role in any one of the several human hereditary retinopathies or, based on its localization to chromosome 4p 16.3, in the pathogenesis of Huntington disease.

Isolation and characterization of new highly polymorphic DNA markers from the Huntington disease region

The defect causing Huntington disease (HD) has been mapped to 4p16.3, distal to the DNA marker D4S10. Subsequently, additional polymorphic markers closer to the HD gene have been isolated, which has led to the establishment of predictive testing programs for individuals at risk for HD. Approximately 17% of persons presenting to the Canadian collaborative study for predictive testing for HD have not received any modification of risk, in part because of limited informativeness of currently available DNA markers. Therefore, more highly polymorphic DNA markers are needed, which will further increase the accuracy and availability of predictive testing, specifically for families with complex or incomplete pedigree structures. In addition, new markers are urgently needed in order to refine the breakpoints in the few known recombinant HD chromosomes, which could allow a more accurate localization of the HD gene within 4p16.3 and, therefore, accelerate the cloning of the disease gene. In this study we present the identification and characterization of nine new polymorphic DNA markers, including three markers which detect highly informative multiallelic VNTR-like polymorphisms with PIC values of up to .84. These markers have been isolated from a cloned region of DNA which has been previously mapped approximately 1,000 kb from the 4p telomere.

Identification of multiple CpG islands and associated conserved sequences in a candidate region for the Huntington disease gene

The HD locus has been assigned to 4p16.3 distal to the DNA segment D4S10. However, the precise location of this gene is still unknown. At least three regions, together encompassing more than 3.5 Mb of DNA, can still be considered as candidate regions for the HD gene. Our efforts are directed toward the cloning and the complete characterization of one of these regions. Thus far we have cloned 460 kb of DNA in contiguously overlapping cosmids distal to D4S111 and have developed a detailed long-range restriction map orienting the contig within the terminal region of 4p16.3. We characterized 15 CpG-rich islands defined by tightly clustered rare cutter restriction sites for the enzymes NotI, BssHII, EagI, NruI, and SacII. In addition, we show that the sequences associated with the CpG-rich islands detect cross-species conservation. The detailed genetic analysis of the 460-kb contig provides a framework for the identification of genes, which can be assessed for the characteristics expected for the HD gene.

Genomic organization and complete sequence of the human gene encoding the beta-subunit of the cGMP phosphodiesterase and its localisation to 4p 16.3

As part of the search for the Huntington disease (HD) gene we have cloned and sequenced 34 kb of genomic DNA containing the full-length gene for the beta-subunit of the human cGMP phosphodiesterase (beta-cGMP PDE). This gene is localized to 4p16.3 about 700 kb proximal to the 4p telomere and represents the most telomeric gene characterized on 4p to date. We show that this gene is comprised of 22 exons spanning approximately 43 kb of genomic DNA. We also provide 400 bp immediately 5' to the putative initiator methionine and 700 bp of 3' flanking sequences. Northern blot analysis of several human tissues revealed a highly abundant 3.5 kb transcript and a minor signal of 4.5 kb in retinal tissue. Alignment of the deduced amino acid sequence to the previously identified beta-subunits of the cGMP PDEs of mouse and cow demonstrates highly significant similarities and, therefore, confirms the identity of the cloned gene. A defect in the beta-subunit of the cGMP PDE gene has been shown recently to be the cause for the retinal degeneration in the rd mouse. The cloning of the human homolog and the knowledge of its genomic organization with exon/intron boundaries will allow rapid assessment of the role of this gene in the causation of human retinopathies.