Alicia F. Scheffer

The Fine-Scale and Complex Architecture of Human Copy-Number Variation

Despite considerable excitement over the potential functional significance of copy-number variants (CNVs), we still lack knowledge of the fine-scale architecture of the large majority of CNV regions in the human genome. In this study, we used a high-resolution array-based comparative genomic hybridization (aCGH) platform that targeted known CNV regions of the human genome at approximately 1 kb resolution to interrogate the genomic DNAs of 30 individuals from four HapMap populations. Our results revealed that 1020 of 1153 CNV loci (88%) were actually smaller in size than what is recorded in the Database of Genomic Variants based on previously published studies. A reduction in size of more than 50% was observed for 876 CNV regions (76%). We conclude that the total genomic content of currently known common human CNVs is likely smaller than previously thought. In addition, approximately 8% of the CNV regions observed in multiple individuals exhibited genomic architectural complexity in the form of smaller CNVs within larger ones and CNVs with interindividual variation in breakpoints. Future association studies that aim to capture the potential influences of CNVs on disease phenotypes will need to consider how to best ascertain this previously uncharacterized complexity.

Detection of a de novo interstitial 2q microdeletion by CGH microarray analysis in a patient with limb malformations, microcephaly and mental retardation

We describe the cytogenetic diagnosis using BAC‐ and oligonucleotide microarrays of a 16‐year‐old Laotian‐American female, who first presented at 2½ years of age with microcephaly, developmental retardation, and skeletal abnormalities of the upper limb including mild syndactyly of the second and third and the third and fourth fingers, short middle phalanges and clinodactyly of the fifth digit at the distal interphalangel joint on both hands, and symphalangism of the metacarpal‐phalangeal joints of the second and fifth digits bilaterally. Her lower limbs displayed symphalangism of the metatarsal‐phalangeal joint of the second, third, and fourth digits on both feet, with fusion of the middle and distal phalanges of the second and fifth digits and hallux valgus bilaterally. G‐banded chromosomal study at age 4 was normal. However, comparative genomic hybridization at age 15 with the Spectral Genomics 1 Mb Hu BAC array platform indicated a microdeletion involving two BAC clones, RP11‐451F14 → RP11‐12N7 at 2q31.1. The maximal deletion on initial analysis comprised the HOXD cluster, which is implicated in limb development. Florescence in situ hybridization (FISH) using the RP11‐451F14 probe confirmed the deletion. Both parents were negative for the deletion. Additional FISH using BAC RP11‐387A1, covering the HOXD cluster, limited the maximal deletion to approximately 2.518 Mb, and excluded involvement of the HOXD cluster. The Agilent 44K and 244K platforms demonstrated a deletion of approximately 2,011,000 bp, which did not include the HOXD cluster. The malformations in our patient may be caused by deletion of a regulatory element far upstream of the HOXD cluster.

Comparative genomic hybridization using oligonucleotide arrays and total genomic DNA

Array-based comparative genomic hybridization (CGH) measures copy-number variations at multiple loci simultaneously, providing an important tool for studying cancer and developmental disorders and for developing diagnostic and therapeutic targets. Arrays for CGH based on PCR products representing assemblies of BAC or cDNA clones typically require maintenance, propagation, replication, and verification of large clone sets. Furthermore, it is difficult to control the specificity of the hybridization to the complex sequences that are present in each feature of such arrays. To develop a more robust and flexible platform, we created probe-design methods and assay protocols that make oligonucleotide microarrays synthesized in situ by inkjet technology compatible with array-based comparative genomic hybridization applications employing samples of total genomic DNA. Hybridization of a series of cell lines with variable numbers of X chromosomes to arrays designed for CGH measurements gave median ratios for X-chromosome probes within 6% of the theoretical values (0.5 for XY/XX, 1.0 for XX/XX, 1.4 for XXX/XX, 2.1 for XXXX/XX, and 2.6 for XXXXX/XX). Furthermore, these arrays detected and mapped regions of single-copy losses, homozygous deletions, and amplicons of various sizes in different model systems, including diploid cells with a chromosomal breakpoint that has been mapped and sequenced to a precise nucleotide and tumor cell lines with highly variable regions of gains and losses. Our results demonstrate that oligonucleotide arrays designed for CGH provide a robust and precise platform for detecting chromosomal alterations throughout a genome with high sensitivity even when using full-complexity genomic samples.