Abstract
Chromosomal structural abnormalities in ALL are powerful independent predictors of prognosis, and directly impact choice of therapy. Currently, clinical detection of these abnormalities is based on karyotype and fluorescent in-situ hybridization (FISH), but these methods have limitations. Under optimal circumstances, structural abnormalities are detectable in well over 90% of ALL cases, but in actuality, typical cytogenetic laboratories demonstrate only a 50–60% abnormality detection rate. Karyotype may fail due to unsuccessful cell growth in culture and/or relative overgrowth of normal lymphocytes. FISH is limited by the expense and labor intensity of performing a separate assay for each probe used. Array comparative genomic hybridization (CGH) may have clinical utility as a complementary diagnostic tool in pediatric ALL. Its advantages include the ability to detect copy number changes in regions too small to be identifiable by karyotype; to identify novel abnormalities for which specific FISH probes do not exist in current diagnostic laboratories; and to provide information in as many as 50% of cases which show a failed or normal karyotype. In addition to its clinical utility, array CGH provides a wealth of information which may be mined for discovery of new pathways in leukemogenesis and additional prognostic factors within existing disease subgroups. The main limitation of array CGH is its inability to detected balanced translocations. We evaluated the diagnostic utility of a bacterial artificial chromosome (BAC) array CGH platform, the SpectralChip 2600, with an average resolution of 1.0 MB across the genome. We analyzed 50 pediatric ALL bone marrow specimens obtained at diagnosis, and compared the findings to the clinical results based on karyotype and standard 5-probe FISH panel. The cases ranged from 1–15 years (mean 5 years), with marrow containing between 33–94% leukemic blasts (mean 77%). Each sample was hybridized to the chip with a healthy control of the opposite gender. The sensitivity of array CGH in detecting abnormalities identified by karyotype and FISH was approximately 88%. Several of the abnormalities “missed” by CGH, which lowered the sensitivity score, were subsequently found to be erroneous karyotype calls when followed up with specific FISH probes. In addition, array CGH detected numerous additional areas of amplification and deletion which were subsequently validated by FISH, including in 10 cases for which cytogenetics was either normal or unsuccessful. Loss of 1p31, loss of 7p21, and gain of 16p13 were aberrations that were each noted to occur in three or more different cases, and hence may be worthy of further study. In the future, development of a customized ALL chip which is enriched for probes at sites of known amplification and deletion could further heighten diagnostic sensitivity, obviate the need for performance of multiple FISH tests, and provide valuable information in the substantial number of cases with a normal or failed karyotype analysis. Balanced translocations would still require testing via a multiplex PCR assay or a combination of available FISH probes.
Application of array comparative genomic hybridization (array- CGH) for detection of chromosomal imbalances in children with developmental delay/congenital malformations in Saudi Arabia
