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MINI 23 - Lung Cancer Risk: Genetic Susceptibility and Airway Biology (ID 135)
- Event: WCLC 2015
- Type: Mini Oral
- Track: Screening and Early Detection
- Presentations: 1
MINI23.10 - Subtraction of Allelic Fractions (Delta-θ): A Sensitive Metric to Detect Chromosomal Alterations in Heterogeneous Premalignant Specimens (ID 2434)
16:45 - 18:15 | Author(s): R.S. Stearman
Lung squamous carcinoma is believed to arise from premalignant bronchial epithelial dysplasia, which demonstrates progressive histologic changes leading up to invasive cancer. However, only a small subset of these lesions progress to carcinoma. Recent studies have shown that somatic chromosomal alterations (SCAs) status is a better biomarker than premalignant histology alone. Single-nucleotide polymorphism microarray (SNP array) has been frequently used to delineate these genomic alterations across the whole genome. However, the cellular heterogeneity, from clinical samples such as endobronchial specimens, is a basic obstacle to perform sensitive and accurate detection of SCAs.
We used: 1) a lung cancer cell line (NCI-H1395) and its matched lymphoblastoid (NCI-BL1395) cell line; 2) frozen lung tissues containing different percentage of invasive cancer cells surgically resected from a patient; and 3) biopsies and brushings obtained at the visually concerning areas during bronchoscopy. The histology of the clinical samples were graded by the study pathologist. Genomic DNA was isolated from each sample, quantified, and labeled for Illumina SNP array (HumanOmni 2.5-Quad BeadChip). Data analysis and visualizations were performed using Partek Genomic Suite 6.6 software.
Our study focused on the detection of SCAs by the comparison of genomic DNAs from cancer/premalignant cells (subject) to blood/normal cells (reference) from the same individual. We tested a B allele frequency metric, the subtraction of allelic fractions (delta-θ), on a standardized mixture of genomic DNAs from a lung cancer cell line and its matched lymphoblastoid cell line. Delta-θ proved to be a sensitive parameter to clearly delineate SCAs present in the tumor cell line even with a large proportion of normal cells (up to 90%). To explore the utility of using delta-θ for heterogeneous samples, we used clinical lung cancer specimens with known cancer cell content. In comparison to the other publicly available analytical metrics/algorithms (conventional Log R Ratio plot, mirrored B Allele Frequency plot, and GAP algorithm), delta-θ performed as well or better (with lower computational power needed), and enabled the detection of SCAs even in highly heterogeneous clinical samples (<30% tumor cell content). In addition, we completed a study using a number of bronchial biopsies and brushings with histologic grade ranging from normal to squamous cell carcinoma. SCAs were rarely detected in those of low to mild dysplasia, while they were detected in approximately 25% of moderate or severe dysplasia, and in all carcinoma in situ (CIS) and squamous cell carcinoma specimens. Longitudinal, repeated samplings from a high risk patient who persistently showed high grade dysplasia across the bronchus, revealed that delta-θ could identify SCAs continuously across the whole genome. The fact this individual had highly overlapping SCAs between different bronchial locations indicates genomic field cancerization may occur, along with the histological field effect in premalignant epithelium.
In SNP microarray studies, delta-θ is a highly sensitive metric for detecting SCAs even in heterogeneous dysplastic bronchial specimens. SNP array may be a powerful tool to understand premalignant genetic alterations and field cancerization.
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