SNP array analysis reveals novel genomic abnormalities including copy neutral loss of heterozygosity in anaplastic oligodendrogliomas

Abstract

Anaplastic oligodendrogliomas (AOD) are rare glial tumors in adults with relative homogeneous clinical, radiological and histological features at the time of diagnosis but dramatically various clinical courses. Studies have identified several molecular abnormalities with clinical or biological relevance to AOD (e.g. t(1;19)(q10;p10), IDH1, IDH2, CIC and FUBP1 mutations).To better characterize the clinical and biological behavior of this tumor type, the creation of a national multicentric network, named "Prise en charge des OLigodendrogliomes Anaplasiques (POLA)," has been supported by the Institut National du Cancer (InCA). Newly diagnosed and centrally validated AOD patients and their related biological material (tumor and blood samples) were prospectively included in the POLA clinical database and tissue bank, respectively.At the molecular level, we have conducted a high-resolution single nucleotide polymorphism array analysis, which included 83 patients. Despite a careful central pathological review, AOD have been found to exhibit heterogeneous genomic features. A total of 82% of the tumors exhibited a 1p/19q-co-deletion, while 18% harbor a distinct chromosome pattern. Novel focal abnormalities, including homozygously deleted, amplified and disrupted regions, have been identified. Recurring copy neutral losses of heterozygosity (CNLOH) inducing the modulation of gene expression have also been discovered. CNLOH in the CDKN2A locus was associated with protein silencing in 1/3 of the cases. In addition, FUBP1 homozygous deletion was detected in one case suggesting a putative tumor suppressor role of FUBP1 in AOD.Our study showed that the genomic and pathological analyses of AOD are synergistic in detecting relevant clinical and biological subgroups of AOD.

Figure 1. Heat map with genomic profiles of anaplastic oligodendrogliomas.

Each column indicates a tumor. Each row indicates a genomic locus. Tumors were clustered based on the Euclidean distance between their copy number vectors. The color code on the left-upper corner indicates the genomic status: yellow, green and red indicate a normal status, loss and gain, respectively. In addition, the IDH1 mutation (pink indicates mutated IDH1/2, while IDH1/2 indicates non-mutated IDH1/IDH2), patient age (blue and pink indicate younger and older, respectively, than the median age of the entire population, 49.9 years old) and patient gender (purple indicates male, while brown indicates female) are reported at the top of the figure. The p-value on the right indicates the distribution of the variables between the 1p19q- and non-1p19q-co-deleted tumors. Panel A. 1p/19q-co-deleted anaplastic oligodendrogliomas, with chromosomes 1 and 19 centromeric breakpoints. Panel B. Non-1p/19q co-deleted anaplastic oligodendrogliomas. The legend is the same as the one used in Panel A.

Figure 2. Frequency of genomic alterations in the 1p/19q-co-deleted anaplastic oligodendrogliomas on the top part of the panel and non-1p/19q-co-deleted anaplastic oligodendrogliomas on the bottom part.

Panel A. Genomic gain, genomic loss and uniparental disomy are indicated in red, green and blue, respectively. Panel B. High-level amplification and homozygous deletion are indicated in red and green, respectively. Panel C. Genomic breakpoints are indicated with a black dot across the genome.

Figure 3. An anaplastic oligodendroglioma with CDKN2A expression and normal CDKN2A gene copy number and allelic statuses.

Panel A. Top part: Genomic profile with the copy number status. Middle part: Genomic profile with the allelic frequencies. Bottom part: The genomic profile including genomic loss (in green), normal copy number status (light blue) and copy neutral loss of heterozygosity (dark blue). Panel B. Chromosome 9 and the allelic frequencies (the arrow indicates the CDKNA locus). Panel C. Microsatellite analysis showing the allelic status of three markers (D9S171 and D9S1121) in the blood DNA (top part) and paired tumor DNA (bottom part) Panel D. CDKN2A expression using immunochemistry.

Figure 4. An anaplastic oligodendroglioma with CDKN2A silencing, normal CDKN2A gene copy number status and copy neutral loss of heterozygosity.

Panel A. Top part: Genomic profile with the copy number status. Middle part: Genomic profile with the allelic frequencies. Bottom part: The genomic profile including genomic loss (in green), normal copy number status (light blue) and copy neutral loss of heterozygosity (dark blue). Panel B. Chromosome 9 and the allelic frequencies (the arrow indicates the CDKNA locus). Panel C. Microsatellite analysis showing the allelic status of three markers (D9S1684, D9S171, D9S1121) in the blood DNA (top part) and paired tumor DNA (bottom part). Acquired allelic loss is observed in the tumor DNA Panel D. CDKN2A silenced using immunochemistry.