1p/19q Deletion in Oligodendrogliomas

1p/19q Deletion by FISH 3001309
Method: Fluorescence in situ Hybridization

Use when oligodendrogliomas are suspected

IDH1 R132H by Immunohistochemistry 2005857
Method: Immunohistochemistry

Use when morphology indicates tumor may be a glioma

Use to differentiate tumor from reactive gliosis

Includes pathologist interpretation

Malignant gliomas (oligodendrogliomas) are the most common type of primary brain tumors. Identification of the 1p/19q deletion is useful in differentiating oligodendrogliomas from astrocytomas and assists in the diagnosis and prognosis of both low- and high-grade oligodendrogliomas, as well as in predicting response to therapy. The 1p/19q deletion should not be used alone for diagnosis.

Disease Overview

Prevalence

  • Second most common glioma in adults
  • Accounts for 2% of central nervous system (CNS) tumors

Diagnostic Issues

Malignant gliomas are the most common type of primary brain tumors (>70% of all CNS tumors).

  • Subtypes are astrocytoma and oligodendroglioma
    • Differentiating astrocytoma from oligodendroglioma is crucial
  • Treatment and prognosis differ between tumors
  • Combined loss of chromosomal arms 1p and 19q is diagnostic for oligodendrogliomas
  • Gain of chromosome 19 supports diagnosis of high-grade astrocytoma (glioblastoma)
  • Loss of 1p may identify treatment-sensitive high-grade oligodendroglioma (for both chemotherapy and radiotherapy)
    • Prognostic relevance in low-grade tumors is less well characterized

Genetics

Gene

Chromosomes 1 and 19 involved

Structure/Function

Deletion of short arm of 1p and long arm of 19q results in loss of mediators of resistance to therapy

Variants

  • 1p/19q codeletion is mutually exclusive for TP53 and ATRX mutations and EGFR amplification
  • 1p/19q codeletion is frequently associated with IDH1 or IDH2 variants

Test Interpretation

Results

  • Positive
    • Tumors with 1p/1q ratio <0.80 and ≥24% deleted cells are deemed deleted for 1p
    • Tumors with 19q/19p ratio <0.80 and ≥26% deleted cells are deemed deleted for 19q
    • Both deletions are associated with a better prognosis
      • Codeletion has better prognosis than single deletion
    • Presence of codeletion establishes diagnosis of oligodendroglioma
  • Negative
    • Effectively rules out diagnosis of oligodendroglioma

Limitations

Test should not be used alone for the diagnosis of malignancy

Last Update: September 2019