Tuesday, May 26, 2015

The Tumor Biomarker Series: EGFR

Epidermal growth factor receptor (EGFR) is the most frequently amplified oncogene in astrocytic tumors (>40% or GBMs and 5-10% of anaplastic astrocytomas). EGFR is far more often amplified in de novo GBMs as compared to secondary GBMs. About one-half of those GBMs with EGFR amplification also have specific EGFR mutations (the vIII mutant), which produce a truncated receptor with constitutive activity. Both EGFR amplification and EGFRvIII mutant are mutually exclusive with IDH mutations. So, what is the utility of EGFR testing? First, astrocytomas with EGFR amplification tend to be of higher grade. So if, for example, the diagnostician is vacillating between a WHO grade II and a WHO grade III tumor, positive EGFR amplification status would favor the latter. Secondly, EGFR amplification can also help distinguish between a small cell GBM (which would potentially harbor the amplification) from anaplastic oligodendrogliomas (which do not exhibit the amplification).

Wednesday, May 20, 2015

"Bad Ass" CrossFitter and Neuropathologist Greg Fuller set to compete in kettlebell competition for brain cancer research

Elite Neuropathologist
Elite Athlete
It's no secret that Dr. Greg Fuller (pictured) is the fittest neuropathologist in the world. What his colleagues might not know is that he is the 67th fittest man in the world for his age class (60+). Greg's wife, Tina, describes him as a "bad ass CrossFitter". As a former owner of a CrossFit "box" myself, I know what it takes to achieve Greg's level in the Sport of Fitness. It requires true grit. There are many of us who would also consider Greg a "bad-ass neuropathologist" -- in the very best sense of that phrase. He's bringing together his love of fitness with his commitment to the study of CNS malignancies by participating in the Kettlebells For Brain Cells competition in Boerne, Texas on June 13. (So, he has a legitimate reason for not showing up at the concurrent AANP annual meeting. in Denver.) All proceeds from the competition will be going to the American Brain Tumor Association. The event is being organized by a brain tumor patient at MD Anderson Cancer Center, where Greg is Chief Neuropathologist. This is a shining example of a neuropathologist reaching out to a patient in a very personal way. Best of luck in the competition, Greg. Crush it!

Monday, May 11, 2015

The Tumor Biomarker Series: Ki-67

An immunohistochemical marker of cellular proliferation, the nuclear antigen Ki-67 is positive in cells that are actively engaged in cell cycle (i.e., not in G0). Results are expressed as a percent index of positively staining cells. Several studies have shows a correlation between Ki-67 indices in various astrocytomas, oligodendrogliomas, and mixed gliomas. Among grade II and grade III diffuse gliomas, the Ki-67 index provides prognostic value. However, investigations have consistently shown that Ki-67 proliferation indices have no prognostic value on patient outcomes for GBM. So, if you have an unmistakable GBM under your microscope, you are not practicing evidence-based parsimonious medicine by ordering Ki-67 immunohistochemistry on that tumor.  On the other hand, if you are debating between diagnosing a grade III or a grade IV astrocytoma, Ki-67 can be helpful in swaying your decision. The Ki-67 index is not used in the WHO grading system because of the high degree of technical variability between laboratories, making standardization difficult.

Friday, May 8, 2015

The Tumor Biomarker Series: ATRX

Alpha Thalassemia/Mental Retardation Syndrome X-linked (ATRX) is a gene that encodes a protein involved in chromatin remodeling. ATRX mutations are a marker of astrocytic lineage among the IDH-mutant gliomas and are mutually exclusive with 1p/19q codeletion. Present in 57% of secondary GBMS, ATRX mutations are uncommon in primary glioblastomas. Nearly all diffuse gliomas with IDH and ATRX mutations also have TP53 mutation and are associated with the Alternative Lengthening of Telomeres (ALT) phenotype. Immunohistochemistry for ATRX demonstrates loss of protein expression in neoplastic cells harboring the inactivating mutations, while expression is retained within non-neoplastic internal controls (such as endothelial cells).

Tuesday, May 5, 2015

The Tumor Biomarker Series: PTEN and LOH of chromosome 10

Loss of heterozygosity (LOH) of chromosome 10 occurs in most GBMs and less frequently in grade II and III diffuse astrocytomas. The phosphatase and tensin (PTEN) gene at 10q23.3 has been most strongly implicated as a glioma-related tumor suppressor on chromosome 10q, with PTEN mutations identified in about 25% of GBMs and less frequently in grade III astrocytomas. Losses of chromosome 10 and mutations in PTEN are considered to be specific for astrocytic differentiation and are rare in oligodendrogliomas. They are also markers of high-grade progression and aggressive clinical behavior in astrocytomas.

Tuesday, April 28, 2015

The Tumor Biomarker Series: BRAF

Aberrant constitutive activation of BRAF tends to be seen in cerebellar and midline pilocytic astrocyomas whereas the activating point mutation at BRAF V600E is more likely to be seen in cerebral examples. The V600E point mutation is also observed in other low-grade gliomas and glioneuronal neoplasms, including approximately two-thirds of pleomorphic xanthoastrocytomas, and lower percentages of ganglioglioma, desmoplastic infantile ganglioglioma, dysembrioplastic neuroepithelial tumor, and papillary craniopharyngioma. Although less common, diffusely infiltrative gliomas including glioblastoma, particularly the epithelioid variant, may also demonstrate the V600E point mutation -- making this biomarker potentially less useful as a diagnostic tool in distinguishing low-grade gliomas from high-grade ones.

Tuesday, April 21, 2015

The Tumor Biomarker Series: MGMT

MGMT stands for O6-methylguanine-DNA methyltransferase. I must admit that this is my favorite biomarker only because of its cool mechanism of action.  The standard chemotherapy for gliomas is temozolomide, which acts by cross-linking DNA through alkylating multiple sites including the 06 position of guanine. Crosslinking at this site is reversed by the DNA repair enzyme MGMT. Thus, low levels of MGMT activity by GBM cells is associated with enhanced response to alkylating agents such as temozolomide. To a large degree, the activity level of MGMT is determined by the methylation status of the gene's promoter. MGMT can be epigenetically silenced by hypermethylation. About half of all GBMs are epigenetically silenced in this manner and are therefore more susceptible to the alkylating action of temozolomide.  The methylation status of MGMT can be assessed by PCR-based testing. In addition to predicting better response to temozolomide, investigators have shown that epigenetic gene silencing of MGMT is a strong predictor of prolonged survival independent of treatment.

Thursday, April 16, 2015

The Tumor Biomarker Series: TP53

A few month ago, the College of American Pathologists released a Template for Reporting Results of Biomarker Testing of Specimens from Patients with Tumors of the Central Nervous System. Therefore, I thought it would be a good idea to review as succintly as possible the various tumor biomarkers one could use to interrogate CNS tumors. Not all neuropathologists would agree as to which ones, if any, are essential. So, comments are most welcome! The first biomarker I'd like to address is TP53, mainly because I have doubts about it's utility -- except for cases where there is a question regarding whether or not an oligodendroglial component is present. However, immunohistochemical p53 testing is performed on virtually all high-grade astrocytomas at many institutions. Here's a summary of the CAP consensus description of the TP53 test:

Found in a majority of high-grade astrocytic tumors, TP53 mutation is rare in oligodendrogliomas. Mutation of TP53 is highly correlated with IDH mutation. As a surrogate for testing the actual TP53 mutation, p53 immunohistochemistry is typically performed. As for the utility of this test, the CAP template makes the following statement: "[T]here is a strong association between IDH1 mutation and TP53 mutation in diffuse astrocytomas and this combination of mutations is helpful in distinguishing astrocytomas from oligodendrogliomas."

My feeling is that if you have a histomorphologically classic pure astrocytic neoplasm, there is no need for p53 immunohistochemistry. And, yet, you almost always see p53 immunohistochemistry results on reports for classic glioblastomas, anaplastic astrocytomas, and infiltrative astrocytomas. I just don't get it. Even in cases where you have a question about the presence of an oligodendroglial component, testing for IDH1 mutation and 1p/19q deletion would be more helpful than testing for IDH1 and TP53. I would love to hear from people who could dissuade me of this opinion regarding the utility of p53 immunohistochemistry. Feel free to enter comments below.