MTAP (MSVA-741R)

S-methyl-5′-thioadenosine phosphorylase (MTAP) is an enzyme with a major role in polyamine metabolism. It supports the recovery of both adenine and methionine by catalyzing the phosphorylation of methylthioadenosine (MTA) to adenine and 5-methylthioribose-1-phosphate. Adenine is one of the purine bases required for both DNA and RNA. As an essential protein for the adenine synthesis in human cells, MTAP plays a critical indirect role for the synthesis of DNA and RNA. The MTAP gene is located at 9p21.3. This is in the immediate vicinity of the CDKN2A gene which is homozygously deleted in about 15% of all human cancers. Homozygous co-deletion of MTAP occurs in 80%–90% of tumors with CDKN2A deletion. MTAP deletion have a significant impact on purine biosynthesis because the MTAP substrate MTA accumulates in affected cells and inhibits PRMT5. As a result critical vulnerabilities of affected cells to targeting of the MAT2A/PRMT5/RIOK1 axis develop. Inhibitors of these enzymes are now being tested in clinical trials recruiting patients with homozygously 9p21 deleted and MTAP inactivated tumors.

Images describing the MTAP staining pattern in normal tissues obtained by the antibody MSVA-741R are shown in our “Normal Tissue Gallery – Images coming soon”.

The ubiquitous staining of MTAP across all tissue types is largely consistent with the RNA data described in the Human Protein Atlas (Tissue expression MTAP).

Positive control = Ovary: At least a moderate, nuclear and/or cytoplasmic MTAP staining should be seen in ovarian stroma cells.

Negative control = Bladder cancer with homozygous 9p deletion: MTAP staining should be absent in cells from urothelial tumors with homozygous 9p deletion.

No data available at the moment

IHC users have different preferences on how the stains should look like. Some prefer high staining intensity of the target stain and even accept some background. Others favor absolute specificity and lighter target stains. Factors that invariably lead to more intense staining include higher concentration of the antibody and visualization tools, longer incubation time, higher temperature during incubation, higher temperature and longer duration of the heat induced epitope retrieval (slide pretreatment). The impact of the pH during slide pretreatment has variable effects and depends on the antibody and the target protein.

All images and data shown here and in our image galleries are obtained by the manual protocol described below. Other protocols resulting in equivalent staining are described as well.

Manual protocol

Freshly cut sections should be used (less than 10 days between cutting and staining). Heat-induced antigen retrieval for 5 minutes in an autoclave at 121°C in pH 7,8 Target Retrieval Solution buffer. Apply MSVA-741R at a dilution of 1:50 at 37°C for 60 minutes. Visualization of bound antibody by the EnVision Kit (Dako, Agilent) according to the manufacturer’s directions.

• The diagnostic utility of MTAP immunohistochemistry in tumors and in preneoplastic disease needs to be investigated. 
• The prognostic relevance of MTAP expression in tumors should be explored. 
• The predictive role of homozygous and heterozygous MTAP deletions for cancer treatment by drugs targeting the MAT2A/PRMT5/RIOK1 axis needs to be evaluated.

There are two ways how the specificity of antibodies can be documented for immunohistochemistry on formalin fixed tissues. These are: 1. Comparison with a second independent method for target expression measurement across a large number of different tissue types (orthogonal strategy), and 2. Comparison with one or several independent antibodies for the same target and showing that all positive staining results are also seen with other antibodies for the same target (independent antibody strategy). 

Orthogonal validation: For the antibody MSVA-741R orthogonal validation is not suited due to the ubiquitous expression of the protein. Ubiquitous MTAP protein expression is consistent, however, with data from three independent RNA screening studies, including the Human Protein Atlas (HPA) RNA-seq tissue dataset, the FANTOM5 project, and the Genotype-Tissue Expression (GTEx) project, which are all summarized in the Human Protein Atlas (Tissue expression MTAP). 

Comparison of antibodies: True MTAP protein expression in all cell types found by MSVA-741R is corroborated by identical stainings obtained by another commercially available independent antibody (termed “validation antibody”).  – Images coming soon