Colorectal cancer (CRC) is the most frequent malignant disease in Europe according to an estimation of cancer incidence and mortality by the International Agency for Research on Cancer in Lyon, France[1
]. In 2008, 436 000 persons were diagnosed with CRC, followed by breast cancer with 421 000 cases, lung cancer with 391 000 cases and prostate cancer with 382 000 cases. Approximately 212 000 patients died due to CRC that year, which makes it the second most common death from cancer (after lung cancer with approximately 342 000 deaths in 2008)[1
]. Worldwide, in the developed countries about 1.167 million new cases of CRC and about 603 000 deaths due to CRC were estimated for 2007[2
However, due to the long process of carcinogenesis in CRC (adenoma-carcinoma sequence), CRC has an overall good prognosis when diagnosed at an early stage. For that reason different CRC screening programs have been developed and are offered in various European countries.
The gold standard for early detection of colorectal neoplasia is colonoscopy. A great advantage of colonoscopy is that adenomas, the potential precursors of carcinogenesis, can be simultaneously detected and removed. However, the acceptance of screening colonoscopy among patients is low. For example, in Germany only 2.7% of insured people exercise their right to a colonoscopy even though it is reimbursed for people over 55 years old[3
]. The most common in-vitro
diagnostic method for CRC screening is the detection of occult blood in the stool using the guaiac-based faecal occult blood test (gFOBT). This test is based on the peroxidase activity of haemoglobin, which induces an oxidation and blue colouration of guaiac in the presence of hydrogen peroxide. Red meat and a number of vegetables may result in false positive results whereas vitamin C may result in false negative staining. Therefore, dietary restriction is recommended for three days prior to and during testing. A widespread criticism of gFOBT is its low sensitivity for adenomas and carcinomas (13%-50%)[4-8
]. The immunological faecal occult blood tests (iFOBTs) specifically quantify human haemoglobin with antibodies. Comparative evaluations of immunochemical faecal occult blood tests from different manufacturers have revealed great variations in their respective sensitivities for colorectal adenoma detection[9,10
The faecal pyruvate kinase isoenzyme type M2 (faecal M2-PK) test recognises a key enzyme controlling the metabolism of cells with a high proliferation rate, such as tumour cells, and thereby detects specific alterations in intestinal cells, such as polyps and CRC, as well as high-risk patients with acute or chronic inflammatory bowel diseases (IBD) (i.e., ulcerative colitis, Crohn’s disease).
M2-PK is a special isoenzyme of pyruvate kinase, a key enzyme within glycolysis which catalyzes the ATP-producing conversion of phosphoenolpyruvate (PEP) to pyruvate. Depending upon the metabolic functions of the tissues, different isoenzymes of pyruvate kinase are expressed. During tumour formation the tissue-specific isoenzymes disappear and the pyruvate kinase isoenzyme type M2 is expressed[11
]. In contrast to all other pyruvate kinase isoenzymes (type L, M1 and R) which consist of four subunits, the M2 pyruvate kinase isoenzyme may occur in a highly active tetrameric form as well as in a dimeric form with low activity. The dimeric form is nearly inactive and favours the channelling of glucose carbons into synthetic processes, such as nucleic acid, amino acid and fatty acid synthesis. The tetrameric form is highly active and favours the energy-regenerating conversion of PEP to pyruvate and lactate (the Warburg effect). In tumour cells, M2-PK is mainly found to be in the dimeric form and has therefore been termed “Tumour M2-PK”. The dimerisation of M2-PK is induced by interaction with different oncoproteins, including pp60v-src-kinase, oncogenic fibroblast growth factor1 and human papilloma virus 16 E7[11
The dimeric form of M2-PK is released from tumours into the blood and can be quantified by a sandwich enzyme-linked immunosorbent assay (ELISA; ScheBo Biotech AG, Giessen, Germany). About 40 studies have been published on M2-PK concentrations in blood since 1997. These demonstrate a significant increase in M2-PK and correlation with staging for the following tumours: melanoma, thyroid, breast, lung, kidney, oesophageal, gastric, pancreatic, colorectal, ovarian, cervical and renal cell cancer[12-19
]. The long-term determination of M2-PK in EDTA-plasma is used as a tool for follow-up studies to monitor failure, relapse or success during therapy. In CRC and adenoma M2-PK is also released into the patients’ faeces. A sandwich ELISA and a lateral flow rapid test (for doctor’s office, point-of-care and laboratory use), both based upon two monoclonal antibodies which specifically recognise the dimeric form of M2-PK, are commercially available for the quantification of M2-PK in stool. The potential of the faecal M2-PK test for CRC screening has been evaluated in at least 17 different independent international studies. The objectives of this review were to obtain an overview of the currently available studies with faecal M2-PK and to present a critical discussion of the efficacy of the faecal M2-PK test for CRC screening.