Heterogeneity in alkaline phosphatase isozyme expression in human testicular germ cell tumours: An enzyme-/immunohistochemical and molecular analysis

In humans, alkaline phosphatases are encoded by one tissue-non-specific alkaline phosphatase (TNAP) gene and three tissue-specific alkaline phosphatase genes, intestinal, placental (PLAP), and germ cell-specific alkaline phosphatase (GCAP). Although the presence of alkaline phosphatases in testicular germ cell tumours (TGCTs) of adolescents and adults has been utilized for both detection and patient monitoring, it is not known in detail which isozymes are expressed. Since alkaline phosphatase is detected in carcinoma in situ (CIS), the common precursor of all TGCTs, it might provide a marker for the early diagnosis of TGCTs. Testicular cancers of germ cell and non-germ cell origin along with testicular parenchyma with and without CIS have been analysed for the expression of the different alkaline phosphatase isozymes. Antibodies to TNAP and PLAP/GCAP showed positivity in CIS, seminoma, and embryonal carcinoma. The heterogeneous staining pattern detected in frozen tissue sections was similar to the pattern found in formalin-fixed, paraffin-embedded material, indicating a biological phenomenon and not a handling artefact. Since PLAP and GCAP cannot be distinguished using immunohistochemistry, the expression of these isozymes was studied at the molecular level using a reverse transcriptase-polymerase chain reaction (RT-PCR) approach, in combination with a primer extension assay. The results show that CIS and seminoma predominantly express GCAP, while in embryonal carcinoma the expression of GCAP versus PLAP varies. Due to the presence of alkaline phosphatase transcripts in normal testicular parenchyma, an RT-PCR-based analysis of alkaline phosphatase is not informative for the early detection of TGCTs in biopsy samples.