Molecular genetic changes in human male germ cell tumors

An isochromosome for the short arm of chromosome 12, i(12p), is the most common and characteristic cytogenetic aberration in testicular germ cell tumors. Little is known about the molecular genetic abnormalities of these neoplasms.

A total of 32 loci were studied in DNA from 31 primary testicular germ cell tumors and compared with corresponding normal DNA. The loci map to 17 different chromosome arms, including seven that contain known tumor suppressor genes. Southern blot analysis and PCR-based methods were used. Several microsatellite loci were included to investigate instability (seen as new alleles) at repeat loci. The TP53 tumor suppressor gene was analyzed for point mutations by constant denaturant gel electrophoresis and for expression by immunohistochemistry. Histologic sections of the tumor biopsies were evaluated with regard to components and percentage of intact tumor cells. The growth fraction, representing one component of proliferative activity of the tumor, was assessed by the Ki-67 index.

Changes were found at all chromosome arms investigated but at very different frequencies, 5-56% of all tumors. The most frequently affected chromosome arms, those showing loss of heterozygosity or allelic imbalance in more than 40% of the tumors, were 2q, 3p, 3q, 11p, 12p, 18q, and 22q. Complete loss of one allele was often seen at 3p and 11p loci, whereas allelic imbalances dominated on the 2p, 3q, 12p, 18q, and 22q loci tested. No mutations were detected within four known mutational hot spots of TP53, but positive immunostaining with two TP53 Ab was seen in 9 of 14 tumors. Most tumors (26 of 31) showed positive immunostaining with Ki-67. Microsatellite instability was not observed.

High frequencies of loss of heterozygosity and allelic imbalance at several loci indicate that inactivation of several tumor suppressor genes may be of importance in developing testicular germ cell tumors. The increase in copynumber of 12p alleles seen in several tumors is likely to reflect one or more 12p isochromosomes. Our findings do not indicate that TP53 plays any major pathogenic role in this tumor type, nor was there any indication that defect repair genes, causing microsatellite instability in other cancers, participate in the progression of testicular cancer.