Molecular genetics of kidney cancer : chromosomal regions of importance in the development of renal cell carcinoma

Abstract

Renal cell carcinoma (RCC) is a malignancy of the human kidney, occurring most commonly during the seventh and eighth decades of life. The incidence is high in the Nordic countries and the five year survival is 40-50%. Smoking is a risk factor inferring a minor increase in risk. The best established risk factor is obesity. It is usually sporadic and unilateral but can also occur in hereditary forms characterized by an earlier age of onset, multiple primary, bilateral tumors. The best established prognostic factor for RCC is stage. Other factors like cell type, age at diagnosis, and tumor size, have been implicated to affect survival.

We wanted to establish, what chromosomal abnormalities are associated with the development of RCC. Tumor and normal kidney tissues were examined from sporadic, non-hereditary RCC cases. 86% of the examined patients had a detectable anomaly of chromosome 3p, manifested as a deletion, distal to band 3pl 1.2-pl3 combined with the non reciprocal translocation of a segment from another chromosome or monosomy 3. Restriction fragment length polymorphism (RFLP) analysis showed loss of D3F15S2 heterozygosity in 76%. D3S2 hetaozygosity was lost in 18%. The variability of the breakpoint between 3pl 1.2 and 3pl3 and the absence of a consistently translocated segment from another chromosome suggested a genetic loss mechanism. With the previously demonstrated involvement of the 3pl4.2 region in a familial case, these findings suggested that RCCs arise by the deletion of a tumor suppressor gene located at the short arm of chromosome 3.

Physical maps were constructed around loci D3F15S2, D3S2 and c-rafl on the short arm of human chromosome 3 using pulsed field gradient gel electrophoresis. The normal restriction pattern was not altered by the t(3;8)(pl4.2;q24.1) characteristic for a hereditary form of renal cell carcinoma, indicating that the breakpoint itself is not included in any of the mapped areas. We found a CpG island within the D3F 15S2 locus, suggesting, the presence of a functional gene in the region.

The screening of a human placenta cDNA library with DNA probes derived from D3F15S2 led to the isolation of several cDNA clones. They identified a 2.9 Kb long message in human placenta and kidney. A DNA homology search in GENBANK revealed that the gene encoded human acylpeptidehydrolase (APEH). The nucleotide identity between the rat and the human genes was 88%. We found an APEH expression below 20%, compared to normal kidney, in 73% of the studied primary RCCs. No tumor had an expression above half of that in normal kidney. It was expressed in all cell lines and normal tissues examined, except in one Burkitt's Iymphoma cell line (DG75). RCC appearsto be associated with a decrease of this enzyme activity. This reduction may reflect the existence of a small, acetylated growth factor of importance in renal tumors.

Since tumors in animal models are easier to manipulate we wanted to determine the chromosomal location of APEH in rat and mouse. It is localized at mouse chromosome 9 and rat chromosome 8. This increases the number of conserved genes in this region. Other regions, beside chromosome 3, have also been found to harbour non-random losses in RCC. They might be associated with the progression of the tumor. Tumor and constitutional DNA pairs were analysed with RFLP markers from all chromosomal arms and compared with clinical classifications and survival. Allelic imbalance (Al) for loci on 5q was found to be a negative prognostic factor for RCC. Grade correlated with fractional allelic loss, loss of 14q and 17p Al but not with specific aberrations on other chromosomes.