Carcinogenesis modifier loci in rat tongue are subject to frequent loss of heterozygosity

Abstract

Rats of the DA strain are highly susceptible to 4NQO-induced TCs, whereas WF rats are barely susceptible. In (DA ؋ WF)F 2 rats, 5 QTL, Tscc1-5, are responsible for most of the phenotypic variations, though they do not account for all of the phenotypic differences between WF and DA rats. Analysis of 40 tongue tumors >5 mm in diameter from (DA ؋ WF)F 1 rats for LOH at the Tscc loci revealed a high frequency of LOH in chromosomal regions where the Tscc2, -3 and -4 loci map. In most cases of LOH, the allele of the barely susceptible WF strain was lost, suggesting that these loci in the WF strain encode tumor-suppressor genes. Analysis of the same tumors for somatic mutations in oncogenes indicated frequent alteration of Ha-ras, which maps in the Tscc3 region, but rare mutation of the p15 INK4B and p16 INK4Agenes or the p53 and Msh2 genes. Frequent LOH was also found on rat chromosomes 5 (RNO5) and 6 (RNO6 Squamous cell carcinoma of the oral cavity is the sixth most common solid tumor in the Western world. The overall survival rate of patients with this carcinoma has not improved significantly in the last decade, and the disease continues to be a serious health problem. 1,2 Rat TC induced by 4NQO is an experimental model for human TC since both tumors share several morphologic and molecular biologic properties. For instance, mutations in Ha-ras in rat or mouse TCs are as prevalent as those in human TCs. 3,4 Our recent studies have shown that susceptibility to 4NQO-induced rat tongue carcinogenesis is determined by several QTL, Tscc1-5. 5-7 DA strain rats are very susceptible to TCs, and DA-derived alleles at Tscc loci are the primary determinants of disease susceptibility. 8,9 The gene products encoded by the Tscc loci as well as the molecular mechanisms responsible for their allele-specific cancer-modifying activities are, for the most part, unknown. The difference in the Tscc alleles may rest in genetic polymorphisms, in either the regulating or the coding regions of the candidate genes. To test whether Tscc genes display either an oncogene or allele-specific tumor-suppressor-like activity on TC, we examined 4NQO-induced TCs in (DA ϫ WF)F 1 hybrid rats for LOH at these loci and for mutations at genes involved in cancer to characterize genetic alterations in TC and their possible relationship with the Tscc loci. MATERIAL AND METHODS AnimalsDA rats were purchased from the Shizuoka Laboratory Animal Center (Hamamatsu, Japan). The origin and history of these rats have been described. 10 WF rats were originally obtained from Hiroshima University (Hiroshima, Japan) and maintained by brother-sister matings for more than 90 generations in our laboratory. A total of 88 F 1 rats (40 females, 48 males) were derived by reciprocal mating between DA and WF rats. All rats were weaned at 35 days after birth, individually numbered and housed in plastic cages at 22°C in an air-conditioned room. Animals were fed on a commercial CE2 pellet diet (Nippon Clea, Tokyo, Japan) and given tap water ad libitum. No spontaneous tumors were obse...

Genetic Predisposition to 4NQO-Induced Tongue Carcinogenesis in the Rat

“…The high incidence of Ha-ras mutations raises the possibility that they represent an early event in TC development. In our parallel study with 4NQO-induced TCs in F 1 rats, frequent loss of the WF allele and point mutations in the remaining DA allele at the Ha-ras gene were observed [16] .…”

“…Polymorphism in the germline as well as somatic changes may contribute to the progression of TCs. Unlike human TCs [16] , genetic changes in p53 were not frequent in the 4NQO-induced rat TCs except in some large TCs.…”

“…In contrast, Tscc1 (RNO19) and Tscc5 (RNO14) were not involved in LOH. In Tscc2 , Tscc3 , and Tscc4 , the WF allele was preferentially lost and LOH frequency increased in parallel with the size of the TC, raising the possibility that the functions of these QTL are to suppress tumour progression [16] . Other hot spots showing 1 30% frequency were on RNO5 and 6, bearing p15 INK4B /p16 INK4A and Msh2 , respectively.…”

“…Among them, special attention should be given to the QTL on RNO5, as frequent loss of the WF chromosomal segment of RNO5 was found where this QTL and the tumour suppressor genes p15 and p16 were mapped. In larger TCs, point mutations and methylation of p15 and p16 also occurred [16] . Polymorphism in the germline as well as somatic changes may contribute to the progression of TCs.…”

Pthlh, a promising cancer modifier gene in rat tongue carcinogenesis

“…Positions of loci were mapped on the Rat Genome Database (6). LOH analysis was performed for tumor samples from the F1 progeny of DA and WF rats as previously described (11,12). Genomic DNA from TCs of 100 reciprocal F1 rats (50 females and 50 males) was used to survey LOH.…”