Alterations in the cellular genome affecting the expression of function of genes controlling cell growth and differentiation are considered to be the main cause of cancer.
Over 30 oncogenes can be activated by insertional mutagenesis, single point mutations, chromosomal translocations and gene amplification.
The ras oncogenes have been defected in 15∼20% of human tumors that include some of the most common forms of human neo-plasia and are known to acquire their transforming properties by single point mutations in two domains of their coding
sequences,
most commonly in codons 12 and 61.
The ras gene family consists of three functional genes, N-ras, K-ras and H-ras which encode highly similar proteins of 188 or 189 amino acid residues generically known as P21. ras proteins have been shown to bind GTP and GTP, and possess
intrinsic
GTPase activity.
Experimental study was performed to observe the mutational change of the ras gene family and apply the results to the clinical activity.
36 Golden Syrian Hamster each weighing 60∼80ｇ were used and painted with 0.5% DMBA by 3 times weekly on the right buccal cheek(experimental side) for 6, 8, 10, 12, 14 and 16 weeks.
Left buccal cheek (control side) was treated with mineral oil as the same manner of the right side. The hamsters were sacrificed on the 6, 8, 10, 12, 14 & 16 weeks.
Normal and tumor tissues from paraffin block were completely dissected by microdissection and DNA from both tissue were isolated by proteinase K/phenol/chloroform extraction. Segments of the K-ras and H-ras gene were amplified by PCR using the
oligonucleotide primers corresponding to the homologous region (codon 12 and 61) of the hamster gene, and then confirmational change of ras genes was observed by SSCP and autosequencing analysis.
The results were as follows:
1. Malignant lesion could be found in the experimental side from the experimental six weeks.
2. One hamster among six showed point mutation of the H-ras codon 12(G→A transition) at the experimental 10 and 14 weeks.
3. One of six at a 6 weeks, two of six at 8 weeks and one of six at 12 weeks revealed the confirmational change of the H-ras codon 61(A→T transversion).
4. The incidence of point mutation of H-ras codon 12 and 61 were 5.5%(2 of 36) and 11%(4 of 36) respectively.
5. Point mutation of the K-ras could not be seen during the whole experimental period.
From the above results, these findings strongly support the concept that H-ras oncogenes may have the influence of the DMBA induced carcinoma of hamster buccal pouch.
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