Elsevier

Oral Oncology

Volume 35, Issue 6, November 1999, Pages 561-563
Oral Oncology

Low incidence of H-, K- and N-ras oncogene mutations in cytological specimens of laryngeal tumours

https://doi.org/10.1016/S1368-8375(99)00032-9Get rights and content

Abstract

Laryngeal cancer is a rare type of neoplasia, constituting approximately 2% of all human cancers. Mutations of the ras gene family is one of the main activating mechanisms in human cancer. Their involvement in head and neck cancer has been mainly demonstrated at the level of the overexpression whereas ras mutations in these cancers are rare in the Western world. In the present study we explored the incidence of codon 12-point mutation in the H-, K- and N-ras genes, in 41 laryngeal cytological specimens. These specimens corresponded to 19 benign and 22 malignant lesions of the larynx. Only two specimens carried a codon 12-point mutation in the K-ras gene (4.8%) while no mutation was detected in the H- and N-ras genes. K-ras mutations were detected in one benign and one malignant specimen. These results indicate low incidence of ras oncogene mutations in laryngeal cytological specimens.

Introduction

Laryngeal cancer constitutes approximately 2% of all cancers and 90–95% are squamous cell carcinomas. Alcohol and tobacco abuse seem to be closely related to laryngeal cancer. Oncogenes are involved in a wide range of human tumours, including laryngeal tumours. Loss of heterozygosity in several chromosomal arms is also a common feature in these tumours, indicating involvement of putative tumour suppressor genes in the development of the disease [1].

Ras family genes (H-, K- and N-ras) encode for a 21-kDa (p21ras) protein which possesses GTPase activity and participates in a signal transduction pathway [2]. Activated ras oncogenes have been detected in several human cancers and this alteration has been associated with the development of the disease [3]. Mutations in codons 12, 13 and 61 constitute one of the most frequent alterations in human cancer [3], [4]. Overexpression of the ras genes has also been shown to cause a step towards malignant transformation [5]. The role of the ras genes in squamous cell carcinomas of the head and neck (SCCHN) has been investigated at the level of the RNA and protein expression, as well as by mutational analysis [6], [7], [8]. The rate of ras gene mutations has been reported as 4–14% in head and neck carcinomas but in SCCHN it is less than 5% in the Western world [8], [9], [10], [11], [12], [13]. A high frequency of ras gene mutations has been found in tumours diagnosed in India, probably related to the action of specific exogenous factors [14], [15].

Another important feature in head and neck tumour cells is loss of heterozygosity (LOH) and microsatellite instability (MI) of a hexanucleotide repeat located in intron 1 of the H-ras gene, providing further evidence for the involvement of the H-ras gene in this disease [16]. LOH of H-ras gene in human tumours may indicate that this gene has a tumour suppressor role, as suggested previously [17], [18], since normal H-ras suppresses the malignant phenotype of transformed cell lines, acting as a tumour suppressor gene.

In the present study, we analyzed 41 laryngeal cytological specimens for mutations in codon 12 of H-, K- and N-ras genes. We employed a non-radioactive polymerase chain reaction (PCR)-based method for our analysis. Only two point mutations in K-ras and none in the H- and N-ras genes were detected, suggesting that ras mutations are a relatively rare phenomenon in laryngeal tumours.

Section snippets

Tumour specimens and DNA extraction

Forty-one laryngeal cytological specimens were obtained from the General Hospital Nikaias, Piraeus, Greece, and from the Otorhinolarygology Department of the University Hospital Heraklion, Crete, Greece (22 squamous cell carcinomas of the larynx and 19 benign neoplasias). The biological material was obtained through direct laryngoscopy, by a brush. Part of the material was smeared on slides and fixed by alcohol for Papanicolaou stain and conventional cytological diagnosis. The remaining part

Results

Among the 41 cytological laryngeal specimens analysed, only two were found to carry a ras mutation, in codon 12 of K-ras. (Table 1). Representative examples of specimens carrying a K-ras mutation are shown in Fig. 1. The K-ras mutations were detected in one benign and one malignant lesion of the larynx. No mutations were found in codon 12 of H- or N-ras. These results are in agreement with previous studies, which show less than 5% ras mutations in SCCHN in the Western world [8], [9], [10], [11]

Discussion

Genetic alterations in oncogenes [20] and tumour-suppressor genes have been implicated in the initiation, promotion and progression of cancer. The ras family of genes (H-, K- and N-ras) encodes a 21-kDa membrane protein (p21ras) which possesses GTPase activity and participates in a signal transmission pathway [2], [3], [21]. Hot spots for ras mutations are found in codons 12, 13 and 61, causing the mutant protein to lose its activity to exchange GTP with GDP, which is related to the promotion

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