Sequential molecular abnormalities are involved in the multistage development of squamous cell lung carcinoma

To understand the molecular pathways involved in the pathogenesis of squamo us cell lung carcinoma, we obtained DNA from 94 microdissected foci from 12 archival surgically resected tumors including histologically normal epithe lium (n = 13), preneoplastic lesions (n = 54), carcinoma is situ (CIS) (n = 15) and invasive tumors (n = 12). We determined loss of heterozygosity (LO H) at 10 chromosomal regions (3p12, 3p14.2, 3p14.1-21.3, 3p21, 3p22-24, 3p2 5, 5q22, 9p21, 13q14 RE, and 17p13 TP53) frequently deleted in lung cancer, using 31 polymorphic microsatellite markers, including 24 that spanned the entire 3p arm. Our major findings are as follows: (1) Thirty one percent o f histologically normal epithelium and 42% of mildly abnormal (hyperplasia/ metaplasia) specimens had clones of cells with allelic loss at one or more regions; (2) There was a progressive increase of the overall LOH frequency within clones with increasing severity of histopathological changes; (3) Th e earliest and most frequent regions of allelic loss occurred at 3p21, 3p22 -24, 3p25 and 9p21; (4) The size of the 3p deletions increased with progres sive histologic changes; (5) TP53 allelic loss was present in many histolog ically advanced lesions (dysplasia and CIS); (6) Analyses of 58 normal and non-invasive foci having any molecular abnormality, indicated that 30 proba bly arose as independent clonal events, while 28 were potentially of the sa me clonal origin as the corresponding tumor; (7) Nevertheless, when the all elic losses in the 30 clonally independent lesions and their clonally unrel ated tumors were compared the same parental allele was lost in 113 of 125 ( 90%) of comparisons. The mechanism by which this phenomenon (known as allel e specific mutations) occurs is unknown; (8) Four patterns of allelic loss in clones were found. Histologically normal or mildly abnormal foci had a n egative pattern (no allelic loss) or early pattern of loss while all foci o f CIS and invasive tumor had an advanced pattern. However dysplasias demons trated the entire spectrum of allelic loss patterns, and were the only hist ologic category having the intermediate pattern. Our findings indicate that multiple, sequentially occurring allele specific molecular changes commenc e in widely dispersed, apparently clonally independent foci, early in the m ultistage pathogenesis of squamous cell carcinomas of the lung.