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Whereas most carcinomas occur through a sequential step, atypical adenomatous hyperplasia and bronchioloalveolar carcinoma pathway is known for pulmonary adenocarcinoma. This type is known as terminal respiratory unit adenocarcinoma. Based on our observation of transitions from normal ciliated columnar cells to adenocarcinoma via dysplastic mucous columnar cells, we reviewed our archive of pulmonary adenocarcinoma. Terminal respiratory unit type adenocarcinoma was defined as adenocarcinoma with type II pneumocyte, Clara cell, or bronchiolar cell morphology according to previous reports. Among 157 cases, 121 cases have been identified as terminal respiratory unit type adenocarcinoma and 36 cases as non-terminal respiratory unit type adenocarcinoma. Among non-terminal respiratory unit type adenocarcinoma, 24 cases revealed mucous columnar cell changes that were continuous with bronchial ciliated columnar cells. The mucous columnar cells became dysplastic showing loss of cilia, disorientation, and enlarged nuclei. Adenocarcinoma arose from these dysplastic mucous columnar cells and, characteristically, this type of adenocarcinoma showed acute inflammation, and honeycombing changes in the background. TTF1 immunostaining was consistently negative. In a case study with 14 males and 10 females, including 12 smokers or ex-smokers, EGFR and KRAS mutations were detected in 3 and 6 patients, respectively. We think that this kind of adenocarcinoma arising through mucous columnar cell change belongs to non-terminal respiratory unit type adenocarcinoma, and mucous columnar cell change is a precursor lesion of pulmonary adenocarcinoma.

Activating epidermal growth factor receptor (EGFR) gene mutations are frequently detected in lung adenocarcinomas, especially adenocarcinomas with a nonmucinous bronchioloalveolar carcinoma component. EGFR-mutated lung adenocarcinomas respond well to EGFR tyrosine kinase inhibitors. We previously found that most (88%) pure nonmucinous bronchioloalveolar carcinomas (adenocarcinoma in situ) already harbor EGFR mutations, indicating that the mutations are an early genetic event in the pathogenesis. We examined 54 atypical adenomatous hyperplasias, precursor lesions of lung adenocarcinomas, obtained from 28 Japanese patients for the hotspot mutations of EGFR exons 19 and 21 and K-ras codon 12. EGFR mutations were observed in 17 of the 54 (32%) atypical adenomatous hyperplasias examined: Ten and seven atypical adenomatous hyperplasias had deletion mutations at exon 19 or point mutations (L858R) at exon 21, respectively. We did not observe apparent histological differences between atypical adenomatous hyperplasias with and without EGFR mutations. K-ras mutation (G12S) was detected in only one atypical adenomatous hyperplasia. As EGFR mutational frequency of atypical adenomatous hyperplasias was much lower than that of nonmucinous bronchioloalveolar carcinomas, we surmise that EGFR-mutated atypical adenomatous hyperplasias, but not atypical adenomatous hyperplasias with wild-type EGFR, are likely to progress to nonmucinous bronchioloalveolar carcinomas.

Matrix metalloproteinases (MMPs) are believed to play an essential role in cancer invasion, although detailed differences between noninvasive and invasive lung carcinomas are still unclear. To elucidate the expression and activity patterns of MMPs in noninvasive and invasive carcinoma of the lung, we performed in situ hybridization and real-time reverse transcription-polymerase chain reaction to detect messenger RNAs (mRNAs) of MMPs and their tissue inhibitors (TIMPs). The basement membrane was evaluated by immunohistochemistry for type IV collagen. Gelatinase activity was examined by zymography and in situ zymography. A total of 14 surgically resected primary pulmonary adenocarcinomas were used for this study. All the tumors were adenocarcinoma mixed bronchioloalveolar carcinomas according to the 1999 WHO classification. MMP and TIMP2 mRNAs were detected by in situ hybridization in all samples, in both noninvasive and invasive carcinoma components. Signals for MMP mRNAs were significantly higher in both noninvasive and invasive carcinomas than in tumor-free lung tissue. However, the differences were small between noninvasive and invasive carcinomas, not only in the amount of mRNA but also in the activity of the MMPs. In most carcinomas, stromal fibroblast-type cells tended to express levels of MMP and TIMP2 mRNAs that were higher than or at least similar to those expressed in epithelial cells. Our data on mixed adenocarcinoma suggest that noninvasive carcinoma areas already express a molecular mechanism involving MMPs similar to that expressed by invasive carcinoma areas. Stromal fibroblast-type cells seem to be the most important source of MMPs, from the earliest event of tumor invasion by pulmonary adenocarcinomas.

Mutations of p53 tumor suppressor gene have been shown to contribute to tumorigenesis in a variety of human cancers. Normally, p53 protein degrades rapidly and is not detected by immunohistochemical procedure, but mutant p53 and wild-type p53 stabilized by certain viral oncoproteins can accumulate to immunohistochemically demonstrable levels. Conventional pulmonary adenocarcinomas and bronchioalveolar carcinomas, although morphologically similar, exhibit different biological behavior and clinical prognosis. To explore the differences in the expression of p53 protein in these two tumor types, we performed immunohistochemistry on 10 conventional pulmonary adenocarcinomas and 12 bronchioalveolar carcinomas on formalin-fixed and paraffin-embedded material, using the commercially available monoclonal antibody against the mutant p53 protein. Intense nuclear staining was observed in 80% (8/10) of conventional pulmonary adenocarcinomas, whereas all 12 bronchioalveolar carcinomas were negative for p53 protein. These observations indicate that altered p53 protein (probably mutant) is overexpressed in conventional adenocarcinomas and may be involved in its tumorigenesis or progression. On the other hand, the lack of p53 expression in bronchioalveolar carcinomas suggests that an alternative pathway is likely to be responsible for its tumorigenesis. Furthermore, p53 protein immunostaining may be useful as an adjunct in differentiating conventional pulmonary adenocarcinomas from bronchioalveolar carcinomas.

Bronchioloalveolar lung carcinoma (BAC) is a unique type of lung cancer with distinguishing pathologic, biologic, epidemiologic, and perhaps etiologic features that set it apart from all other forms of lung cancer, including general adenocarcinoma, into which it is traditionally grouped. Recent studies at our institution have demonstrated a near exponential increase in BAC cases with 25% showing evidence of multifocality. Although some theories suggest that this multifocality is caused by intrapulmonary aerosol/aspiration or lymphatic spread, this study provides evidence for multiclonality as the basis for some cases of multifocal BAC by exploiting a novel strategy for clonality determinations that involves polymerase chain reaction amplification of a 511-base pair region located within the first intron of the human hypoxanthine phosphoribosyltransferase gene, a site that contains inactive X chromosomal obligately methylated HpaII/MspI sites and single-base allelic polymorphisms in 5 to 10% of females. BAC cells, obtained by enzymatic dissociation of different fresh/paraffin-embedded tumoral foci from polymorphic individuals with multilobar or bilateral BAC, were sorted to homogeneity with a fluorescein-conjugated anticarcinoembryonic antigen and then subjected to genomic DNA extraction and HpaII digestion before polymerase chain reaction amplification and subsequent analysis of the product on denaturing gradient gel electrophoresis. The differing migrations of the single homoduplexes generated were indicative of BAC clonal nonidentity or multiclonality in three separate cases. The demonstration of multiclonality in some cases of BAC provides an alternate explanation for multifocality.