¹Section of Medical and Molecular Genetics, Department of Reproductive and Child Health, University of Birmingham, The Medical School, Edgbaston, Birmingham, B15 2TT, UK

²Department of Obstetrics and Gynecology, Washington University School of Medicine, St Louis, Missouri, USA

³Breast Cancer Research Unit, University of Leicester, Clinical Sciences, Glenfield Hospital, Leicester, UK

⁴Department of Internal Medicine, National Kinki-Chuo Hospital, Osaka, Japan

⁵Laboratory of Immunobiology, NCI-Frederick Cancer Research and Development Center, Frederick, Maryland, MD, USA

⁶Hamon Center for Therapeutic Oncology Research, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, TX, USA

Correspondence to: F Latif, Section of Medical and Molecular Genetics, Department of Reproductive and Child Health, University of Birmingham, The Medical School, Edgbaston, Birmingham, B15 2TT, UK

Previously we analysed overlapping homozygous deletions in lung and breast tumours/tumour lines and defined a small region of 120 kb (part of LCTSGR1) at 3p21.3 that contained putative lung and breast cancer tumour suppressor gene(s) (TSG). Eight genes including RASSF1 were isolated from the minimal region. However, extensive mutation analysis in lung tumours and tumour lines revealed only rare inactivating mutations. Recently, de novo methylation at a CpG island associated with isoform A of RASSF1 (RASSF1A) was reported in lung tumours and tumour lines. To investigate RASSF1A as a candidate TSG for various cancers, we investigated: (a) RASSF1A methylation status in a large series of primary tumour and tumour lines; (b) chromosome 3p allele loss in lung tumours and (c) RASSF1 mutation analysis in breast tumours. RASSF1A promoter region CpG island methylation was detected in 72% of SCLC, 34% of NSCLC, 9% of breast, 10% of ovarian and 0% of primary cervical tumours and in 72% SCLC, 36% NSCLC, 80% of breast and 40% of ovarian tumour lines. In view of the lower frequency of RASSF1 methylation in primary breast cancers we proceeded to RASSF1 mutation analysis in 40 breast cancers. No mutations were detected, but six single nucleotide polymorphisms were identified. Twenty of 26 SCLC tumours with 3p21.3 allelic loss had RASSF1A methylation, while only six out of 22 NSCLC with 3p21.3 allele loss had RASSF1A methylation (P=0.0012), one out of five ovarian and none out of six cervical tumours with 3p21.3 loss had RASSF1A methylation. These results suggest that (a) RASSF1A inactivation by two hits (methylation and loss) is a critical step in SCLC tumourigenesis and (b) RASSF1A inactivation is of lesser importance in NSCLC, breast, ovarian and cervical cancers in which other genes within LCTSGR1 are likely to be implicated. Oncogene (2001) 20, 1509-1518.

3p; tumour suppressor gene; methylation; lung cancer; breast cancer