Koos Boeve

145 Methylation biomarkers for the detection of tumor DNA in saliva BACKGROUND Oral Squamous Cell Carcinoma (OSCC) is the most common subtype of head and neck cancer. It is the sixth most common cancer worldwide, accounting for 650,000 new cases and 350,000 related deaths annually [1]. Over the last 30 years, the incidence of OSCC has almost doubled, while the 5-year survival increased by 10% [2], reaching a 5-year survival of only 48% [3]. Risk factors for recurrence of OSCC are locally residual cancer after treatment of the first primary tumor or field cancerization of the oral mucosa [4,5]. Residual tumor cells are isolated cells of the first primary tumor which can remain after treatment and have the potential to develop into a local recurrence. Due to the small size of isolated cells and often submerged location, these residual tumor cells are often discovered late by regular clinical examination or imaging [5]. Due to the long-term exposure to tobacco and alcohol the epithelium of the upper aerodigestive tract might harbor areas with accumulation of pre-cancerous (epi)genetic changes [6,7], with or without clinical manifestation which is known as field cancerization [5]. These (epi)genetic changes drive carcinogenesis (6) and therefore areas with field cancerization are at risk of developing a new malignant tumor [8]. Besides the difficulty in detecting residual tumor cells/precancerous epithelial cells and the challenge of detecting the conversion of clinical visible precancerous fields (e.g. leukoplakia and erythroplakia) into new tumors as early as possible, the detection of local recurrences at an early stage is complicated by the consequences of earlier treatment. The resection area of the first primary tumor might be reconstructed with tissue from extra-oral donor sites and fibrosis is induced by surgery and irradiation [9]. Although local recurrences and new primary tumors are clinically difficult to detect at an early stage, (epi)genetic alterations in DNA from residual primary tumor cells or field cancerization cells released into saliva might be detectable before clinical manifestation of recurrent disease [10]. Using (epi)genetic alterations to detect tumor DNA in saliva is therefore a promising new non-invasive strategy for the early detection of local recurrences. Alteration in DNA methylation status is one of the epigenetic aberrations that drives tumor genesis in OSCC [11]. Changes in DNA methylation are associated with etiological factors such as cigarette smoking and alcohol consumption [6,7] through regulation of DNA methyltransferases (DNMT) [8,12,13]. Changes in DNMT expression might result in genome- wide hypermethylation associated with one of the hallmarks of cancer, chromosomal instability [14] as well as the downregulation of tumor suppressor genes [14]. Moreover, DNA methylation changes occurs early in tumorigenesis [14]. Therefore, DNA methylation