Relevance of chromosomal band 11q13 in oral carcinogenesis: An update of current knowledge

Highlights

• 11q13 amplification is a key event in oral carcinogenesis.

• We examine the 11q13 amplicon and its amplicon cores and drivers in OSCC.

• We review the oncogenic implications of oncogenes in the 11q13 amplicon.

• We analyze the therapeutic targeting of chromosome band 11q13 in HNSCC and OSCC.

• We discuss future research lines.

Abstract

An important event in oral carcinogenesis is the amplification of chromosomal band 11q13, in which numerous oncogenes and some tumor-suppressor genes are localized and frequently co-amplified during the malignant transformation of oral epithelium. The objectives of this study were to review published data on the involvement of 11q13 amplification in oral cancer, to provide an update on novel concepts and terminology related to gene amplification, and to explore the composition of the 11q13 amplicon in OSCC, including its most relevant amplicon cores and potential drivers. We report on the critical oncogenes and tumor-suppressor genes in 11q13 that may play a major role in oral cancer, focusing on their functions, on the characteristics acquired by their amplification, and on their clinicopathological implications. Finally, we discuss the possible usefulness of the 11q13 region as a therapeutic target in oral cancer.

Introduction

DNA copy number aberrations [1] in the form of microscopic (>500 kbp) or submicroscopic (<500 kbp) genetic material gains or losses represent a major genetic disorder that can promote cancer development [2]. The aberrations are frequently observed as the net gain or loss of a complete chromosome (aneuploidy) or as the partial gain or loss of a chromosome, producing an amplification or deletion, respectively, of genetic material [1], [3], [4]. Gene amplification is defined as an increase in copy number in a restricted region of a chromosome arm [4] and is a common mechanism for oncogenesis, which is frequently associated with the overexpression of amplified genes [4]. Gene amplification also has programmed and strictly regulated physiological functions during development, as in Drosophila oogenesis, the development of salivary glands of Sciara Cophrophila larvae, or chicken myogenesis [5], [6]. It is considered to begin with a double-strand DNA break [4], while the aberrant progress of the cell through its cycle would likely require the evasion of some robust checkpoints, including the p53-mediated checkpoint [4], [7], [8]. The most plausible hypotheses to explain the initial double-strand DNA break have related it to fragile chromosome sites, telemetric dysfunction, or DNA replication errors [4]. Once established, amplification can appear in three different forms: extrachromosomal copies, also known as “double minutes”; intrachromosomal copies, forming homogeneously stained regions (HSRs); and copies dispersed throughout the genome [4], [6], [9]. The different forms of presentation can coexist in the same cell, which often carries >5 copies, with some reports of >500 copies [6].

The amplification of a region frequently produces the expression of genes associated with human tumor development. The degree of causal relationship between amplification and cancer development depends on the amplified region and tumor type. Thus, the alteration proto-oncogene v-myc avian myelocytomatosis viral oncogene neuroblastoma derived homolog (MYCN), located in 2p24, amplified in neuroblastomata, is known to cause neuroblastoma development [9]. In contrast, the amplification of chromosome band 11q13 frequently houses various co-amplified genes and is considered a prime example of the complexity of amplification in cancer [4], [9], [10]. The identification and quantification of amplified DNA in tumor cells permits the isolation of new candidate oncogenes involved in cell growth regulation and possibly in tumor development. Comparative genome hybridization (CGH), fluorescent “in situ” hybridization (FISH), Quantitative microsatellite analysis (QuMA), BAC end sequencing, and digital karyotyping are the most widely applied techniques for mapping and measuring amplification [4]. Increases in the resolution of amplification mapping techniques have led to the development of specific concepts related to amplification, including: amplicon, i.e., a focal DNA region with copy number increase; and amplicon driver, i.e., an amplified gene within an amplicon, whose expression confers advantages to the host cell and contributes to the maintenance of the malignant phenotype [11]. Recent studies demonstrated that an amplicon can house more than one amplicon driver [11], [12]. However, the characterization of a gene as an amplicon driver is challenging, because the transcriptional activation of some oncogenes is not regulated by the amplification mechanism alone. For instance, CCND1 amplification in 11q13 is an essential mechanism for regulating overexpression of cyclin D1 protein in oral squamous cell carcinoma (OSCC); however, this overexpression is also regulated by other mechanisms such as mitogen-activated protein kinase (MAPK) or phosphatidylinositol 3-kinase (PI3K) signaling pathways, which are often altered in oral cancer, hampering identification of CCND1 as driver in the 11q13 amplicon. Another novel amplification-related concept is the amplicon core, i.e., the minimum unit within an amplicon that can be amplified independently from other regions in the same amplicon [11].

Genetic material amplification has diagnostic and prognostic implications and can serve as a therapeutic target, while research on MYC, erb-b2 receptor tyrosine kinase 2 (ERBB2), epidermal growth factor receptor (EGFR), and CCND1 genes appears especially promising. Various publications have described gene amplification as an acquired drug-resistance mechanism. Technological advances over the past decade have achieved the genomic characterization of 11q13 region amplification, which is prevalent in numerous tumor types, including cancers of breast (15–20%), ovary (16%), bladder (11%), lung (9%), and pancreas (14–25%), melanoma (9–44%), and esophagus squamous cell (33%) and head-and-neck (30–62%) carcinomas [11]. It is of particular importance in OSCC, in which amplification of chromosome band 11q13 is highly frequent and has major clinicopathological repercussions.

The objectives of this study were: to review published data on chromosome band 11q13 amplification in human cancer, particularly in OSCC; to provide an update on key terminology and concepts; and to explore the composition of 11q13 amplicon in OSCC, including its most relevant cores and potential drivers. We report on the critical oncogenes and tumor-suppressor genes located in 11q13, which may play a major role in oral cancer, describing their functions, the characteristics conferred by their amplification, and the clinicopathological implications. Finally, we evaluate the possible usefulness of the 11q13 region as a therapeutic target in oral cancer.

Locus 11q13 is one of the most frequently amplified and important chromosomal regions in human cancer. The 11q13 amplicon is formed by long sequences of amplified DNA. Almost all of its genes can be simultaneously co-amplified, and it often contains neighboring co-amplified genes, making elucidation of its role in cancer highly complex [4], [9], [10]. The amplification values of 11q13 are frequently low (3–10 copies) [9], with the amplified DNA localized intrachromosomally as an HSR, and all copies are usually, although not always, maintained in the original chromosomal region of 11q13 [4], [6], [13].

11q13 amplification was first reported around three decades ago in fibroblast growth factor 3 (FGF3) gene (also known as INT2) in breast cancer [14] and was subsequently described in other cancers, including squamous cell carcinomas [15]. Shortly afterwards, fibroblast growth factor 4 (FGF4) gene (HSR1), also mapped in band 11q13, was found to be co-amplified with FGF3 in most tumors with 11q13 amplification, including squamous cell carcinomas [16], [17]. However, despite the co-amplification of FGF3 and FGF4, the protein expression of these genes is frequently low or undetectable [10]. This has prompted the search for the true driver genes of 11q13, i.e., those that possess the capacity to overexpress proteins and offer the host cells an oncogenic advantage. Thus, CCND1 gene [18], which is invariably found to be amplified and overexpressed in squamous cell cancer [19], has been proposed as 11q13 amplicon driver [11]. It encodes cyclin D1 protein, which promotes G1/S transition in the cell cycle, it is overexpressed in numerous tumors [20] and it plays a key role in OSCC development [21]. Likewise, CTTN gene (aka EMS1), which can be co-amplified alongside its neighbor CCND1 in 11q13 and overexpressed in various human cancers [22], also appears to play a major role in tumor development. Other genes mapped in 11q13-q14 (EMSY, p21 (RAC1) activated kinase 1 [PAK1], and GRB2 associated binding protein 2 [GAB2]) may also confer selective advantages to cells in which they are amplified [11]. It appears likely that other co-amplified genes are involved in tumor development, underscoring the complexity of 11q13 amplicon. In the late 1990s, comparative genomic hybridization (CGH) studies observed the independent amplification of genes in different loci mapped in 11q13, not only neighboring genes, and demonstrated the presence of four distinct cores within the amplicon [11]. A more precise mapping of these cores in tumor tissue has recently been achieved using high-resolution microarray-based CGH, revealing the composition and borders of the 11q13 amplicon [11]. The author reported that Core 1 (66.4–67.3 Mb) contained genes with a possible role in human cancer, e.g., RNA binding motif protein 4 (RBM4), aryl hydrocarbon receptor interacting protein (AIP), or cyclin dependent kinase 2 associated protein 2 (CDK2AP2), as did Core 2 (68.8–70.3 Mb), e.g., CCND1, CTTN, Fas associated via death domain (FADD), oral cancer overexpressed 1 (ORAOV1) or FGF3/4, some of which (CCND1, CTTN, and FADD) have been proposed as potential amplicon drivers. Core 3 (75.4–76.4 Mb) was also found to contain relevant genes, e.g., Wnt family member 11 (WNT11) and EMSY, and the latter is also a candidate amplicon driver. Interestingly, Core 3 houses UV radiation resistance associated (UVRAG) gene, which has been attributed with tumor-suppressor activity, because it is an essential component of the Beclin1-PI3KC3 complex, an important tumor cell growth and autophagy signaling checkpoint [23]. In other words, the amplification patterns of genes in the 11q13 amplicon do not necessarily have oncogenic functions. Finally, core 4 (77.0–79.2), which extends to region 11q14.1, includes PAK1 and GAB2 genes, both proposed as possible amplicon drivers. Characteristic 11q13 amplification patterns have been defined in tumors at different sites. Thus, amplification of cores 1 and 2 is less frequent in ovarian than breast cancer, in which all cores are usually amplified [24], [25], [26]. Moreover, CCND1 amplification has been reported in adenocarcinomas, melanoma, and squamous cell carcinomas [4], [21], [27], [28], whereas EMSY amplification appears to be frequent in breast adenocarcinoma but rare in squamous cell carcinomas [4], [29].