Acute Lymphoblastic Leukemia (ALL) is characterized by primary and secondary genetic aberrations, which lead the initiation and progression of the leukemic clone (1,2). Primary abnormalities are often chromosomal translocations, whereas secondary abnormalities are usually copy number alterations (CNAs) and point mutations, which may be present in only a subset of leukemic cells (2).

Chromosomal abnormalities are used as biomarkers to provide subtype, outcome and therapeutic response information. The application of genomic tools either in cases with or without an established abnormality revels copy number alterations, which can be used alone or in combination as prognostic information (3).

Initially, the role of a disintegrin and metalloproteases (ADAMs) proteins was limited to the fusion of gametes; however, due to their adhesion properties in intercellular interactions, their involvement in tumor biology has also been suggested (4).

Members of the ADAM family are currently an object of considerable scientific attention, due to their role in numerous signaling pathways associated with carcinogenesis, such as phosphoinositide 3-kinase (PI3K), Notch and transforming growth factor (TGF)-β (5-7). Research concerning ADAMs often focuses on their role in carcinogenesis and as potential targets of novel anticancer therapies (8,9).

The properties of ADAMs mentioned above render them an important object of interest in cancer. Thus, the present study investigated CNAs in 12 different ADAM genes that have been implicated in carcinogenesis and associated their CNA status with the clinicopathological data of ALL pediatric patients. The findings of the present study demonstrate that the deletion of ADAM3A is significantly related to leukocytosis and high-risk cases.

All samples exhibited at least one aberration to one of the investigated ADAM genes (Table III). These genes are described in the literature as being associated with the carcinogenesis of numerous types of cancer. For the 12 genes investigated, only ADAM29 exhibited no changes. The most frequent aberrations were amplifications of ADAM6 (94%). Notably, ADAM3A was deleted in 31% of the samples whilst it was amplified in 31%. It is noteworthy that 8 of these gene alterations have not been previously associated with ALL (Table III).

The occurrence of gene aberration according to the NCI risk group, sex, age and cytogenetic findings in at least 2 samples is presented in Table IV. Deletions involving ADAM3A were significantly associated with male patients (P=0.025), leukocytosis (P=0.007) and NCI-HR cases (P=0.004). We did not find significant results correlating any other genes (Table IV).

A total of 5 patients (31%) were hyperdiploid. The majority of the detected chromosomal gains corresponded to trisomies, gain of chromosomes X, 6 and 3 were the most frequent. A hypodiploid patient with loss of chromosomes 2, 3, 9, 11, 12, 18, 19 and 20 was also identified (data not shown). However, no significant results were found associating any numerical chromosomal abnormalities.

In the present study, the ADAM genes investigated exhibited a low frequency of CNAs, with the exception of the ADAM6 gene, which was amplified in 94% of the samples and ADAM3A (amplified in 31% and also deleted in 31% of cases) (Table III). This similar frequency between ADAM3A aberrations is probably due the intra- and inter-heterogeneity of malignant cells or the reduced sample size.

The ADAM protein family includes 29 members that are known to play an important role in the regulation of cell adhesion, the activation of oncogenic receptors (Notch and HER2), tumorigenesis, in cell migration and in the production of cytokines and growth factors; however, the specific functions of the majority of ADAM genes are not yet fully understood (11-13).

Concerning other members of ADAM family, overexpression of ADAM28 was found in lung and breast carcinomas, while loss of expression of ADAM23 was found in breast tumors (14). ADAM17 has been shown to be involved in EGFR regulation and their overexpression in astrocytes promotes an increase in cell proliferation and invasion (14).

ADAM3A is located at chromosome 8p11.23, the locus that exhibits a strong association with cancer (13). ADAM3A amplifications have been observed in squamous cell carcinoma of the conjunctiva and in a subtype of B-cell lymphoma (15,16). The amplification of genes located on 8p11.23 has been linked to tumor development and metastasis (17,18); however, in the present study, no significant clinicopathological association with ADAM3A amplification was found.

As regards deletions of ADAM3A, these have been previously identified in high-grade gliomas, cribriform neuroepithelial tumors and extranodal NK/T cell lymphoma of the nasal type (12,14,19). Dun et al (20) found frequent 8p11.23 deletion as secondary genetic abnormalities in cases ETV6-RUNX1-positive leukemia. However, to the best of our knowledge, the present study is the first to describe ADAM3A deletion in leukemia. It is important to note that all samples with ADAM3A deletion investigated herein were negative for the gene fusions ETV6-RUNX1, BCR-ABL1, MLL-AF4 or TCF3-PBX1 (Table I).

Furthermore, deletions on the short arm of chromosome 8 (8p) are common in different tumor types (20-24), suggesting that tumor suppressor genes on 8p are frequently co-deleted reinforcing the functional role of those genes in carcinogenesis (24).

In the present study, the deletion of ADAM3A was associated with high-risk cases (NCI-risk) and leukocytosis. This finding is consistent with the observation that CNAs in 8p11 (both amplification and deletion) are commonly associated with a more aggressive tumor phenotype (24), which is the case for high-risk ALL patients.

Of note, ADAM3A deletion also exhibited an association with male patients. These results indicate a potential sex-specific association between ADAM3A deletion in the study population of the present study. One explanation could be the fact that males are generally more exposed to carcinogenesis than females (25). However, the exact reasons for this apparent sex-specific association and the risk of leukemia cannot be fully explained.

Leukocytosis typically occurs in response to hematological malignancies and inflammation, among others conditions (25,26). Among the mechanisms that connect inflammation to cancer are intrinsic factors, which include acquired genetic alterations affecting oncogenes, tumor suppressors and genome stability genes that contribute to the activation of the inflammatory pathways (26,27). Several molecular and cellular signaling pathways have been identified as links between inflammatory processes and cancer development (27,28).

Moreover, in different types of cancer, the inflammatory process often determines the development of a tumor and has an impact on the course and prognosis of the disease (28,29). Elevated levels of certain metalloproteinases have been reported in some types of inflammatory responses (29,30). ADAM proteins are expressed among others, by human lymphocytes, and they can interact with adhesion proteins located on the surface of other leukocytes (30,31).

The capacity of certain ADAMs to differentiate immunologically competent cells suggests that they play an important role in immunological processes (31,32). Dendritic cells, B cells and monocyte subpopulations also express these proteins, which indicates the important roles of ADAMs in cancer prognosis (4,32,33).

Thus, it was hypothesized that ADAM3A may act as a tumor suppressor in ALL. It was suggested that ADAM3A deletion, alone or combined with other tumor suppressor genes in this genomic region, plays an important role in leukemic transformation, contributing to the activation of inflammatory pathways.

There are few studies available regarding the role of ADAM6 in cancer; however, they demonstrate a potential association in cancer development, similar to the other members of the ADAM family (34-37).

Studies in vitro on different types of tumor cells (6,38-40) have demonstrated that the biological function of ADAM10 can be cell type-specific, that is, depending on the substrate activated by this gene.

Liu and Chang (41) demonstrated that the protease PILP-1-induced death of leukemia cells was mediated through the downregulation of ADAM17 and the subsequent inactivation of Lyn and Akt. Several studies have focused on the importance of ADAM17 upregulation in tumor malignancy (42-44). Thus, based on these studies, the suppression of ADAM17 protein expression may have potential for cancer therapy.

ADAM28 is overexpressed in several cancer types and is related to cell proliferation and lymph node metastasis (45-48). The overexpression of ADAM28 is asscoiated with relapse and is potentially regulated by the PI3K/Akt pathway, suggesting that ADAM28 may be a novel biomarker for evaluating relapse in B-ALL and as a potential therapeutic target in B-ALL patients (49).

In B-CLL culture, ADAM28 knockdown has been shown to decrease the release of CD200 (a membrane glycoprotein of the immunoglobulin superfamily), indicating that ADAM28 plays a role in the shedding of CD200 from B-cell CLL cells (50).

In conclusion, the present study reinforces that aCGH allows the identification of novel genes associated with cancer and emphasizes the need for including the investigation of submicroscopic aberrations as additional markers for risk stratification. Through this technique, recurrent aberrations in ADAM genes were identified in the present study, particularly in ADAM3A and ADAM6, suggesting that these genes may have important functions in carcinogenesis. Thereby, ADAM3A deletion can be related to leukemic process in patients with high-risk characteristics. Although the sample size was limited, the results of the present study should be considered, taking into account that the associations observed were concordant with those of previous studies mentioned above. There is substantial evidence supporting the involvement of ADAMs in cancer formation or progression. Thus, the effects of the aberration on these genes need to be fully defined in hematological malignancies, particularly in leukemia.