Review

10.1586/17474124.2.1.125 © 2008 Future Drugs Ltd ISSN 1747-4124 125www.future-drugs.com

The novel oncogene CD24 and its arising role in the carcinogenesis of the GI tract: from research to therapyExpert Rev. Gastroenterol. Hepatol. 2(1), 125–133 (2008).

Eyal Sagiv and Nadir Arber†

†Author for correspondenceIntegrated Cancer Prevention Center, Tel Aviv Medical Center, Tel Aviv, IsraelTel: +972 3697 4968Fax: +972 3695 0339narber@post.tau.ac.il

CD24 was first described in the early 1980s and only attributed to scattered publications,referred to as a cell surface molecule in hematopoiesis. Recently, studies are accumulating toshow that CD24 conveys a function in cell-to-cell interaction and regulation of proliferationand adhesion. CD24 appears to be highly expressed in a large variety of human cancers and tocontribute to the acceleration of tumor growth and metastases shedding by binding toplatelet (P)-selectin, L1 and by evoking – to date unknown – intracellular signal pathways.Anti-CD24 monoclonal antibodies thus act as a promising cancer treatment as was shown inthe setting of gastrointestinal cancers. Recent articles also correlate CD24 expression with theidentification of ‘tumor stem cells’.

KEYWORDS: cancer treatment • CD24 • GI cancer • monoclonal antibody • oncogene • P-selectin • signal transduction • tumor stem cell

Colorectal cancer (CRC) typically develops overa period of decades. It is a multistep process,consisting of several genetic alterations that cre-ate a change in the protein milieu of the cell anddrive it to malignant transformation [1–3]. Thereis more information on the sequence of geneticand epigenetic events that characterizes themultistep process of the transformation ofcolonic epithelium, through adenomas (pre-malignant lesion) to invasive carcinoma, thanfor any other tumor type. Much less is knownabout the molecular pathways involved in thecarcinogenesis of the more devastating cancersof the pancreas or the liver, yet similar onco-genic pathways are found impaired at high fre-quency among these tumors as well. However,as this prototype presented at 1989 for thecarcinogenesis of the gastrointestinal (GI)tract is being thoroughly investigated, andwith the completion of the Human GenomeProject, it becomes clear that many new playersin carcinogenesis are yet to be defined.

Recently, a novel gene is being reported moreand more in relation to cancers, the gene forCD24. The protein that was, up until the lastfew years, regarded solely as a cell marker on

hematopoietic cell lineages, due to it being ashort mucin-like peptide presented at the outercell surface with no transmembrane domain. Itis now being shaped as an important signaltransducer that manifests a strong regulation oncell growth and adhesion. The CD24 moleculeis of great importance in GI malignancies, asthis review aims to demonstrate [4].

The glycoproteins of tumor cells are oftenabnormal, both in structure and quantity. Inparticular, the mucin-type O-glycans, such asCD24, is enriched with several cancer-associ-ated structures [5]. These structural changes canalter the function of the cell, and its antigenicand adhesive properties, as well as its potentialto invade and metastasize as they mediate achange in the microenvironmental interactionsof the cell. Cancer-associated mucin antigenscan be exploited in diagnosis and prognosis,and in the development of cancer vaccines [5].Studies have shown that interference with theactivity of CD24 is harmful for cancer cellprosperity. Since clinical data collected show ahighly specific expression of the protein in can-cer tissues, these data strongly suggest CD24 asa target for treatment.

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The presented data summarized in this review are of a spe-cial interest, since CD24, in the setting of GI cancers beforeall others, is revealed as a potential target for treatment.

CD24 gene & protein

The human CD24 gene was first cloned in 1990, revealing a veryshort peptide, of 31 amino acids, homologous to the mouseCD24a or heat-stable antigen (HSA) [6]. CD24 is a short mucin-like protein, as it is heavily glycosylated; its peptide backbone har-bors multiple sites for both N- and O-glycosylations (FIGURE 1). Theglycosylation pattern diverges vastly within cells [7], and amongcells from different tissues [8]. Thus, the molecular weight of theprotein ranges from 25 to 75 kDa, roughly concentrating atapproximately 37 kDa in epithelial cells, as will be discussed in thisreview. CD24 is presented at the outer surface of the cytoplasmicmembrane, yet it has no transmembrane domain but is anchoredto the membrane by glycosyl–phosphatidyl–inositol (GPI).

The open reading frame that encodes the protein is 243 basepairs long, translated into an 81 amino acid peptide given topost-translational modification at the endoplasmic reticulum(ER). The signal peptide is similar to known signal-peptidesdirecting for localization at the membrane and is cleaved at theER. The 23 C-terminal amino-acids form a GPI displaced tail,enhancing the binding to GPI and are then cleaved.

Considering the short peptide chain, the CD24 mRNA is longerat 2180 base-pairs, as it includes a comparatively long 5´UTR; nospecific function for this mRNA structure is evident. Whileresearching, we hypothesized that the uniquely long 3´UTR regionis of importance in capping and stabilizing the mRNA. Prelimi-nary, unpublished data show that ectopic expression with a fulllength CD24 mRNA expression vector is more successful thanwith the open reading frame-only expression vector. Wang et al.provide preliminary data that suggest a modified expression ofCD24 influenced by the mRNA stability [9].

The chromosomal location of the CD24 gene is still debatedamong researchers, although most probably it is located onchromosome 6q21 [10]. The debate emerges from the presenceof several highly identical homologous genetic sequences alongthe human genome, found at the Human Genome Project whilethe specific location at chromosome 6, which is supposed toencompass the CD24 gene, was not fully mapped, probably dueto multiple regions of repetitive patterns in the vicinity; particu-larly high homology is observed on chromosomes 15q21 andYq11. Several evidences support the hypothesis that the chromo-some 6 copy of CD24 is the only coding copy: the gene copy onchromosome 6 is located in a region that is homolog to a wholeset of genes in the mouse chromosome 10 that includes themouse CD24 gene equivalent (CD24a, which encodes the HSAprotein; -b and -c homologue genes are found in the mouse aswell). The gene on chromosome 15 is a pseudogene as it does nothave a starting codon, but instead has a GTG codon [11]. Thecopy on chromosome Y might hypothetically transcribe a pro-tein; however, since it does not contain the intron that is present

at chromosome 6 [11], and does not have functional responsiveelement sequences or a TATA box, as present on the CD24 pro-moter on chromosome 6 [12], and since CD24 expression levels donot seem to differ among men and women, the copy on the Ychromosome is suggested to be inactive as well. Blast to the CD24mRNA sequence yields several additional locations along thehuman genome that are in high homology to large parts of thesequence, thus suggesting it as an evolutionary conserved gene.

CD24 gene population heterogeneity

A single polymorphism in the CD24 gene was described as amodifier for higher risk for multiple sclerosis (MS) [13]. Thevariance is a C→T substitution at position 280 of the gene

Figure 1. (A) A model of CD24. (B) Protein sequence comparison between human and mouse CD24.* Putative N-linked glycans.° Putative O-linked glycans.GPI: Glycosyl–phosphatidyl–inositol; P-selectin: Platelet-selectin.Adapted from Kristiansen et al. 2004 [7].

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sequence, resulting in a missense mutation (GCG [Alanine] →GTG [Valine]) at position 57 of the CD24 protein set forthimmediately after the putative cleavage site of the GPI anchor.Notably, the ω-1 position in GPI anchors is evolutionary con-served for one of a few amino acids, among which alanin is one,but valine is not. Among familial MS, the CD24v allele is prefer-entially transmitted into affected individuals, and carriersshowed a more rapid progression of the disease [13]. A very recentstudy by Sanchez et al. shows an increased risk for systemic lupuserythemat in carriers of the same polymorphism (in homozy-gous more than in heterozygous) that varied according to ethnic-ity [14]. A more recent report from the same group finds a dinu-cleotide deletion in the CD24 gene that confers protectionagainst autoimmune diseases. Two more SNPs are described bythe authors, positioned at the 3´UTR, but they do not conveyany clinical significance so far [9]. No correlation between theCD24 genotype to cancer has been published to date.

CD24 physiological expression

Expression of HSA in the mouse was described under nopathologic conditions in specific cells as early as the 1970s.Expression was observed in B-lymphocytes especially, but alsoin erythrocytes, neutrophils and at lower levels on T-lympho-cytes at certain stages of hematopoiesis. Later on, the expressionwas also observed in the developing brain and, to some extent,on different epithelial cells [7]. The protein was thus used as amarker for developing lymphocytes at the bone marrow andspecific anti-CD24 poly- and monoclonal antibodies weredeveloped [15]. The function of CD24 was not clear, and CD24knock-out in mice revealed an overall normal health with amild decrease in the number of B-cells, leading to the hypo-thesis that CD24 can serve as a marker for the primal develop-ment of B-cells; erythrocytes in these mice shows mainly aslight shortening in their half-life [16].

In humans, CD24 does not appear to be expressed in thesame cells as in the mouse; it is not, for example, expressed inerythrocytes. The protein was first reported to be expressed onspecific B-cell populations in the bone marrow, being lost attheir progression to mature B-cells and totally absent on plasmacells [17]. Thus, CD24 is nondetectable on the earliest definedB-cells until the first D–J rearrangement, reaching its highestlevels in the interleukin-7-responsive pro- and pre-B cells [18].CD24 is also re-expressed out of the bone marrow, in differenti-ating B-cells in the spleen, yet further progression required itscomplete absence [19]. Among B-cells precursor populations,absence of CD24 is a unique marker for a memory cell-enriched population [20]. Besides B-cells, CD24 is also detect-able on developing T cells, in their CD4+CD8+ double positivethymocytes, where it is shown to play a crucial role in prolifera-tion (tested in lymphopenic environment) [21] and on variousepithelial and muscle cells. At the embryonic stage, CD24expression was evident in the developing pancreas [22] and inthe nervous system [23].

CD24 is widely expressed in human cancers

In recent years, a large body of literature has evolved describ-ing CD24 overexpression in various human malignancies,solid and hematological, arising from all three germ layers,and often correlated with poor survival [7]. CD24 overexpres-sion in malignant transformation was evident in many gene-profiling analyses using microarray technology, and since2001, when a new monoclonal anti-CD24 antibody (Ab-2,clone 24C02, Neomarkers) became commercially available;allowing immunohistochemical staining in paraffinized tis-sues, wider descriptions of the protein expression were feasiblein existing tumor collectives.

Mainly, CD24 overexpression was described in non-HodgkinB-cell lymphomas and leukemia [24] and acute lymphoblasticleukemia (ALL) [25]. Valet et al. described CD24 among a pat-tern of genes to identify a high-risk group in patients with acutemyeloid leukemia [26]. Overexpression in large collections oftumor tissues was also reported in neural cancer [22], renal cell [27],breast [28,29], ovarian [30, 31], prostate [32,33], non-small-celllung [34], Merkel cell [35] and uterine carcinomas [36], and inother tissues in smaller samples as well (including bladder andurothelial). Increased expression of CD24 is usually tied with amore aggressive course of the disease [37].

CD24 expression in GI cancers

Overall, we have found a significant overexpression of CD24protein in adenomas and adenocarcinomas from the esophagus,stomach, small and large bowel [38]. Using immunohistochem-istry with the previously described AB-2 MAb by Neomarkers,increased expression of CD24 was seen in 70–80% of tumors,already at the early stage of the benign polyp, and to the sameextent as in carcinomas. Only weak expression was observed in16% of the adjacent normal mucosa samples (FIGURE 2).

In addition, using a northern blot analysis, CD24 mRNAwas found to be overtranscribed in 66–68% of hepatocellularcarcinomas (84 patients), where it was significantly predomi-nant among young patients (<50 years old; p < 0.025) [39]. Thesame authors found a significant correlation between elevationin the transcripts level and harboring of p53 mutations, as wellas with the diagnosis of a poorly differentiated tumor.

In CRC, Nestl et al. were the first to report CD24 overexpres-sion in a set of ten patients using in situ hybridization [40].Weichert et al. have found, using immunohistochemistry,increased membrane and cytoplasmic stainings of CD24 in 147CRC cases [41]. We have found that the wide overexpression ofmembrane CD24 protein was even more significant amongtumors of the colon and rectum where a strong signal of CD24,both in adenomas and carcinomas, was seen in about 90% ofthe tumors, with the same low level of expression in normaladjacent mucosa [38]. A stronger expression was observed at theluminal polar of the cells, but, unlike the former authors, cyto-plasmic staining was less significant. Since CD24 is a

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membrane receptor, the debate regarding whether cytoplasmicor membrane staining is of clinical importance is interesting. Itis possible, however, that cytoplasmic staining may represent amarker of poorer prognosis, as was described by Weichert et al.for CRC [41], by Chou et al. for diffused gastric cancer [42] andin other types of human cancers [29,43]. The correlation ofCD24 overexpression with a worse prognosis sets the groundfor the therapeutic potential of CD24 and does not contradictits being expressed at an early stage of carcinogenesis. In addi-tion, CD24 was expressed, presenting a distinctively high colorintensity, in four out of five human CRC cell lines [38].

In the pancreas, Cram et al. were the first group to report theexpression of CD24 in rat insulinoma cell lines (using microarrayanalysis) and in the human pancreas during the embryonicperiod [22]. Using cDNA microarray, Han et al. showed that CD24is overexpressed in pancreatic cancer cell lines in comparison withnine normal pancreatic cell lines (these data were not confirmed byqPCR) [44]. Finally, using immunohistochemistry, Jacob et al. [45]

have recently demonstrated that CD24 is commonly expressed inadenocarcinomas of the pancreas in a sample of 95 patients. In thisstudy no correlation between CD24 expression and prognosis wasobserved. We have found a strong CD24 expression in two out ofthree pancreatic cancer cell lines using western blot and cell-basedELISA assay for CD24 membrane availability [46].

The mucin-like structure of CD24 features it as an adhesion molecule

CD24 is largely described as a structurally heterogeneous dif-ferentiation antigen, found on hematopoietic and neuralcells, and also on cancer cells. During the last 10 years, evi-

dence emerged to suggest that CD24 also has a functionalrole in cell proliferation and differentiation which mightcontribute in carcinogenesis. Very little is known about thefunction of CD24 and the protein milieu with which itinteracts in the cell or cell microenvironment. CD24 wasproven to bind to platelet- (P-)selectin in a specific manner,as no binding at all was demonstrated for endothelium- (E-)or leukocyte- (L-)selectin [47]. The role of human CD24 as aP-selectin ligand depends on the appropriate modification ofglycans as only sialylLex-modified CD24 can bind to P-selec-tin. CD24 thus potentiated homotypic B-cell aggregationand heterotypic adhesion to activated endothelium [48].Under physiological conditions, CD24 overexpression wasthus sufficient to endow the cells with the capacity of rollingon and invading through vessel walls, by adhering to plateletsand endothelial cells [49].

The selectin family features calcium-dependent type-Itransmembrane glycoproteins with extracellular lectin-likedomains that interact, for example, with sialylated carbohy-drate determinants and mucin-like glycoproteins [50]. Inmammals, three structurally similar family members havebeen identified: E-selectin, L-selectin and P-selectin. Com-mon to all selectins is the mediation of initial tethering of cir-culating blood cells with endothelial cells of the intima oramong each other [50]. P-selectin (CD62P, LECAM-3) isexpressed on platelets but also in endothelial cells. It is consti-tutively expressed and stored in secretory granules (α-gran-ules in platelets and Weibel–Palade bodies in endothelialcells). Upon stimulation (e.g., by histamine or thrombin)these secretory granules fuse with the plasma membrane,causing rapid surface expression. Furthermore, a soluble formof P-selectin, lacking the transmembrane domain, is found inserum and plasma [50]. Expression of selectins was proven tobe in correlation with bad prognosis in several epithelial can-cers: P-selectin in the tumor microenvironment was found tobe crucial for tumor development; in P-selectin deficientmice versus normal mice, human CRC cells injected subcuta-neously into mice proliferated slower and gave less lungmetastases following intravenous injection [51], suggestingthat the mucin-dependent interaction with P-selectin is animportant feature of the aggressiveness of CRC cells. None ofthese functions has yet been proven to exist in cancer cells ofthe GI tract.

In neural cells, CD24 alone does not contribute to celladhesion, but through sialic acid sites on its glycan chains itcan bind and potentiate the adhesion molecule L1 and thusaffects cell adhesion and subsequently neuronal outgrowth indorsal root ganglion [52]. In these experiments, anti-CD24monoclonal antibodies inhibited the growth inhibitory effectthat the CD24 had on the neurons in culture. It is to benoted that L1, was recently reported to be overexpressed incancer tissues, including in CRC, as a target gene of theβ-catenin oncogenic pathway and accelerates cell growth andinvasiveness [53].

Figure 2. Summary of studies reporting CD24 expression in collections from patients with gastrointestinal cancers. Bars represent the percentage of tissues that were positively stained for CD24. *Cytoplasmic staining in CR cancer.‡Membrane staining in CR cancer.§Unicentric HCC.¶Multicentric HCC.ad.: Adenoma; cr.: Carcinoma; CR: Colorectal; HCC: Hepatocellular carcinoma; PC: Pancreatic cancer; SB: Small bowel; UGI: Upper GI tract.

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Jacob et al. Sagiv et al.

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CD24 is involved in evoking signal transduction pathways

CD24 was demonstrated to be involved in intracellular signaltransduction, although the specific pathway involved is stillvague. Even though CD24 has no transmembrane portion thatcan evoke an intracellular signal on its own, it was shown to belocalized in glycolipid-enriched membrane (GEM) domainsrather than appearing sporadically across the cytoplasmic mem-brane, where enhanced association to Lyn protein tyrosinekinase was shown [54]. Following excitation with monoclonalantibodies to CD24 or its Fab fragment, B cells showed a con-centration- dependent transient increase in cytosolic Ca2+ levels,within 3–5 min and lasting 10 min [55].

Very few studies have managed to describe the direct effect ofCD24 over- or underexpression. The expression of CD24 wasassociated with downregulation of the CXCR-4 chemokinereceptor expression in GEM regions. This localization is impor-tant for its function. Thus, CD24 modified the response ofbreast cancer cells to stromal-cell-derived factor (SDF)-1α, thusresulting in slower proliferation and migration [56]. Theseresults diverge from the findings of two studies by two differentgroups, published closely in the same journal, claiming thatCD24 expression contributes to the acquisition of a malignantphenotype. Ectopic expression of CD24 in breast cancer cellsresulted in increased proliferation rates and the activation of theα3β1 and α4β1 integrins, which induces binding to selectins,collagen and laminin and thus cell migration [57]. Similarly,transient underexpression of CD24 using addition of CD24targeted siRNA molecules to the growth medium of several epi-thelial cancer cell lines (breast, urothelial and prostate carcino-mas, and osteosarcoma) conducted slower cell growth andlower clonogenicity in soft agar, as well as observed changes inthe actin cytoskeleton that resulted in impaired motility [58].The mechanism of growth regulation by CD24 is, as yet, unde-fined. Runz et al. have found that within the lipid raft domain,CD24 expression augments cell motility by increasing thelocalization of β1-integrin [59]. No studies of CD24 over- orunderexpression were performed so far to prove the validity ofthese results in GI cancers.

Anti-CD24 antibodies convey a therapeutic potential in cancers of the GI tract

Studies have shown that anti-CD24 monoclonal antibodies donot only inhibit aggregation but also induce growth inhibitionin Burkitt’s lymphoma cells (which express CD24) via theGEM-dependent mechanism, accelerated by cross-linking withthe B-cell receptor [60]. In solid tumors, Sagiv et al. haverecently discovered that anti-CD24 monoclonal antibodies area potential therapeutic agent [38,45], as they inhibit cancer cellgrowth in up to 90% efficiency, in time- and dose-dependentmanners, and more importantly, in dependence of the CD24expression levels of the cells (nonexpressing cells did not

respond to the antibodies). Results were repetitive for three dif-ferent monoclonal antibodies (SWA-11 and ML-5, and thecommercially available c-20 antibody [Santa Cruz Biotechnol-ogy, CA, USA]). Experiences on cancers of other human tissuesare currently under investigation. It should be considered thatthese are preliminary results only, shown so far by our groupalone and still in vitro – further research is necessary.

Under these condition, pre-B cells also underwent apoptosisvia the activation of caspases 3 and 8 and in a p38 (a MAPKfamily member usually associated with signaling for apoptosis)dependent mechanism [61]; Erk1 and 2 were also activated, butamong the MAPK family, the main effect was on the p38,whose specific inhibitor blocked the death effect. Jung et al.showed this effect in double negative (CD4- CD8-) and doublepositive (CD4+CD4+) mouse thymocytes [62], which are twodifferentiation stages that were shown to express CD24. Theauthors claimed that the apoptotic effect is not dependent onthe activation of caspases (and also not with p53, CD95 orTNF receptor), and is inhibited by scavengers of reactive oxy-gen or overexpression of BCL2, which correlates its mechanismwith the mitochondrial regulation [62]. Anti-P-selectin mono-clonal antibodies, as well as the targeting of the selectincounter-receptor instead of the selectin itself, were shown to beeffective in cancer therapy [50].

CD24 is a marker for pancreatic cancer stem cells

Emerging evidence has suggested that the ability of a tumor togrow and propagate is dependent on a small subset of cellswithin a tumor, termed cancer stem cells, described so far forhematological brain, breast and prostate malignancies [63–66].Interestingly, and with no relation to the study of its function,CD24 is suggested as a marker for tumor stem cells, specificallyin the breast and the pancreas; however, the question whether itis its absence or overexpression that characterizes this cell popu-lation seems to differ according to the investigated tissue. A firststudy of primary breast cancer cells isolated from pleural infu-sions suggested that CD24 might be a potential marker forbreast tumor stem cells by demonstrating that the ability toform tumors in nonobese diabetic/severe combined immuno-deficiency mice was greater in the CD44+CD24low/–-expressingcell fraction compared with the CD44+CD24high fraction.Owing to the enhanced tumor-forming ability, theCD44+CD24low/– fraction was proposed to represent ‘breasttumor stem cells’ [64]. These cells were shown to be radio-resistant and suggested to be the source of fast tumor renewalduring gaps in radiotherapy [67]. Lawson et al. suggestCD45CD31Ter119Sca-1+CD49f+ as the cell surface profilethat characterizes prostate cancer stem cells [64]; all these cellsare also CD24 positive. A similar profile was described formammary stem cells, with a CD24 median expression [68].

Li et al. have recently shown that the pancreatic adenocarcinomacell population is also not homogenic and, using a xenograft modelof growing small masses of cells in immunocompromised mice [21],

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they looked for correlation between tumorigenicity and theexpression of the same markers, CD24 and CD44, as well as epi-thelial specific antigen (ESA) in order to be able to characterizethese cells. By contrast to the breast, these authors found thatCD44+CD24+ESA+ cells (0.2–0.8% of the cells in the tumormass) are 100-fold more tumorigenic than the others. These cells,while isolated, showed properties of self-renewal and the ability toproduce differentiated progeny, and in dependence of the CD24expression, showed strong activation of the sonic hedgehog(SHH) pathway. Overall, depending on the individual tumor,3–28% of the cells in a tumor mass expressed CD24 [21]. TheSHH was shown to be an important oncogenic pathway in vari-ous GI malignancies, especially in the pancreas (although, not inthe colon) [69]. This growing evidence strengthened our reasoningfor expanding the research to pancreatic cancer as well.

No study searching for markers for CRC stem cells has beenperformed to date. At the intestinal epithelium, a constant processof rapid cell renewal takes place, originating from the bottom ofthe crypts, where progenitor cells are constantly dividing, produc-ing differentiated cells that climb up to the top of the crypts untilthey are shed off, and cells that stay at the bottom keeping theprogenitor potential. According to our observation while per-forming immunohistochemistry to the intestinal epithelium [38],normal colonic epithelium demonstrated no staining for CD24,either in the membrane or cytoplasm. Nevertheless, occasionally,normal cells appeared to display some membranous staining, par-ticularly at the bottom of the crypts or in reactive or inflamed epi-thelium. In the rare cases of normal crypts that were positivelystained, it is the middle-to-deep portion rather than the superficialthat was stained, suggesting some degree of differentiation-relatedchanges (immature or cycling cells were stained more than maturenoncycling cells). This rare staining was not concentrated at a cer-tain polar of the cell. Normal cells in the small bowel, stomach oresophagus were not stained to CD24, besides occasional cytoplas-mic staining in neuroendocrine cells at the small bowel, and more,at the stomach. This description calls for a further research ofcolonic progenitor cells and CD24.

Expert commentary

Recent progress in genomic research that led to the develop-ment of novel antibodies as well as microarray techniquesallows the exposure of new genes that were not known beforeto be related to cancer pathogenesis. Thus revealing newmediators of neoplastic changes that shed new light on ourunderstanding of carcinogenesis, and therefore on ourresearch for early detection, prevention, treatment and selectionof high-risk populations.

In the body of literature presented previously, CD24 is simul-taneously center-staged at several disciplines of cancer research.The protein which was discovered almost 20 years ago, wasimplicated in scattered studies as a cellular-surface marker,mainly on hematopoietic and neural cells, sometimes linked withhematological malignancies but not attributed with its role in the

pathogenesis of the disease. However, since studies have recog-nized its expression in malignant tissues and reported the pheno-mena at very high prevalence and on almost all human tissues, itbecame necessary to further investigate the molecular and cellbiology as well as genetics of CD24 in relation to cancer.

Given that the CD24 protein seems to contribute to tumorprogression, its blockage of action by MAb should be furtherinvestigated for the development of a novel biological treat-ment. Its main promise lies within its high specificity to cancertissues that rises above that of other molecules in use for MAbtreatments (including CD20, VEGF and ErbB2).

Five-year view

The study of CD24 in relation to cancer generates highly inter-esting research projects in molecular cell biology, the clinicalsetting and genetic investigations. We expect that in the nextfew years much more will be uncovered regarding the exact roleof this protein in tumor promotion and maintenance. Mouse-originated MAb directed to CD24 are now under investiga-tions in animal cancer models, showing very promising resultsfor both efficacy and safety for treatment. The currentlyresearched antibodies are mostly noncommercialized and areaimed at academic purposes. We expect that these studies willelucidate which MAb, epitope or combination of MAb are themost efficient in cancer treatment, alone, or in combinationwith currently available nonspecific treatments. The chosenMAb that passes all examinations in animals will have to behumanized and be, as we hope, ready for Phase I trials inhuman patients.

Financial & competing interests disclosureSWA-11 and ML-5 monoclonal antibodies were generously supplied byPeter Altevogt (German Cancer Research Center, Heidelberg, Germany).The authors have no other relevant affiliations or financial involvementwith any organization or entity with a financial interest in or financialconflict with the subject matter or materials discussed in the manuscriptapart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Key issues

• CD24 is highly overexpressed in a large variety of human malignancies while it is barely expressed in normal tissues.

• CD24 expression is highly specific to transformed gastrointestinal epithelium.

• Genetic heterogeneity in the CD24 gene is clinically significant.

• CD24 expression contributes to the aggressive phenotype of cancer cells.

• Anti-CD24 monoclonal antibodies are efficient in the treatment of colorectal and pancreatic cancers.

• CD24 is a marker for tumor stem cells.

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Affiliations

• Eyal Sagiv, PhDResearch student, Integrated Cancer Prevention Center, Tel Aviv Medical Center; and, Tel Aviv University, 6 Weizmann Street, Tel Aviv 64239, IsraelTel: +972 3697 4968Fax: +972 3695 0339sagiveyal@yahoo.com

• Nadir Arber MD, MSc, MHAProfessor of Gastroenterology and Medicine, Integrated Cancer Prevention Center, Tel Aviv Medical Center;and, Tel Aviv University, 6 Weizmann Street, Tel Aviv 64239, IsraelTel: +972 3697 4968Fax: +972 3695 0339narber@post.tau.ac.il