Aflatoxin B1 aldehyde reductase (AFAR) genes cluster at 1p35-1p36.1 in a region frequently altered in human tumour cells

Alterations of the distal portion of the short arm of chromosome 1 (1p) are among the earliest abnormalities of human colorectal tumours. Recently, we have cloned the Aflatoxin B1 aldehyde reductase (AFAR) gene from a smallest region of overlapping deletion that is frequently (48%) hemizygously deleted in sporadic colorectal cancer. AFAR is expressed in a broad range of tissues. Its closely related rat protein is the major factor conferring resistance of rats towards aflatoxin B1-induced liver carcinogenesis. Here, we have identified cDNAs covering two additional human AFAR-related genes localized in close proximity to the previously described AFAR at 1p35-36. We have analysed their structure and tissue-related expression. One of them, AFAR3, carries a Selenocysteine-Insertion Element (SECIS)-like structure that during translation may recode an in-frame TGA-stop codon to a selenocysteine. Two additional AFAR-pseudogenes are localized at Xq25 and 1p12, respectively. AFAR exon sequences share an identity of DNA and amino acids of more than 78%. Also large blocks of intronic sequences can be up to 98.6% identical. Knowledge of the AFAR genes and their structure will be essential in genetic and functional studies, where discrimination of the genes and proteins is a prerequisite for evaluating their individual functions.

Preferential integration of a transfected marker gene into spontaneously expressed fragile sites of a breast cancer cell line

Common fragile sites are non-randomly distributed unstable chromosomal regions thought to be hot spots for recombination. They appear as gaps, breaks and triradial figures when cells are cultured under conditions that inhibit replication or repair of DNA. The removal of replication-inhibitory challenges is followed by repair activation to restore the DNA damage at the fragile site. The breast cancer cell line MDA-MB-436 has a spontaneous and non-random expression pattern of fragile sites that appear to be related to the complex pattern of chromosomal rearrangements. The high frequency of which fragile sites are spontaneously activated should make MDA-MB-436 cells a powerful tool to study in greater detail the DNA sequences of a multiplicity of fragile sites. Here, we have explored if the DNA at spontaneously activated fragile sites in MDA-MB-436 cells can be genetically tagged by the repair-mediated insertion of an exogenously supplied drug resistance gene. The cells were transfected with pSV2Neo, stably transfected clones were selected with neomycin, and the sites of pSV2Neo integration were determined by fluorescent in situ hybridization. Eighty-eight of 100 isolated clones had a non-random distribution of a total of 112 pSV2Neo integrations. Of these, 95 integrations (85%) coincide with the position at which non-random gaps and breaks appear in the MDA-MB-436 cells. Forty-nine (44%) of the 112 integrations appeared to be at position of known fragile sites, 46 (41%) were at the non-random chromosomal sites not previously described as "true" fragile sites. It is possible, however, that these non-random instabilities signal of genomic regions equivalent to fragile sites, that either have not previously been detected due to low level expression or that are activated in a tissue- or cell-type-specific manner. Collectively, our results show a preferential integration of exogenous DNA into fragile sites and other non-random regions of high genomic instability in MDA-MB-436 cells. This approach has provided a platform for the efficient targeted cloning and characterization of a substantial number of both common fragile sites and other non-random instability regions possibly related to breast cancer, and possibly also to other types of cancer.

BRCA2: a genetic risk factor for breast cancer

The identification of the breast cancer susceptibility genes BRCA1 and BRCA2 a few years ago has been greeted with great excitement and has raised hopes that they might illuminate the common mechanisms of this disease. Today we have to recognize that these expectations remain unfulfilled. Mutations in BRCA1 and BRCA2 account only for a relatively small proportion of breast cancers, even within the group of familiar clusters, they seem to be virtually non-existing in sporadic breast cancers. A substantial proportion of familiar breast cancer clusters has failed to provide evidence for an association with mutations in either BRCA1 or BRCA2, thus we have to look forward to the identification of additional breast cancer susceptibility genes. What has been most disappointing is that the mutation status of BRCA1/2 can provide only limited information for cancer risk. Initial assessments had indicated a risk of close to 90% for mutation carriers to develop breast cancer until age 75 - a value that turned out to be restricted to high-risk families in which the BRCA1 and BRCA2 genes had been genomically mapped. In unselected clusters the risk appears much lower, some estimates suggest less than 40%. Both BRCA1 and BRCA2 large encode proteins that appear to have a plethora of functions, with a conspicuous association to DNA repair and DNA recombination, and probably transcription activation. Defects in DNA repair can result in cancer predisposition syndromes and are recognized as being instrumental in cancer progression. Central questions have remained unanswered: What is the function of damaged BRCA1 and BRCA2 genes in breast cancer risk? What is the basis of large variations of risk conferred to the patients by identical mutations? How can the predictive value of mutation surveys be increased?

Smallest region of overlapping deletion in 1p36 in human neuroblastoma: a 1 Mbp cosmid and PAC contig

In human neuroblastomas, the distal portion of 1p is frequently deleted, as if one or more tumor suppressor genes from this region were involved in neuroblastoma tumorigenesis. Earlier studies had identified a smallest region of overlapping deletion (SRO) spanning approximately 23 cM between the most distally retained D1S80 and by the proximally retained D1S244. In pursuit of generating a refined delineation of the minimally deleted region, we have analyzed 49 neuroblastomas of different stages for loss of heterozygosity (LOH) from 1pter to 1p35 by employing 26 simple sequence length polymorphisms. Fifteen of the 49 tumors (31%) had LOH; homozygous deletion was not detected. Seven tumors had LOH at all informative loci analyzed, and eight tumors showed a terminal or an interstitial allelic loss of 1p. One small terminal and one interstitial deletion defined a new 1.7 cM SRO, approximately 1 Mbp in physical length, deleted in all tumors between the retained D1S2731 (distal) and D1S2666 (proximal). To determine the genomic complexity of the deleted region shared among tumors, we assembled a physical map of the I Mbp SRO consisting predominantly of bacteriophage P1-derived artificial chromosome (PAC) clones. A total of 55 sequence-tagged site (STS) markers (23 published STSs and short tandem repeats and 32 newly identified STSs from the insert ends of PACs and cosmids) were assembled in a contig, resulting in a sequence-ready physical map with approximately one STS per 20 Kbp. Twelve genes (41BB, CD30, DFFA, DJ1, DR3, FRAP, HKR3, MASP2, MTHFR, RIZ, TNR2, TP73) previously mapped to 1p36 are localized outside this SRO. On the basis of this study, they would be excluded as candidate genes for neuroblastoma tumorigenesis. Ten expressed sequence tags were integrated in the contig, of which five are located outside the SRO. The other five from within the SRO may provide an entrance point for the cloning of candidate genes for neuroblastoma.

Constitutional genomic instability with inversions, duplications, and amplifications in 9p23-24 in BRCA2 mutation carriers

Germ-line mutations of the BRCA2 gene (13q12-13) account for a large proportion of familial breast cancer cases in females and the majority of familial breast cancers in males. Recent studies provide evidence for a role of the BRCA2 protein in the maintenance of genomic integrity by involvement in DNA repair and recombination. In pursuit of identifying in humans genetic damage resulting from mutated BRCA2, we have analyzed constitutional karyotypes of BRCA2 mutation carriers. The present study establishes that constitutional distal 9p rearrangements without obvious additional gross chromosomal alterations are a recurrent feature of independently ascertained families. From our cytogenetic analyses we have no indication of additional gross rearrangements, but we cannot exclude more subtle recombinations in other genomic regions. We also show that the topography of the 9p rearrangements can differ among family members, even within an individual that can have cell populations with different 9p rearrangements. Collectively these results raise point to an association of mutant BRCA2 with genomic instability and gene alteration in 9p23-24 in at least a subset of BRCA2 mutation carriers.

Amplification of oncogenes revisited: from expression profiling to clinical application

Regulatory or structural alterations of cellular oncogenes have been implicated in the causation of cancers. Amplification represents one of the major molecular pathways by which gene expression is constitutively enhanced above the level of physiologically normal variation. Consequently, the significance of oncogene amplification in tumorigenesis originally had emerged from expression profiling of tumor cells by oncogene arrays. Amplified oncogenes have been found associated with more aggressive tumor variants and in selected settings are clinical markers to determine patient prognosis.

Chromosomal mapping of human genes by radioactive hybridization of cDNAs to CEPH-YAC high density gridded filter sets

Chromosomal assignment of human transcribed sequences has been done mainly by high throughput genome analysis in specialized genome centres and, in a more classical fashion, by fluorescence in-situ hybridization (FISH) analysis. Not every laboratory has the ability to map cDNAs by FISH analysis. We here report a rapid mapping approach that is based on the hybridization of cDNA probes to high density gridded CEPH-YAC filters followed by subsequent computational analysis by database searches in the internet. Not only transcribed sequences but also genomic DNA could be subjected to this mapping approach. The presented approach allows to map human transcribed and genomic DNAs within 1-3 days and with a high level of resolution that will constantly increase in line with the incorporation of data deriving from high throughput genome mapping.

Cloning of the human homologue of the metastasis-associated rat C4.4A

We have previously described a rat metastasis-associated molecule, C4.4A, which has some common features with the uPAR. Because of its restricted expression in non-transformed tissues a search for the human homologue became of interest. Human C4.4A was cloned from a placental cDNA library. As in the rat, the human uPAR and the human C4.4A genes appear to belong to the same family. Both genes are located on chromosome 19q13.1-q13.2 and both molecules have a glycolipid anchor site and are composed of three extracellular domains. Only domains one and two of the human C4.4A and the uPAR protein show a significant degree of identity. Expression of the human C4.4A was observed by RT-PCR and Northern blotting in placental tissue, skin, esophagus and peripheral blood leukocytes, but not in brain, lung, liver, kidney, stomach, colon and lymphoid organs. Yet, tumors derived from the latter tissues frequently contained C4.4A mRNA. As demonstrated for malignant melanoma, C4.4A mRNA expression correlated with tumor progression. While nevi were negative and only a minority of primary malignant melanoma expressed C4.4A, all metastases were C4.4A-positive. Taking into account the high degree of homology between rat and human C4.4A, the conformity of the expression profiles and the association of rat C4.4A with tumor progression, human C4.4A might well become a prognostic marker and possibly a target of therapy.

Topology of double minutes (dmins) and homogeneously staining regions (HSRs) in nuclei of human neuroblastoma cell lines

Amplification of the MYCN gene is a characteristic feature of many neuroblastomas and is correlated with aggressive tumor growth. Amplicons containing this gene form either double minutes (dmins) or homogeneously staining regions (HSRs). To study the nuclear topology of these tumor-specific and transcriptionally active chromatin structures in comparison to chromosome territories, we performed fluorescence in situ hybridization with a MYCN probe and various chromosome paint probes, confocal laser scanning microscopy, and quantitative three-dimensional image analysis. The dmins formed dot-like structures in interphase nuclei and were typically located at the periphery of complexly folded chromosome territories; dmins noted in the chromosome territory interior were often detected within an invagination of the territory surface. Interphase HSRs typically formed extremely expanded structures, which we have never observed for chromosome territories of normal and tumor cell nuclei. Stretches of HSR-chromatin often extended throughout a large part of the cell nucleus, but appeared well separated from neighboring chromosome territories. We hypothesize that dmins are located within the interchromosomal domain (ICD) space and that stretches of HSR-chromatin align along this space. Such a topology could facilitate access of amplified genes to transcription and splicing complexes that are assumed to localize in the ICD space.

Fine mapping of distal 1p loci reveals TP73 at D1S468

In the present study we establish a FISH fine-map of 1p36.3 loci. This region is frequently altered in different types of human tumors suggesting the existence of cancer-related genes. Identification of cosmids carrying both D1S468 and TP73 sequences leads to the assignment of TP73 to the most frequently deleted locus in colon and breast cancer and integrates this gene in human genetic maps. Localization of other distal loci was determined as follows: distal-CDC2L1-D1Z2-D1S94-TP73/D1S468-D1 S1615-proximal. D1S1615, earlier reported as a telomeric sequence, is considerably more proximal than previously thought.

Chromosomal mapping of human genes by radioactive hybridization of cDNAs to Centre d'Etude du Polymorphisme humain high density gridded filter sets

Chromosomal assignment of human transcribed sequences has been done mainly by high throughput genome analysis in specialized genome centres and, in a more classical fashion, by fluorescence in-site hybridization (FISH) analysis. Not every laboratory has the ability to map cDNAs by FISH analysis. We here report a rapid mapping approach that is based on the hybridization of cDNA probes to high density gridded Centre d'Etude du Polymorphisme Humain filters followed by subsequent computational analysis by database searches in the internet. Not only transcribed sequences but also genomic DNA could be subjected to this mapping approach. The presented approach allows to map human transcribed and genomic DNAs within 1-3 days and with a high level of resolution that will constantly increase in line with the incorporation of data deriving from high throughput genome mapping.

Two regions of deletion in 9p23-24 in sporadic breast cancer

Allelic deletions of 9p including band 21-22 are common in various types of human carcinomas including breast cancer. Our previous cytogenetic studies had identified constitutional chromosomal changes in 9p23-24 in patients of a male-breast-cancer family and 9p23-24 alterations in a cell line established from a sporadic female breast cancer. To find out whether this genomic region is involved more frequently in alterations in sporadic breast cancers, we have surveyed 80 microdissected tumor samples for both loss of heterozygosity (LOH) and homozygous deletion at 22 microsatellite loci spanning 9p22 to 9p24 using fluorescent multiplex PCR. LOH at one or more loci was observed in 32 (40%) of these tumors. Homozygous deletion was detected in four cases. Eleven tumors had LOH at all of the informative loci analyzed, whereas 21 tumors showed partial-terminal or interstitial allelic loss of 9p. Deletion mapping identified two common regions of deletion: (a) 4 cM including D9S281 to D9S286; and (b) 1 cM including D9S1808 to D9S268.

Retention of polysomy at 9p23-24 during karyotypic evolution in human breast cancer cell line COLO 824

Somatic genetic alterations of 9p have been seen in a wide range of human cancers, including breast cancer. Loss of heterozygosity analysis of primary breast cancer tumors has revealed a high frequency of deletion of DNA from 9p21-22 encompassing the MTSI (P16/CDKN2A) gene. We report the approximately tenfold increase in copy number of DNA from 9p23-24, which is far distal to P16/CDKN2A in female breast cancer cell line COLO 824, as revealed by fluorescence in situ hybridization, comparative genomic hybridization, and microsatellite analysis. Amplification of DNA has been reported previously to encompass multiple sites of the genome of the breast cancer cell, but increase in DNA copy number has not been seen in distal 9p.

Cloning of the human aflatoxin B1-aldehyde reductase gene at 1p35-1p36.1 in a region frequently altered in human tumor cells

Alterations of the distal portion of the short arm of chromosome 1 (1p) are among the earliest abnormalities of human colorectal tumors. Loss of heterozygosity analysis has previously revealed a smallest region of overlapping deletion (SRO) B, at 1p35-36.1, deleted in 48% of sporadic tumors. From this region we have now cloned a gene encoding a protein of 330 amino acids that is 78% identical with the Rattus norvegicus aflatoxin B1 aldehyde reductase (Afar) and, therefore, likely represents its human homologue. In rat liver, Afar is strongly inducible by the antioxidants ethoxyquin and butylated hydroxyanisole, which protect the rat against aflatoxin B1-induced liver tumorigenesis by detoxifying its genotoxic and cytotoxic dialdehyde. Human AFAR is expressed in a broad range of tissues and, therefore, is likely involved in endogenous detoxication pathways. Impaired detoxication of genotoxic aldehydes and ketones, which are involved in tumorigenesis of the colon and breast, may be a crucial factor both for tumor initiation and progression. We here provide a detailed contig of 1.5-2 Mbp/2.7 cM encompassing part of SRO B, including known genes and previously unmapped expressed sequence tags. PLA2G2A (secretory type II phospholipase A2), described previously as a candidate, is localized outside SRO B.

Isolation and characterization of human SGT and identification of homologues in Saccharomyces cerevisiae and Caenorhabditis elegans

We have recently isolated a rat cDNA encoding a novel cellular protein able to interact with the major nonstructural protein NS1 of parvovirus H-1 and have termed this protein SGT, for small glutamine-rich tetratricopeptide repeat (TPR)-containing protein. Here we report the isolation of a cDNA from human placenta encoding the human homologue, human SGT. SGT from rat and human contain 314 and 313 amino acids, respectively, and share 91% sequence identity at the protein level. The highest degree of similarity is present within the central region containing three TPR motifs in tandem array. The similarities, however, also extend beyond this region. Human SGTtranscript was found to be ubiquitously present in all human tissues tested. By fluorescence in situ hybridization analysis we have mapped the human gene to chromosome 19p13. The SGT-coding sequences are evolutionarily conserved, since we could identify genes encoding proteins of similar size and structure in the genomes of Saccharomyces cerevisiae and Caenorhabditis elegans.

Association between the rat homologue of CO-029, a metastasis-associated tetraspanin molecule and consumption coagulopathy

Recently, we have described a panel of metastasis-associated antigens in the rat, i.e., of molecules expressed on metastasizing, but not on nonmetastasizing tumor lines. One of these molecules, recognized by the monoclonal antibody D6.1 and named accordingly D6. 1A, was found to be abundantly expressed predominantly on mesenchyme-derived cells. The DNA of the antigen has been isolated and cloned. Surprisingly, the gene product proved to interfere strongly with coagulation. The 1.182-kb cDNA codes for a 235-amino acid long molecule with a 74.2% homology in the nucleotide and a 70% homology in the amino acid sequence to CO-029, a human tumor-associated molecule. According to the distribution of hydrophobic and hydrophilic amino acids, D6.1A belongs to the tetraspanin superfamily. Western blotting of D6.1A-positive metastasizing tumor lines revealed that the D6.1A, like many tetraspanin molecules, is linked to further membrane molecules, one of which could be identified as alpha6beta1 integrin. Transfection of a low-metastasizing tumor cell line with D6.1A cDNA resulted in increased metastatic potential and provided a clue as to the functional role of D6.1A. We noted massive bleeding around the metastases and, possibly as a consequence, local infarctions predominantly in the mesenteric region and all signs of a consumption coagulopathy. By application of the D6.1 antibody the coagulopathy was counterregulated, though not prevented. It has been known for many years that tumor growth and progression is frequently accompanied by thrombotic disorders. Our data suggest that the phenomenon could well be associated with the expression of tetraspanin molecules.

Informal payments for health care in the Former Soviet Union: some evidence from Kazakstan

An important feature of the health care system of the Former Soviet Union (FSU) and Central and Eastern Europe is the presence of informal or under-the-table payments. It is generally accepted that these represent a significant contribution to the income of medical staff. Discussions with medical practitioners suggest that for certain specialities in certain hospitals a doctor might obtain many times his official income. Yet little empirical work has been done in this area. Informal payments can be divided into those paid to health care providers and those that go directly to practitioners. They can be further divided into monetary and non-monetary. The complexity of these payments make obtaining estimates using quantitative survey techniques difficult. Estimates on contributions to the costs of medicines in Kazakstan suggest that they may add 30% to national health care expenditure. Payments to staff are likely to add substantially to this figure, although few reliable statistics exist. Research in this area is important since informal payment is likely to impact on equity in access to medical care and the efficiency of provision. The impact of attempts to reform systems using Western ideas could be reduced unless account is taken of the effect and size of the informal payment system.

An interstitial tandem duplication of 9p23-24 coexists with a mutation in the BRCA2 gene in the germ line of three brothers with breast cancer

Germ-line mutations of the BRCA2 gene account for the majority of families with both male and female breast cancer. However, among independently ascertained families with the same mutation, cases of male breast cancer often appear to cluster in a single family or in a particular branch of one family. This suggests that the risk of male breast cancer conferred by BRCA2 mutations may be modified by other genetic or environmental factors. We report a family in which three brothers with breast cancer carry in their germ line two genetic abnormalities: an insertion A at nucleotide 2041 in exon 10 of BRCA2, which leads to premature termination of the encoded protein at codon 615, and a tandem interstitial duplication involving chromosome bands 9p23-24. We propose that the coexistence of this rare chromosomal abnormality with BRCA2 mutation may be augmenting the risk of male breast cancer conferred by the BRCA2 mutation.

New genetic loci in neuroblastoma

Neuroblastoma has been associated genetically with amplification of the MYCN gene and with alteration of the short arm of chromosome 1 (1p). In pursuit of determining the spectrum of genetic loci damaged recurrently in neuroblastoma cells we have recently encountered two additional types of genomic abnormalities: i.) duplication of the MYCN gene on chromosome 2p24; and ii.) amplification of the gene MDM2. These alterations extend the spectrum of genetic lesions in neuroblastoma cells, although their incidence in primary tumor tissues has not been determined as yet.

Patterns of oncogene activation in human neuroblastoma cells

Human neuroblastoma is the most frequent solid tumor in children. Recent studies suggest that a multiplicity of genomic alterations contributes to neuroblastoma, the most frequent and well studied being deletion of the short arm of chromosome 1 and amplification of N-MYC. We here present and discuss different patterns of oncogene activation including, amplification of N-MYC, duplication of N-MYC and amplification of MDM2.

A highly amplified mouse gene is homologous to the human interferon-responsive Sp100 gene encoding an autoantigen associated with nuclear dots

In human cells, three proteins are currently known to colocalize in di screte nuclear domains (designated nuclear dots): Sp100, a transcription-activating protein autoantigenic primarily in patients with primary biliary cirrhosis; PML, a tumor suppressor protein involved in development of acute promyelocytic leukemia; and NDP52, a protein of unknown function. Here we report sequence similarities between the Sp100 protein and a putative protein encoded by a highly amplified mouse gene which is visible as an inherited homogeneously staining region (HSR) on chromosome 1 of some mouse populations. By in situ hybridization, the Sp100 gene was mapped to locus 2q37, the syntenic region of the HSR on mouse chromosome 1. Unlike the highly amplified mouse gene, Sp100 was found to be a single-copy gene and showed no restriction fragment length polymorphisms. Sequence similarities in the promoter regions and similar exon-intron organizations of the two genes were revealed. As for Sp100, steady-state levels of the mRNAs of the HSR-encoded genes could be greatly increased by interferon (IFN) treatment. As in human cells, IFN treatment led to an enlargement in both size and number of nuclear dots in mouse cells as visualized by immunofluorescence staining with autoimmune sera from patients with primary biliary cirrhosis. These data indicate that a gene located in the inherited HSR of mice, designated mSp100, is homologous to the human Sp100 gene, has a similar gene organization, and responds similarly to IFN treatment.

Human homologue of a candidate for the Mom1 locus, the secretory type II phospholipase A2 (PLA2S-II), maps to 1p35-36.1/D1S199

Mice heterozygous for the dominant Min mutation in their Apc gene develop multiple intestinal neoplasia. Analogously, family members from familial adenomatous polyposis kindreds inheriting mutations in their human APC homologue develop a similar phenotype. Quantitative trait loci studies have identified the Mom1 locus (for modifier of Min-1), which is responsible for part of the genetic variability in polyp number found among inbred mouse strains. The secretory type II phospholipase [nonpancreatic Pla2s (type II Pla2s or Pla2s-II)] has been demonstrated to be a candidate for Mom1, and a mutation in Pla2s-II in mice carrying the Min mutation has been proposed to account for an increased polyp number compared to mice without the Pla2s-II mutation. In this study, we have mapped the chromosomal position of the human homologue of Pla2s-II. We have identified 3 mega-yeast artificial chromosomes that carry PLA2S-II and localized one of them by fluorescence in situ hybridization to the border between 1p35 and 1p36.1. The presence of the microsatellite marker D1S199 in all three clones integrates PLA2S-II into different genetic maps. This highly polymorphic CA repeat D1S199 has previously been shown by us to identify loss of heterozygosity in 48% of sporadic colorectal tumors, indicating that the human homologue of the Pla2s-II/Mom1 locus might be related to human colorectal cancer.

Duplication of N-MYC at its resident site 2p24 may be a mechanism of activation alternative to amplification in human neuroblastoma cells

Amplification of the human N-MYC proto-oncogene is frequently seen either in extrachromosomal double minutes or in homogeneously staining regions of aggressively growing neuroblastomas. N-MYC maps to chromosome 2 band p23-24, but homogeneously staining regions have never been observed at this band, suggesting transposition of N-MYC during amplification. Previous studies had suggested that in cells with amplified N-MYC the chromosomes 2 appear to be unaltered and to carry one apparently normal copy of N-MYC each. In contrast, the contribution of N-MYC to tumors which lack amplification has been unclear. We here show, by fluorescence in situ hybridization, that N-MYC is occasionally duplicated at its resident site in neuroblastoma cell lines previously thought to have a single copy gene. Additionally, we detected duplication in a neuroblastoma cell line carrying amplification. Our results raise the possibility that duplication may, in some neuroblastomas, either be a prelude to amplification or an alternative pathway by which N-MYC becomes activated.