Germinal centre B-cells, the translocations appears at the pre B-stage of the B-cell differentiation.

t(2;18)(p11;q21) is found in 8 cases (0.5% of all cases with abnormal karyotype) (Konishi H et al., 1990; Leroux D et al., 1990; Bertheas M-F et al., 1991; Juneja S et al., 1997; Horsman DE et al., 2001; Henderson L-J et al., 2004; Bentley G et al., 2005; Babu Rao V et al., 2006). The sex ratio is balanced M:F=1:1. The anomaly is observed mostly in older patients (average age 56 years; range 47-72).

t(2;18) as a sole anomaly is found only in one case. The other cases are with additional anomalies: 5 cases are with 1 to 4 anomalies and 3 cases are with highly complex karyotypes. In three cases the stemline with t(2;18) is evaluated to subclones with secondary chromosome aberrations.
Additional anomalies Most of the additional anomalies frequently associated with t(14;18) and FL are also present in the described cases with t(2;18): +12 (1 case), +7 (3 cases), 6q- (1 case), +X (3 cases; one with additional deleted X in q22), +5 (1 case), +8 (2 cases) and +der(18)t(2;18) (3 cases; one with 2 copies). In one case t(8;14)(q24;q32) is observed and in another two copies of its minor variant t(8;22)(q24q11) are present. Additional abnormalities of the following chromosomes are also described: chromosome 1 (3 cases - all of them are with 1p or 1q gains as a result of duplication of the segment 1q21q42 (one case) and unbalanced translocations of the segments 1p11qter, 1q11qter and 1q21qter to the different recipient chromosomes including 1, 3, 9, 13 and 15), chromosome 3 (3 cases - two with -3, +3 and unbalanced translocations involving 3p11, q12 and q21 and one with t(3;3)(p21;q23) as a second anomaly in the stemline), chromosome 13 (2 cases - one with interstitial deletion of the segment q13q31 and one with translocation involving 13q34) and chromosome 15 (3 cases - one with interstitial deletion of the segment 15q12q15 and translocations involving 15q12, one with der(15)t(1;15)(p11;p11) and one with t(6;15)). Gains of 2p, 18 and 21 as well as losses of 1p, 10q and 17p that are frequently associated with FL with t(14;18) are not reported. , Additional anomalies Some of the additional anomalies associated with t(14;18) are found: +12 (1 case with CLL and 1 with DLBCL), +X (1 case with DLBCL) and +18 (1 case with DLBCL). Abnormalities of chromosome 1 are reported in all cases except in the patient with CLL (1 case is with +der(1)(q12), 1 with t(1;9)(p34;p22), 1 with i(1)(q10) and 1 with add(1)(p36)). Chromosome 3 abnormalities is present in 2 cases with DLBCL - one is with add(3)(p25) and add(3)(q21) and one with t(3;22)(q27;q11). In one case with DLBCL t(8;14)(q24;q32) is described and in another with MBCN the same anomaly in combination with der(14)t(8;14)(q24;q32).

Other B-lymphoproliferative disorders

(2;18) is found in 5 cases: 2 cases (0.14% of all cases with abnormal karyotype) with diffuse large B-cell lymphoma (DLBCL) (females; one with age 56 year)(Hillion J et al., 1991; Macpherson N et al., 1999), 1 case (0.14% of all cases with abnormal karyotype) with mature B-cell neoplasm, NOS (MBCN) (male) (Tomita N et al,. 2009), 1 case (0.11% of all cases with abnormal karyotype) with Burkitt lymphoma/leukemia (BL) (37-year-old female) (Hillion J et al.,1991) and 1 case (0.04% of all cases with abnormal karyotype) with chronic lymphocytic leukemia (CLL) (55-year-old male) (Dyer MJS et al., 1994).

Only the case with CLL is with one additional anomaly. All remaining cases are with complex karyotypes.

No hybrid gene is created. T(2;18) IgK/BCL2 leads to the juxtaposition of BCL2 near the enhancer sequences of the IgK gene. In contrast to the distribution of the breakpoints in BCL2 of the classical t(14;18) that are clustered in the majority of cases within the major (MBR) and minor breakpoint cluster region and more rarely within the 5- flanking region of BCL2, the breakpoints in the BCL2 locus in t(2;18) occurs only in the 5 flanking region of the BCL gene termed the variant cluster region (VCR) (Larsen CJ et al., 1990; Hillion J et al., 1991; Bertheas M-F et al., 1992; Yabumoto K et al., 1996). The BCL2 coding region is not affected, because the breakpoints in VCR are distributed upstream of the translational initiation site of the BCL2 gene. On the other hand the locations of the breakpoints in the IgK gene are diverse. DNA breakage as a result of 5-BCL2/IgK junctions have been described in the region of the intronic sequences, joining segments and k-deleting element (Yonetani N et al., 2001). In some cases with t(2;18) head-to-tail configuration of the BCL2 and IgK genes have been recognized. In the molecular variant IgK/KDSR (FVT-1) the breakpoints on 18q21 and 2p11 occurred in the last intron of FVT-1 and within the J4 segment of the Jk region respectively. As a result of the translocation the promoting region and the 5 part of the coding sequence of FVT-1 is juxtaposed to the Vk -Jk region of the kappa light chain on the der(2) chromosome.Fusion protein in the cases with BCL2/IgK rearrangements is not produced. The kappa immunoglobulin enhancer induces BCL2 overexpression.

The consequence of t(2;18) is the same as in the t(14;18). The overproduction of the Bcl2 protein blocks the apoptosis and promotes prolonged B-cell survival. But the differences in the molecular structure of both rearrangements possibly predispose difference in the levels of BCL2 expression. It was found that the cases and tumor cell lines with 5-BCL2/Ig (including 5-BCL2/IgK) rearrangements have markedly higher levels of BCL transcripts (as well as expression of BCL2 protein by immunocytochemistry staining) than those of BCL2/IgH with breakpoints in MBR or 3-MBR (Dyer MJS et al., 1993; Yonetani N et al., 2001). In the molecular variant IgK/FVT-1 the FVT-1 disruption resulted in the constitution of a chimeric Vk -Jk-5 FVT-1 gene in a tail-to-tail configuration. On the other hand the BCL2 is juxtaposed to the kappa light chain locus in the vicinity of the 5 kappa gene enhancer leading to its overexpression. Therefore the molecular pathogenesis of this variant of t(2;18) is also linked to the deregulation of BCL2 and not to the FVT-1.

The dual role of the nighbor genes BCL2 And KDSR: Rimokh et al. (1993) proposed that both genes BCL2 and FVT-1 (KDSR) participate in the pathogenesis of the non-Hodgkin lymphoma with t(14;18)(q21;q32). Their suggestion is based on the proximity of both genes and that in t(14;18) with break at the 3' end of BCL2, the FVT-1 and BCL2 genes are juxtaposed to the Ig heavy chain locus and might be both deregulated. The fact that FVT-1 is expressed in t(14;18) associated with lymphomas and cell lines confirmed this hypothesis. It seems likely that both evolutionary conserved neighbor genes participate not only in oncogenesis but also in the regulation of normal cellular processes. KDSR is crucial for de novo synthesis of ceramide and sphingosine-1 phoshate (S1P) - intermediates of sphingolipid catabolism that are involved as BCL2 in the regulation of apoptosis (cereamide is apoptotic and S1P is anti-apoptotic regulator) and neuronal growth and development (Ryu JR et al., 2016, Riebeling C&Futerman AH 2013, Kono M et al., 2014). Possibly it is non-random that the evolutional step when BCL2 and KDSR became neighbors coincides with the appearance of the vertebrates. During phylogeny with small exceptions the distance between both genes is gradually shortened and reached 7598 bp in humans - approximately twice shorter than the distance between BCL2 and KDSR of our nearest living relative - the chimpanzee (13189 bp). These changes of the distance between both genes is a part of one general regularity of DNA alterations including deletions in the non-coding regions that resulted in the formation of close proximity located genes in the human genome compared with the nonhuman primate genomes (McLean CY et al., 2011). Because the chimpanzee shared 98.5 percent similarity with the human protein coding loci (Mikkelsen TS et al., 2005) and the main difference is the changes of non-coding DNA sequence, it was supposed that this event has an important role in human evolutionary divergence (McLean CY et al., 2011). On the other hand, significantly higher incidence of cancer has been found in humans than in chimpanzees (Beniashvili DS, 1989, Varki A, 2000). Possibly, brain evolution and cancer frequency are mutually linked with the genome changes in the non-coding regions. It was hypothesized that the increased brain size and the propensity for cancer may both be associated at least in part with difference in apoptotic function because the rate of apoptosis in humans is reduced relative to the chimpanzee (Arora G et al., 2012, Arora G et al., 2009). Except the difference in apoptotic regulation a great number of genetic and epigenetic regulatory differences have been found between the two species (Fukuda K et al., 2013, Uddin M et al., 2004, Nowick K et al., 2009, Khan Z et al., 2013) that are linked to the influence of interspersed elements (particularly retrotransposones) in the non-coding regions (Bowen NJ & Jordan IK, 2002, van de Lagemaat LN et al., 2003, Fescotte C, 2008, Polavarapu N et al., 2011). But even an evolutional advantage in the regulation of the neighbors BCL2 and KDSR could not be able to induce production of high level of BCL2 protein and at the same time to modulate the ceramide/S1P rheostat in order to produce high level of S1P necessary for the rapid brain development during human embryogenesis. Obviously, this requires more complex regulatory mechanisms. Moreover, the experiments of Meadows et al., 2010) on fruit fly Drosophila, in which by targeted chromosomal inversion testis-specific neighbors genes were split up, clearly demonstrated that there is no significant difference in gene expression between the flies with the inversion and those without. The negative results of Meadows et al., 2010) showed that the expression of the neighbor genes does not depends on their positioning and directed to another possible explanation of their role in human evolution - the biological relevance of the gene proximity is to provide long range interactions between one or more sets of neighbor genes or between neighbor genes and regulatory elements (promotor-enhancer interactions) from different chromosomes (or chromosome loci) during its expression. The epigenetic alterations in the regulation of the insulators by the transposable elements, as well as the proximity of the neighbor genes may have allowed effective long distance moving of the DNA loops in the nucleus. One genome organization in humans based on the expression of multi gene assembly (active chromatin hubs) could be a prerequisite for a great diversity of expression patterns - an essential condition for the development and maintenance of the human brain and especially of the frontal lobe where the most complex transcription activities were found (Konopka et al., 2012, Pletikos et al., 2014). But the expression mechanisms based on the gene interactions have their fault - a risk of more recombination events which could explain the increased predisposition for cancer in humans. In this view, future studies on KDSR and BCL2 and more precisely on their possible interactions with enhancers from other genes (including immunoglobulin enhancers) during their normal expression could clarify the role of both genes in brain development and apoptotic regulation, as well as the mechanisms of the appearance of lymphoma associated anomalies as t(14;18) and its variants.

IGK/BCL2

IGK/KDSR

t(2;18)(p11;q21) is a very rare reciprocal translocation described only in a 13 cases with B-lymphoproliferative disorders mainly follicular lymphomas (FL). The anomaly represents a minor variant of the classical t(14;18)(q32;q21) which is the most frequent translocation associated with follicular lymphoma (FL). In contrast to t(14;18) that juxtapose BCL2 with the heavy chain locus, t(2;18) resulted to the juxtaposition of the BCL2 to the kappa light chain locus. Despite of this difference the consequence of t(2;18) is the same as that in t(14;18) - deregulation of BCL2 leading to inhibition of apoptosis and respectively accumulation of a long living B-cells. A molecular variant of t(2;18)(p11;q21), in which the kappa light chain instead with BCL2 is rearranged with the coding region termed FVT-1 (for follicular lymphoma variant translocation - IGK/FVT-1), is also described Rimokh et al. (1993). It was ascertained later that FVT-1 coded the gene of 3-ketodihydroshingosine reductase (KDSR) - a key enzyme in the "de novo" synthetic pathway of ceramide.