BMI1 as oncogenic candidate in a novel TCRB-associated chromosomal aberration in a patient with TCR|[gamma]||[delta]||[plus]| T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a malignant disease of progenitor T cells that have undergone multiple genetic alterations. Translocations between T-cell receptor (TCR) genes and genes with oncogenic properties are a genetic hallmark of this disease. Most of these oncogenes encode transcription factors, the deregulated expression of which causes a block in the development of progenitor T cells. Until recently, most of these TCR chromosomal rearrangements were attributed to erroneous recombinations involving the / (TCRA/D) locus. However, new approaches uncovering cryptic chromosomal rearrangements revealed a higher incidence of TCRB locus-associated aberrations leading to the identification of new oncogenes involved in T-ALL leukemogenesis.1, 2 Here, we present a novel chromosomal aberration involving the TCRB locus and the 10p12 region in a previously described TCR+ T-ALL (T003/MA).3 Cytogenetic analysis on bone marrow material of T003, a 7-year-old male patient, revealed a complex karyotype: 46,XY,add(5)(q13),der(7)?inv(7)(p1?q3?),add(10)(q21),del(10)(q25)[ 26] /45,idem,–Y[ 4] ,46,XY[ 6] . Chromosomal rearrangements involving the q arm of chromosome 7 usually suggest the involvement of TCRB genes in T-ALL cases. Southern blot analysis of the TCRB gene revealed the presence of a 5.35 kb band representing a D1–J2 rearrangement on one allele, while the second allele contained an atypical TCRB gene rearrangement of 4.4 kb within the 2 region (Figure 1).4 In order to resolve the nature of this unknown TCRB gene rearrangement, we performed a ligation-mediated PCR, as previously described,5 using D2-specific nested primers. Sequencing revealed the presence of a fusion of the 3' end of the D2 gene segment and the p12 region of chromosome 10, representing the derivative chromosome der(7) (Figure 2a). Furthermore, a second ligation-mediated PCR analysis performed with nested primer sets directed to the J2.2, J2.5 and J2.7 regions revealed the reciprocal der(10) containing a fusion between the centromeric side of 10p12 and the 5' side of the J2.3 gene segment (Figure 2a). Additionally, our data showed that the break on chromosome 10 occurred in the absence of cryptic recombination signal sequences, indicative of a 'type 2' translocation.6 The breakpoint was localized between the p11 and p12 bands on chromosome 10, proximal to the centromere (reference sequence: NT_ 008705.15) (Figure 2b). This area encompasses the genes DNAJC1 and AF10/MLLT10, which are located 35 and 504 kb telomeric to the break, respectively, and COMMD3 and BMI1, which are located 277 and 282 kb centromeric to the break, respectively (Figure 2b). Our data suggest that the der(7) resulted from a possible translocation with the distal part of chromosome 10p, thereby juxtaposing TCRB to DNAJC1 (Figures 2c and d). The reciprocal chromosomal exchange giving rise to der(10) resulted in positioning of J2.3, C2 segments and TCRB enhancer elements upstream of COMMD3 and BMI1 involving an area of over 100 kb. The 10p12 region has previously been shown to be involved in other lineage-specific t(10;11)(p13;q14–21) translocations resulting in CALM-AF10 fusions. Breaks within this area have been shown to cause the aberrant expression of BMI1 and COMMD3, which is postulated to be induced by transcriptional cis activation. BMI1 plays a critical role in tumorigenesis, and its expression is decisive for the proliferative potential and self-renewing ability of stem cells;7 BMI1 overexpression is associated with poor prognosis and influences the aggressiveness of tumors in general.7 As in the CALM-AF10+-TCR+ T-ALL cases, BMI1 stands out as the candidate gene involved in the development of T-ALL in our T003 TCR+ case. Although we did not have remaining cell material to perform mRNA analysis on T003, we speculate that this translocation might have had a significant effect on the expression of BMI1 and COMMD3 either as the result of the presence of strong TCRB enhancer activity upstream of these genes (Figures 2c and d) or due to conformational changes caused by the break.8 The fact that patient T003 was diagnosed as having high-risk T-ALL, with a white blood cell count above 50 106 per ml blood, strengthens our speculation that this novel translocation has led to the upregulated expression of BMI1 causing the aggressiveness of the disease. In summary, we identified a novel TCRB chromosomal aberration in a TCR+ T-ALL sample, possibly leading to activation of the BMI1 oncogene. This case illustrates the importance of using new approaches to analyze and uncover cryptic chromosomal rearrangements, which were undecipherable in the past, and adds to the growing list of TCRB-associated chromosomal translocations and our understanding of T-ALL leukemogenesis.