Sub-Clonal Genetic Heterogeneity in BCR-ABL1 Positive Acute Lymphoblastic Leukaemia

Abstract 1348

Philadelphia-positive (Ph⁺) acute lymphoblastic leukaemia (ALL), characterised by the BCR-ABL1 fusion gene, occurs in approximately 30% of adult and 5% of childhood ALL and is associated with a poor prognosis. It is considered a single clinical entity with identifiable and recurrent copy number alterations (CNA); notably deletions of the lymphoid transcriptional regulator IKAROS (encoded by IKZF1), PAX5, and CDKN2A/B that are presumed to cooperate with BCR–ABL 1 in lymphoid leukaemogenesis. In particular, IKZF1 deletions are present in 80% of BCR-ABL1 positive ALL cases, and have been implicated as an independent indicator of poor prognosis in childhood ALL. Our previous studies of twin pairs either concordant or discordant for BCR-ABL1+ ALL indicate that the fusion gene is a first hit that occurs prenatally. However, the order and sequence of acquisition of CNA is unknown. We recently reported a complex sub-clonal genetic architecture for leukaemic blasts and leukaemia-propagating (‘stem’) cells in childhood ETV6-RUNX1-positive ALL (Anderson et al., Nature 469: 356–361, 2011). In the present study, we aimed to determine whether similar sub-clonal genetic diversity occurs in BCR-ABL1+ ALL. We carried out five colour FISH to diagnostic blast cells from eight BCR-ABL1 positive cases with differentially-labelled probes for BCR, ABL1, IKZF1, CDKN2A and PAX5. In a subset of cases we also performed Affymetrix single nucleotide polymorphism (SNP 6.0) arrays to determine the specific boundaries of deletions. Four out of the eight cases screened had concurrent IKZF1, PAX5 and CDKN2A deletions. In one case the order of acquisition of these deletions was uninformative, with 97% of cells exhibiting a single FISH pattern (BCR-ABL1+ with monoallelic deletions of all three genes). In the second case, a linear clonal progression was observed with IKZF1 deleted first, PAX5 second and CDKN2A third. In the two remaining cases a branching sub-clonal pattern was observed. In one of these monoallelic IKZF1, CDKN2A and PAX5 deletions all arose independently in different sub-clones; i.e. IKZF1 was deleted first in one subclone, CDKN2A first in another and PAX5 first in a third sub-clone. In the final case we also studied matched diagnosis and relapse samples. Here, SNP array analysis revealed different deletions in all three genes at diagnosis and relapse. Genomic fusion breakpoint analysis revealed an identical BCR-ABL1 genomic sequence at diagnosis and relapse, confirming the same clonal origin of leukaemia. The different deletion boundaries in IKZF1, PAX5 and CDKN2A permitted us to design specific FISH probes to distinguish between ‘diagnostic’ and ‘relapse’ deletions and to track their evolution. The predominant clone at relapse was not a direct evolutionary product of any of the major clones found at diagnosis. The dominant sub-clone at diagnosis was BCR-ABL1+, with a large 9p deletion (encompassing PAX5 and CDKN2A) and a focal CDKN2A deletion, all sub-clonal to a focal IKZF1 deletion. At relapse, the dominant sub-clone had acquired a different IKZF1 deletion, which was sub-clonal to two different (focal, biallelic) deletions of CDKN2A and a different monoallelic PAX5 deletion. The large 9p deletion was not present at relapse. These results indicate the existence of a pre-leukaemic BCR-ABL1 fusion gene positive clone that has given rise to at least two sub-clones, each with different IKZF1, PAX5 and CDKN2A deletions, that have evolved independently. These data indicate that the sub-clonal architecture in this poor prognosis subtype of ALL is genetically diverse, and that key ‘driver’ CNA can arise independently and in no preferential order.