High hyperdiploid acute lymphoblastic leukemia (HeH-ALL) is characterized by 51-67 chromosomes and nonrandom gains of specific chromosomes (X, 4, 6, 10, 14, 17, 18, and 21). It presents the most frequent numerical cytogenetic alteration in childhood pre B-cell ALL occurring in 25-30% of cases. Recurrent disease will affect 15-20%. Pre-leukemic HeH clones are generated in utero, but cooperating oncogenic lesions are necessary for overt leukemia and remain to be determined. Recently, a phenomenon termed chromothripsis has been described in which massive structural variations occur in a single aberrant mitosis. Whole or partial chromosomes are shattered and some fragments are lost in the process of rejoining. Thus, characteristically, chromosomal copy numbers oscillate between two copy number states. Chromothripsis has been suggested to be a tumor-driving alteration that may be present in 2-3% of all human cancers. Its role as a potential cooperating or initiating lesion in HeH-ALL has not been determined.
We applied state-of-the-art whole-genome next-generation-sequencing to analyze structural variations in six pediatric patients with recurrent HeH-ALL. Matched sample sets taken at diagnosis, remission and/or relapse were compared. Paired end sequencing was carried out on a Genome Analyzer IIx or a HiSeq 2000 (Illumina), respectively. Reads were aligned against the human reference genome (GRCh37) using BWA. Translocations were detected by GASV. Copy number variations were analyzed by FREEC. Structural variations were validated by PCR/Sanger sequencing and FISH.
Of the six patients analyzed, five harbored on average one interchromosomal translocation or intrachromosomal inversion, but one patient presented with massive genomic rearrangements (Figure). These affected chromosome 3, 11, 12 and 20. Ten copy number shifts on chromosome 3 oscillating between two copy number states (2 and 3) indicated that these rearrangements were caused by chromothripsis. Breakpoint sequencing revealed that one of the identified translocations (t(12;20)(p13.1;p12.3)), was indeed a three-loci-rearrangement composed of small fragments derived from chromosomes 3, 12 and 20. Characteristically for chromothripsis, the breakpoints clustered closely. Three breakpoints separated by 224 bp and 64 kb were located in the transducin (beta)-like X-linked receptor 1 (TBL1XR1) gene. Other genes repeatedly targeted included the MACRO domain-containing protein 2 (MACROD2) gene (a deacetylase involved in deacetylation of lysine residues in histones and other proteins), the KIAA1467 gene (a transmembrane protein of the integrin alpha FG-GAP repeat containing 3 (ITFG3) family), and a novel regulatory lincRNA (ENSG00000243276). MACROD2 was previously observed as a target of chromothripsis in a colorectal carcinoma. Thus, the characterized breakpoints may identify fragile genomic sites prone to chromothriptic rearrangement. DNA repair was effectuated by non-homologous-end-joining as typical addition of non-template nucleotides with microhomologies of two to four nucleotides at the breakpoints demonstrated. Copy number profiles of this patient showed that at least two distinct leukemic clones could be identified at diagnosis. One had acquired chromothriptic alterations and presented the dominant clone at relapse indicating chemotherapy resistance and tumor-driving potential. Prior whole-exome sequencing did not reveal mutations in known oncogenes or tumor suppressor genes. Therefore, loss of function or expression of genes affected by chromosomal rearrangements, such as TBL1XR1 that is recurrently mutated in childhood ALL with ETV6-RUNX1 translocation, may account for the tumor-driving effect. All leukemic cells at diagnosis showed conformity concerning number and pattern of whole chromosome gains demonstrating that chromothripsis was not an initiating oncogenic event, but occurred secondary to high hyperdiploidy. Further aberrations (t(4;7), loss of 4q) were gained by the chromothriptic clone and could be detected by FISH in minor subclones pointing at ongoing clonal evolution. Taken together, our study reveals chromothripsis as a novel assisting and tumor-driving lesion in HeH ALL.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.