The Sub-Clonal Complexity of STIL-TAL1 T-ALL

Introduction

The STIL-TAL1 fusion is found in 16% cases of paediatric and adolescent T-ALL, making it one of the most common T-ALL subgroups. Our study considers this leukaemia subtype in the context of a complex ecosystem that is diverse, evolving and subject to selective pressures. We used single cell methods to understand the order of co-operating mutational events and the clonal evolution of mutations in genes that are re-iteratively targeted, such as PTEN.

Methods

Diagnostic DNA from five STIL-TAL1 positive T-ALL cases was exome sequenced using Agilent SureSelect Human all Exon kit plus Illumina paired end sequencing. Driver copy number alterations and NOTCH1/PTEN exon 7 mutation status had been identified in a previous study and candidate driver mutations for inclusion in single cell experiments were validated by sequencing or Q-PCR using custom assays. Where more than one mutation was present within the same exon of a candidate driver gene, cloning experiments were carried out to verify the independent mutation sequences. Material from xenograft transplants was available in three of the five cases to assess their clonal heterogeneity in the leukaemia initiating cell compartment. Single cell multiplex Q-PCR was used to examine the single cell genetics of the pre-defined mutation events. Briefly, single cells were sorted and lysed prior to multiplex specific (DNA) target amplification and Q-PCR using the 96.96 dynamic microfluidic array and the BioMark HD (Fluidigm, UK). Copy number assays for the 1p33 deletion and custom assays for the patient specific STIL-TAL1 fusion breakpoints were used to confirm that the 1p33 deletion leading to this gene fusion was a clonal event.

Results

The only aberrant events common to all five samples were CKDN2A copy number loss and the 1p33 deletion that results in the STIL-TAL1 fusion. Exome sequencing revealed further mutations in known T-ALL drivers including NOTCH1, PTEN and PHF6 as well as candidate driver mutations in FREM2, PIK3CD, RPL14, BMPR1A and CDH18. Both NOTCH1 and PTEN demonstrated re-iterative inactivation and this was investigated in detail for PTEN. Case 1 had multiple PTEN exon 7 mutations and sub-clonal copy number loss. Case 2 had parallel frameshift mutations in PTEN exons 5 and 7. Case 3 contained an exon 8 mutation and multiple PTEN exon 7 mutations. In this case the three most frequent PTEN exon 7 indels were validated and tracked in a single cell multiplex Q-PCR experiment. This revealed a branching sub-clonal genetic architecture (see figure 1) in which all malignant cells at the proposed apex of the branching architecture harboured the STIL-TAL1 fusion and CDKN2A deletion with copy number losses of 4p, 6q and FREM2 and PTEN mutations occurring as sub-clonal events. PTEN indels 2 and 3 were found co-localised in the same sub-clone. Preliminary analysis of the paired mouse xenograft bone marrow did not detect PTEN exon 7 indels 1 – 3 in 84 single cells. However, bulk Sanger Sequencing analysis did identify the PTEN exon 8 mutation in the mouse. Ongoing work is in progress to determine whether single cells of the xenograft carry alternative PTEN exon 7 mutations detected in the diagnostic sample exome data and to characterise in which diagnostic sub-clone the PTEN exon 8 mutation resides.

Conclusions

This study demonstrates how exome sequencing and single cell multiplex Q-PCR can be used as complementary tools to understand the sub-clonal complexity of STIL-TAL1 T-ALL. PTEN inactivation is sub-clonal by single cell analysis, demonstrating the parallel evolution of multiple independent PTEN inactivated sub-clones, highlighting PTEN inactivation as a key event in this T-ALL subgroup. In a wider cohort of 20 patients collected by our group at least 50% had PTEN inactivation as assessed by sequencing of exon 7 and copy number data alone. Results indicate a strong evolutionary pressure selecting for mutational events that result in inactivation of the PTEN-PI3Kinase pathway. These events occur via multiple mechanisms, including copy number loss and truncating mutations, which are not limited to the known T-ALL hotspot in exon 7. Current work is focussing on using a similar approach to examine the clonal evolution of NOTCH1 mutations in STIL-TAL1 T-ALL samples in diagnostic and xenograft samples of cases 4 and 5.

Figure 1

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Figure 1

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