Pathogenesis of MONOSOMY 7 In BONE MARROW FAILURE SYNDROMES

Abstract 2411

One of the most serious complications of aplastic anemia (AA) is the clonal evolution to myelodysplastic syndrome (MDS). Monosomy 7 (del[7]) is the most common chromosomal aberration encountered in this setting, but it can also occur in primary MDS along with deletion 7q (del[7q]). In our whole cohort of AA, we detected del[7] in 24 cases of clonal evolution; however, unlike in MDS no cases of del[7q] were observed. In contrast, in MDS or myelodysplastic/myeloproliferative neoplasms, del[7]/[7q] were present in 12% and 11%, respectively. Based on an initial observation of an index AA case with a mutation of CBL, we speculated that certain molecular events such as point mutations can promote or are at least associated with the evolution of del[7]. Given the distinct distribution of del[7] and del[7q] in primary and secondary MDS (derived from AA), we stipulate that the associated mutational pattern will be distinct. This may be due to different driving mechanisms, such as immune escape or a growth factor-rich milieu in AA.

Pursuant to this theory, we performed a screen for CBL, RUNX1, TET2, UTX, EZH2, DNMT3A, ASXL1 and IDH family mutations in patients who developed del[7] after AA (N=24). 14 cases were adults and 10 were pediatric cases. In addition, we analyzed 15 del[7], 13 del[7q] and 10 UPD[7q] abnormalities in patients with MDS without antecedent AA. We identified 3 CBL (13%), 5 RUNX1 (20%), 1 TET2 and 1 DNMT3A mutation in patients with del[7] after AA; all CBL mutations are observed in adult cases of AA (21%). However, 0, 1, and 2 CBL mutations were found in primary MDS with del[7], del[7q] and UPD[7q], respectively. In contrast, RUNX1 mutations were seen in both AA sub-groups (14% and 30% in adults and children, respectively). Also, no differences were seen with regard to the distribution of RUNX1 mutations in either post-AA MDS with del[7] or primary MDS with del[7], del[7q] and UPD[7q]. Most significantly, ASXL1 was found to be the most frequently mutated gene in the cohort without AA (37%), although no mutations were seen in patients with del[7] after AA. Additionally, UTX and EZH2, both involved in the trimethylation of histone H3 lysine 27, were found to be mutated only in cases of MDS with UPD[7q] and del[7q], but no mutations were seen in primary and AA-derived MDS with del[7]. When studied serially, none of the tested mutations was seen in the initial AA phase, but the mutated clone increased gradually during clonal evolution of del[7].

We also investigated the presence of additional chromosomal abnormalities using single nucleotide polymorphism array (SNP-A)-based karyotyping in the cases with del[7] after AA. In addition to del[7], 22 gains, 20 losses and 4 regions of somatic uniparental disomy (UPD) were identified. These abnormalities, including microdeletions, were found in 50% and 60% of adult and pediatric cases, respectively. Recurrent lesions were detected on chromosome 6, 8 and 21. A microdeletion on chromosome X involved the tumor-associated gene PHF6 in a male case. In another patient, trisomy 21 was accompanied by a RUNX1 mutation. Regions of UPD included 6p resulted in loss of heterozygosity of MHC class I and 11q was associated with homozygous CBL mutations in 2 cases.

In sum, our analysis supports the theory that distinct mechanisms of molecular progression during MDS evolution may be operative in AA and primary MDS. Moreover, del[7], del[7q] and UPD[7q] have distinct molecular profiles with frequent TET2 mutations in del[7q] and UPD[7q]. ASXL1 mutations appear to be most ubiquitous promoting event in MDS with LOH [7/7q], suggestive of their “type II' nature. In contrast, other mutations such as those in CBL may be selected for e.g., through cytokine-rich milieu and activation of CBL-regulated receptor tyrosine kinases.