Trisomy 13 Defines a Subgroup of Myeloid Malignancies with an Extremely High Frequency of AML1 Gene Mutations and Newly Defined VEGFR-1 Mutations.

Somatic mutations in the DNA-binding domain, the socalled Runt domain, of the AML1 gene have been identified to occur in AML with the highest incidence in the AML M0 subgroup as well as in myeloid malignancies displaying acquired trisomy 21. To characterize our own cohort of AML M0 patients (n = 83) for AML1 mutations, we analyzed AML1 transcripts by a combination of denaturing high performance liquid chromatography (dHPLC) and direct sequencing. In this cohort 37 cases (45%) carried an AML1 mutation. The AML1 mutated cases showed a remarkable high incidence of trisomy 13 (14/37 pts; 38%). In contrast to previous data only one case had a trisomy 21 (3%). The group without AML1 mutations (n = 46) included only one case with trisomy 13 (2%) and no trisomy 21. To extent this observation beyond the AML M0 subtype, we analyzed 14 additional trisomy 13 cases from other FAB subtypes. 79% (11/14) of the non-AML M0 trisomy 13 cases displayed an AML1 mutation. To evaluate the role of trisomy 13 in the context of AML1 mutations we looked for tyrosine kinase receptors on chromosome 13, which might play a cooperative role in concert with mutated AML1. The fms-related tyrosine kinase 3 (FLT3) and the fms-related tyrosine kinase 1 (FLT1, VEGFR-1) were identified as candidates on chromosome 13. In the AML M0 group 70 of 83 cases were analyzed for activating FLT3 length mutations (FLT3-LM) and/or activating point mutations in the tyrosine kinase domain (FLT3-TKD). 12 cases (18%) were positive for such activating mutations (9 FLT3-LM, 2 FLT3-TKD, 1 FLT3-LM+TKD) with 4 of these cases having an AML1 mutation in parallel. However, none of the 14 analyzed M0 cases with trisomy 13 were affected. FLT3 overexpression has also been suspected as a mechanism for constitutive FLT3 activation. Therefore, we hypothesized that trisomy 13 by means of a gene dosage effect might contribute to an increase in FLT3 expression. When we compared FLT3 RNA expression levels as determined by Affymetrix U133 Plus 2.0 gene expression arrays between AML M0 trisomy 13 samples (n = 9) versus other AML M0 samples (n = 19), we observed a significantly higher FLT3 expression in the trisomy 13 cases (p = 0.048, t-test). In the group of non M0 cases with trisomy 13 (n = 14) 3 cases (21%) had activating FLT3 mutations (2 FLT3-LM, 1 FLT3-TKD). Only 1 of these cases carried an AML1 mutation in parallel. In the trisomy 13 group we detected novel mutations in VEGFR-1 in 3 of 21 cases (14.2%). All three cases had an M0 phenotype as well as an AML1 mutation. Thus, 3 of 5 M0/trisomy 13 cases were positive for this new kind of mutation. In conclusion

1. we confirm the high incidence of AML1 somatic mutations in the AML M0 subgroup (45%).

2. We identified a new entity of AML characterized by trisomy 13 with an incidence of AML1 mutations of about 80%.

3. We suggest that trisomy 13 might be a mechanism for the amplification of FLT3 transcript levels for the activation of FLT3 signaling which collaborates with AML1 somatic mutations in leukemogenesis.

4. We identified a new kind of mutation in the tyrosine kinase domain of VEGFR-1 which might be a specific cooperating mutation for AML1 mutation.