Gain of the Centromeric Region of Chromosome 8, a New Germline Alteration In Juvenile Myelomonocytic Leukemia

Abstract 4830

Juvenile myelomonocytic leukemia (JMML) is an aggressive childhood myeloproliferative disorder characterized by the clonal hyperproliferation of myelomonocytic cells. The pathogenesis of JMML involves disruption of signal transduction through the RAS pathway, which may be an early event during leukemogenesis. Additional genetic lesions may be necessary for full malignant transformation. We applied microarray BAC/PAC- and high resolution 244k oligo-arrayCGH to bone marrow samples from 21 JMML patients in order to identify subtle genomic alterations. In 8 of13 JMML patients with normal karyotype and 2 of 8 patients with monosomy 7, additional copy number alterations were identified. A recurrent deletion of around 1.4 Mb at 17q11.2 targeting the NF1 gene was identified in 2 patients with clinical diagnosis of neurofibromatosis due to germline mutations and LOH in the tumor as described previously. In addition to the monosomy 7, one patient (D600) with a somatic mutation in PTPN11 showed an additional marker chromosome. The origin of this marker chromosome was resolved by arrayCGH. The BAC/PAC array indicated a gain of the centromeric region: arr8p12-q11.21(37,204,000-49,686,000)×3. Breakpoints were refined to 8p12-q11.21(36,673,794- 50,142,678) by high resolution oligo-aCGH. Interestingly, in patient D703 with a heterozygous CBL germline mutation and homozygosity in hematopoietic cells, we detected a nearly identical gain arr8p12-q11.21(40,076,917-49,622,319)×3 not accompanied by -7 and with slightly different breakpoints. The gain was confirmed by means of FISH using the centromere-specific probe for chromosome 8 CEP8 showing three signals. This patient was also found to have a small additional marker chromosome. In order to rule out that the marker chromosome was inherited, we performed chromosome analyses in the parents of patient D703. Both had a normal karyotype and did not carry an extra copy of the region 8p12-q11.21. However, FISH on buccal epithelial mucosa cells of the patient D703 revealed a mosaic constellation with 34% cells positive for trisomy 8. Likewise, in the second patient, the dup 8p12-q11.21 (D600) was constitutional as 38% of fibroblasts showed 3 CEP8 FISH signals. There has been a longstanding discussion as to whether mosaic trisomy 8 may be constitutional. Our findings provide evidence that at least partial trisomy 8 may indeed be present as a germline mutation. Future work is needed to determine how constitutional partial trisomy 8 might contribute to leukemogenesis in JMML with different mutational subtypes.