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535 Genetic Basis of Myeloid Proliferation Related to Down Syndrome

Program: Oral and Poster Abstracts
Type: Oral
Session: 611. Leukemias - Biology, Cytogenetics and Molecular Markers in Diagnosis and Prognosis: Genetic Subgroups in Acute Myeloid leukemia
Monday, December 10, 2012: 2:45 PM
B206, Level 2, Building B (Georgia World Congress Center)

Kenichi Yoshida, MD1*, Tsutomu Toki, PhD2, Myoung-ja Park, MD, PhD3*, Yusuke Okuno, MD, PhD1*, Yuichi Shiraishi, PhD4*, Masashi Sanada, MD1*, Ayana Kon, MD1*, Yasunobu Nagata, MD1*, Aiko Sato-Otsubo, PhD1*, Yusuke Sato, MD1*, RuNan Wang, MD2*, Kiminori Terui, MD, PhD2, Rika Kanezaki, MS2*, Norio Shiba, MD5*, Kenichi Chiba, BA4*, Hiroko Tanaka, BA4*, Asahito Hama, MD, PhD6*, Daisuke Hasegawa, MD, PhD7*, Kazuhiro Nakamura, MD, PhD8*, Hirokazu Kanegane, MD, PhD9*, Keiko Tsukamoto, MD, PhD10*, Souichi Adachi, MD, PhD11, Satoru Miyano, PhD4*, Seiji Kojima6*, Shai Izraeli, MD12, Yasuhide Hayashi, MD, PhD3, Etsuro Ito2 and Seishi Ogawa, MD, PhD1

1Cancer Genomics Project, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan
2Department of Pediatrics, Hirosaki University Graduate School of Medicine, Hirosaki, Japan
3Department of Hematology and Oncology, Gunma Children's Medical Center, Shibukawa, Japan
4Laboratory of DNA Information Analysis, Human Genome Center, Institute of Medical Science, The University of Tokyo, Tokyo, Japan
5Dept. of Pediatrics, Gunma University Graduate School of Medicine, Maebashi, Japan
6Department of Pediatrics, Nagoya University Graduate School of Medicine, Nagoya, Japan
7Department of Pediatrics, St. Luke's International Hospital, Tokyo, Japan
8Pediatrics, Hiroshima University Graduate School of Biomedical Sciences, Hiroshima, Japan
9Department of Pediatrics, University of Toyama
10Division of Neonatology, National Center for Child Health and Development, Tokyo
11Human Health Science, Graduate School of Medicine, Kyoto University, Kyoto, Japan
12Human Molecular Genetics and Biochemistry, Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel

Background

Transient abnormal myelopoiesis (TAM) represents a self-limited proliferation exclusively affecting perinatal infants with Down syndrome (DS), morphologically and immunologically characterized by immature blasts indistinguishable from acute megakaryoblastic leukemia (AMKL). Although spontaneous regression is as a rule in most cases, about 20-30% of the survived infants develop non-self-limited AMKL (DS-AMKL) 3 to 4 years after the remission. As for the molecular pathogenesis of these DS-related myeloid proliferations, it has been well established that GATA1 mutations are detected in virtually all TAM cases as well as DS-AMKL. However, it is still open to question whether a GATA1 mutation is sufficient for the development of TAM, what is the cellular origin of the subsequent AMKL, whether additional gene mutations are required for the progression to AMKL, and if so, what are their gene targets, although several genes have been reported to be mutated in occasional cases with AMKL, including JAK2/3, TP53 and FLT3.

Methods

To answer these questions, we identify a comprehensive spectrum of gene mutations in TAM/AMKL cases using whole genome sequencing of three trio samples sequentially obtained at initial presentation of TAM, during remission and at the subsequent relapse phase of AMKL. Whole exome sequencing was also performed for TAM (N=16) and AMKL (N=15) samples, using SureSelect (Agilent) enrichment of 50M exomes followed by high-throughput sequencing. The recurrent mutations in the discovery cohort were further screened in an extended cohort of DS-AMKL (N = 35) as well as TAM (N = 26) and other AMKL cases (N = 19) using target deep sequencing.

Results

TAM samples had significantly fewer numbers of somatic mutations compared to AMKL samples with the mean numbers of all mutations of 30 (1.0 /Mb) and 180 (6.0 /Mb) per samples in whole genome sequencing or non-silent somatic mutations of 1.73 and 5.71 per sample in whole exome sequencing in TAM and AMKL cases, respectively (p=0.001). Comprehensive detections of the full spectrum of mutations together with subsequent deep sequencing of the individual mutations allowed to reveal more complicated clonological pictures of clonal evolutions leading to AMKL. In every patient, the major AMKL clones did not represent the direct offspring from the dominant TAM clone. Instead, the direct ancestor of the AMKL clones could be back-traced to a more upstream branch-point of the evolution before the major TAM clone had appeared or, as previously reported, to an earlier founder having an independent GATA1mutation. Intratumoral heterogeneity was evident at the time of diagnosis as the presence of major subpopulations in both TAM and AMKL populations, which were more often than not characterized by RAS pathway mutations.

While GATA1 was the only recurrent mutational target in the TAM phase, 8 genes were recurrently mutated in AMKL samples in whole genome/exome sequencing, including NRAS, TP53 and other novel gene targets that had not been previously reported to be mutated in other neoplasms. The recurrent mutations found in the discovery cohort, in addition to known mutational targets in myeloid malignancies, were screened in an extended cohort of DS-associated myeloid disorders (N=61) as well as other AMKL cases, using high-throughput sequencing of SureSelect-captured and/or PCR amplified targets. Secondary mutations other than GATA1mutations were found in 3 out of 26 TAM, 20 out of 35 DS-AMKL and 4 out of 19 other AMKL cases.

Conclusion

TAM is characterized by a paucity of somatic mutations and thought to be virtually caused by a GATA1 mutation in combination with constitutive trisomy 21.  Subsequent AMKL evolved from a minor independent subclone acquiring additional mutations. Secondary genetic hits other than GATA1 mutations were common, where deregulated epigenetic controls as well as abnormal signaling pathway mutations play a major role.

Disclosures: No relevant conflicts of interest to declare.

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