Abstract 3802: Genetic basis of myeloid leukemogenesis in Down syndrome.
Authors: Yoshida Kenichi , Toki Tsutomu , Park Myoung-ja , Okuno Yusuke , Shiraishi Yuichi , Sanada Masashi , Kon Ayana , Nagata Yasunobu , Sato-Otsubo Aiko , Sato Yusuke , Wang RuNan , Terui Kiminori , Kanezaki Rika , Shiba Norio , Chiba Kenichi , Tanaka Hiroko , Hama Asahito , Hasegawa Daisuke , Nakamura Kazuhiro , Kanegane Hirokazu , Tsukamoto Keiko , Adachi Souichi , Miyano Satoru , Kojima Seiji , Izraeli Shai , Hayashi Yasuhide , Ito Etsuro , Ogawa Seishi 
Published: August 2014

Abstract
BACKGROUND
Transient abnormal myelopoiesis (TAM) is known as a clonal myeloid proliferation affecting ∼10% of neonatal infants with Down syndrome (DS). Although spontaneous regression is as a rule in most cases, about 20-30% of the survived infants develop non-self-limited acute megakaryoblastic leukemia (AMKL) years after the remission. As for the molecular pathogenesis of TAM and DS-AMKL, 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 and trisomy 21 are sufficient for the development of TAM, what is the cellular origin of the subsequent AMKL and whether additional gene mutations are required for the progression to AMKL.
METHODS
To answer these questions, we performed a comprehensive analysis of somatic mutations in TAM/AMKL cases using whole genome sequencing of three trio samples sequentially obtained at the initial presentation of TAM, during remission and at the subsequent relapse phase of AMKL. Whole exome sequencing was also performed for TAM (N=15) and DS-AMKL (N=14) samples. The recurrent mutations in the discovery cohort were further screened in an extended cohort of TAM (N = 41) as well as DS-AMKL (N = 49) and other AMKL cases (N = 19).
RESULTS
TAM samples had significantly fewer numbers of somatic mutations compared to AMKL samples with the mean numbers of non-silent somatic mutations of 1.7 and 5.7 per sample in whole exome sequencing in TAM and AMKL cases, respectively (p=0.001). Whole genome sequencing and subsequent deep sequencing of the individual mutations revealed more complicated pictures of clonal evolutions leading to AMKL. Founding clones in TAM evolved into AMKL in two cases and, on the other hand, the direct ancestor of the AMKL clone in a remaining case could be back-traced to a more upstream branch-point of the evolution before the major TAM clone had appeared.
While GATA1 was the only recurrent mutational target in the TAM phase, 8 genes were recurrently mutated in AMKL samples in whole exome sequencing, including NRAS, TP53 and other novel gene targets. 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=90) as well as other AMKL cases, using high-throughput sequencing of hybrid-selection and/or PCR amplified targets. Secondary mutations other than GATA1 mutations were found in 6 out of 41 TAM, 38 out of 49 DS-AMKL and 10 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. We found two major clonal evolution patterns during DS-AMKL relapse. Secondary genetic hits other than GATA1 mutations were common in DS-AMKL and mutations involving genes such as tyrosine kinase and RAS pathway genes play a major role in clonal evolution into AMKL.
Citation Format: Kenichi Yoshida, Tsutomu Toki, Myoung-ja Park, Yusuke Okuno, Yuichi Shiraishi, Masashi Sanada, Ayana Kon, Yasunobu Nagata, Aiko Sato-Otsubo, Yusuke Sato, RuNan Wang, Kiminori Terui, Rika Kanezaki, Norio Shiba, Kenichi Chiba, Hiroko Tanaka, Asahito Hama, Daisuke Hasegawa, Kazuhiro Nakamura, Hirokazu Kanegane, Keiko Tsukamoto, Souichi Adachi, Satoru Miyano, Seiji Kojima, Shai Izraeli, Yasuhide Hayashi, Etsuro Ito, Seishi Ogawa. Genetic basis of myeloid leukemogenesis in Down syndrome. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3802. doi:10.1158/1538-7445.AM2013-3802