Molecular Genetics of Myeloid Leukemias: From Bench to Bedside

Abstract

Acute myeloid leukemia (AML) is a genetically diverse hematologic malignancy. Myelodysplastic syndrome (MDS) refers to a group of clonal hematopoietic stem cell disorders which may progress to AML and involve a multistep molecular pathogenesis. In the past several years, we have used a full range of molecular technology, (1) to detect and monitor the most prognostically relevant fusion genes involving core binding factor, retinoic acid receptor α, and MLL rearrangements in AML, (2) to characterize the fusion partners of MLL gene, and (3) to analyze the mutations of receptor tyrosine kinase/Ras pathway and myeloid transcripts factors in myeloid leukemias. In AML, the frequencies of t(8;21)/AML1-ETO, inv(16)/CBFβ-MYH11, and t(15;17)/PML-RARα were defined by RT-PCR assays. We used Southern blot analysis to screen MLL rearrangement [MLL(+)] in de novo AML, RT-PCR to detect common MLL fusion transcripts and cDNA panhandle PCR to identify infrequent or unknown MLL partner genes. We identified two novel MLL fusion transcripts (MLL-SEPT6 and MLL-CBL) and 2 novel fusions of MLL-ENL. MLL-PTD was the most common genetic lesion in adult patients with MLL(+) AML, whereas MLL-PTD was rare in childhood AML. We also demonstrated that mutations in FLT3 or Ras genes were highly associated with AML harboring PML-RARα or MLL rearrangement, supporting the two-hit model of leukemogenesis. AML1, CEBPα and PU.1 are the three important myeloid transcription factors of granulopoiesis. Our results showed that CEBPα mutations were infrequent and PU.1 mutation was absent in patients with MDS/AML, whereas AML1 mutations were common in MDS or CMML. We have published the first pediatric AML series with CEBPα and FLT3/TKD mutations. The incidence of FLT3/ITD in childhood AML was lower than adult AML and a high ratio of mutant to wild-type FLT3 predicted poor prognosis. The roles of mutations of FLT3/ITD, FLT3/TKD, N-ras/K-ras and CEBPα genes in de novo AML at diagnosis and relapse, and in the transformation of AML from MDS were also examined. Nine percent of AML patients acquired FLT3/ITD mutations at relapse. We observed heterogeneous patterns, loss or gain, of FLT3/TKD mutations at AML relapse, suggesting the mutation being a secondary event in a subset of AML patients. One-third of MDS patients acquired FLT3 or N-ras mutations during AML evolution. FLT3/ITD was associated with a poor outcome in patients with MDS. Detection of the genetic abnormalities in AML have been translated into diagnostic approach and patient management. Characterization of novel genetic alterations will continue to provide a bench to bedside translation of molecularly targeted therapy in the future.