Acute myeloid leukemia with myelodysplasia related changes (AML-MRC) is a subgroup of AML. It was introduced in 2008 by the WHO classification and updated in 2016 (Weinberg et al., 2015). A diagnosis of AML-MRC requires ≥ 20% blasts and additionally one of the following three criteria:

history of myelodysplastic syndrome (MDS) or myelodysplastic/myeloproliferative neoplasm (MDS/MPN)

MDS-related cytogenetic abnormalities or

multilineage dysplasia (≥ 50% dysplastic cells in≥ two hematopoietic lineages) (Arber et al., 2016a).

There are various risk factors which can cause AML in general. These risk factors can be categorized as genetic disorders, physical and chemical exposure, radiation therapy and chemotherapy. Even though many people are exposed, to various degrees, to carcinogens, only a few cases may present with a known history of carcinogen exposure. Currently, it is not fully understood why some people develop AML and some do not (Estey and Döhner, 2006). Genetically, many researchers favor the "two-hit" hypothesis first proposed by Hartmut Beug (Beug et al., 1978), and later by Gary Gilliland. This hypothesis states that for transformation, a mutation in a kinase coding gene such as RAS, KIT or FLT3 has to occur together with a class II mutation targeting a transcription of nuclear factor such as NPM1, RUNX1 or CEBPA. Today, we know about a high variation of somatic mutations and chromosomal abnormalities characterizing different types of AML. AML-MRC is often associated with the following chromosomal abnormalities (Table 1) (Deschler et al., 2006; Weinberg et al., 2015).

If the WHO classification is strictly applied, 48% of all adult AML cases meet the AML-MRC criteria. It tends to affect more male and is a disease of the elderly with a median age of 68 years. Additionally, it is associated with a higher frequency of unfavorable cytogenetics and, consequently, a worse clinical outcome (Davis et al.,2013; Xu et al., 2014; Weinberg et al., 2009). AML-MRC occurs more often de novo then secondary to myelodysplasia (Vardiman and Reichard, 2015).

AML-MRC cases commonly present with multilineage dysplasia. This is validated best with peripheral blood smears or bone marrow smears for identifying myeloid and erythroid dysplasia and bone marrow biopsy for identifying megakaryocytic dysplasia (Falini et al., 2016). Following the WHO classification an AML-MRC presents with ≥ 50 % dysplastic cells in at least two hematopoietic lineages. The dysplastic features are not unique for AML-MRC, but can be also detected in other hematopoietic diseases, such as MDS (Wu et al., 2013). Dysplastic neutrophils may exhibit a nucleus-cytoplasm dysynchrony, hypogranulation and abnormal nuclear segmentation (pseudo-Pelger-Huët anomaly), e.g. bi-nucleated segments (Weinberg et al., 2015). As mentioned, dyserythropoesis and dysmegakaryopoesis are also commonly seen. Dyserythropoesis presents with odd erythroid precusors and sometimes ring sideroblasts, as well as multinuclearity, megaloblastoid changes and karyorrhexis. Dysmegakaryopoesis commonly shows hypolobulated micromegakaryocytes, non-lobulated nuclei in megakaryocytes of all sizes and multiple widely separated nuclei (Wakui et al., 2008).

Immunophenotype: Because of the heterogeneity and commonly associated cytogenetic abnormalities AML-MRC has no specific immunophenotypic profile. However, the immunophenotype of blasts exhibits similar characteristics as seen in MDS (Ayar et al., 2014). Antigens associated with myeloid differentiation, such as MPO, CD33, CD13, CD64, CD14 and CD15 are seen in various degrees (van Dongen et al., 2012). MDS related AMLs with monocytic differentiation also show expression of CD45 and lack CD34 (Van Dongen et al., 2012). Additionally, the aberrant expression of CD10, CD56 and CD7 in blasts may be found (Weinberg et al., 2016). AML-MRC cases associated with monosomy 5 or 7 may show aberrant expression of TDT and CD7 (Font et al., 2006).

The therapy of AML-MRC is based on regular AML-therapy with a risk-adapted chemotherapy including induction chemotherapy ("7+3"; daunorubicine plus cytarabine) and postinduction therapy, such as high-dose cytarabine or in high-risk patients, allogeneic transplantation in first remission. Using regular assessments, lab studies including cytogenetics and molecular analyses, the patient can be assigned to a risk group and treatment and prognosis can be determined and the decision between curative chemotherapy/allogeneic stem cell transplantation, palliative chemotherapy and best supportive care can be made (Estey and Döhner, 2006). Chemotherapy can be separated into two main parts. First, the induction therapy with the attempt to reach complete remission. Second, the consolidation therapy to prolong the complete remission. If the patient displays characteristics of high risk AML, a hematopoietic stem cell transplantation (HSCT) must be considered (Kumar, 2011). As AML-MRC frequently fulfills the high-risk criteria such as: 3(q21q26) abnormalities, RPN1/MECOM (EVI1), del(5q), del(7q), t(6;9), other 11q23 abnormalities, 17p abnormalities, or complex aberrant karyotypes described as at least 3 unrelated abnormalities excluding cases with t(8;21), inv(16), and t(15;17), an allogeneic stem cell transplantation will be the post-induction therapy of first choice in most adults (Vardiman and Reichard, 2015).

The myelodysplastic changes and MDS associated chromosomal abnormalities of AML-MRC are associated with a worse prognosis and lower rates of event-free survival and overall survival. However, it was shown that there is no difference in prognosis between patients with AML-MRC with a history of MDS and the remaining cases of AML-MRC (Weinberg et al., 2009; Arber et al., 2003). Besides, AML-MRC with the absence of cytogenetic abnormalities seem to have a better prognosis (Weinberg et al., 2009). Additionally, AML-MRC with NPM1 mutations and with or without cytogenetic abnormalities show a better prognosis (Haferlach et al., 2009). This was also shown for CEBPA mutations (Vardiman and Reichard, 2015). Therefore AML-MRC with NPM1 or CEBPA mutations are excluded from the AML-MRC category in the WHO classification 2016 (Arber et al., 2016). The prognosis of AML-MRC particularly in context of multilineage dysplasia remains controversial (Vardiman and Reichard, 2015). In patients who have undergone allogenic HSCT there is no prognostic difference in AML-MRC patients compared to AML not otherwise specified (AML-NOS) (Ikegawa et al., 2016).