Background
According to the current recommendations of the WHO [
1] the large and heterogeneous group of cytogenetically normal AML (CN-AML) is further stratified by the presence or absence of internal tandem duplications of fms-related tyrosine kinase 3 (
FLT3-ITD), mutations of nucleophosmin (
NPM1) and mutations in the CCAAT/enhancer binding protein (C/EBP) alpha (
CEBPA). In fact, AML with mutated
CEBPA has been classified as its own category in the current WHO classification [
1].
The gene encoding for the CCAAT/enhancer binding protein-α (
CEBPA) is located on chromosome 19 band q13.11. It was first full-length cloned in 1997 [
2]. The
CEBPA protein is 42 kDa of size. It is expressed in myelomonocytic cells and upregulated in granulocytic differentiation acting as a myeloid transcription factor. Mutations of
CEBPA in AML were first described in 2001 [
3]. N-terminal frameshift mutations lead to the overexpression of a truncated 30 kDA isoform of
CEBPA that suppresses
CEBPA function in a dominant negative way. C-terminal mutations occur mainly in the basic Zipper (bZIP) domain of
CEBPA, and impair its function to homodimerize and heterodimerize with other proteins as well as its DNA binding [
4].
CEBPA knock-out mice show a selective block of differentiation lacking mature granulocytes while other hematopoietic cells differentiate regularly [
5]. Mutations of
CEBPA have been shown to be associated with CN-AML where they occur with frequencies of 8 -18% [
6]-[
10] and with the French-American-British (FAB) subtypes M1 and M2 [
11].
CEBPA mutations can occur as monoallelic mutations (mo
CEBPA) or as biallelic mutations (bi
CEBPA). Patients with bi
CEBPA mutations usually have a C-terminal mutation on one allele and an N-terminal mutation on the other allele, resulting in a lack of
CEBPA wildtype allele expression [
12],[
13]. We and others have reported earlier that the positive prognostic impact on outcome is restricted to patients with bi
CEBPA mutations [
6],[
13]-[
16]. Except for the studies of Taskesen [
16] and Green [
14] patient numbers with mutated
CEBPA at diagnosis were small (n < 50) and median follow-up times were <10 years in the majority of the upper mentioned analyses.
The objective of this study was to investigate if the effect of moCEBPA versus (vs.) biCEBPA mutations on outcome was true also in a longer follow-up period and to elucidate the clinical course of disease in biCEBPA mutations patients.
Discussion
According to the current WHO classification and the ELN guidelines patients with mutated
CEBPA represent a cohort with a favorable prognosis [
1],[
28]. We and others have shown before, that this favorable prognostic effect is restricted to the group of patients with bi
CEBPA, in contrast to mo
CEBPA[
6] mutations. The aim of our current analysis was to test if the favorable prognostic effect of bi
CEBPA mutations was still evident within a (1) larger patient cohort and (2) after a longer follow-up period.
Due to the low frequency of
CEBPA mutations of about 8 - 18% in CN-AML, studies are often based on small patient numbers [
6],[
13],[
15], which reduces statistical power. Our study includes 88 patients, 75 of whom were treated homogeneously within AMLCG99 and AMLCG2008 trials.
Except for the study of Green et al. [
14] with a medium follow-up for survivors of 11.7 years, median follow-up in the published literature including our previous publication is mostly ≤5 years [
6],[
13],[
15],[
16],[
33]. The present analysis has a long medium follow-up of almost 10 years (9.8 years) and a maximum follow-up of 13.3 years. This allowed to detect late relapses.
In accordance with previous analyses [
13]-[
15], bi
CEBPA-mutated AML patients displayed a significantly longer OS compared to mo
CEBPA-mutated patients. Differences in OS might be caused by differences in early death rate, achievement of a CR and RFS/incidence of relapses. Patients with a bi
CEBPA mutation showed a slightly lower early death rate at day 60 (6.7% versus 9.3%, Table
1) and a higher CR rate (CR rates: 82% versus 70%, p = 0.17, respectively) compared to patients with a mo
CEBPA mutation. Interestingly, overall early death rate until day 60 in all
CEBPA-mutated patients in our cohort was 8.0%, which is lower compared to data in the literature of 10% to 16% [
19],[
20],[
34]. These differences might be caused by the fact that these studies included all cytogenetic risk groups and not only CN-AML patients. Most importantly, we could clearly see a better RFS in bi
CEBPA-mutated patients. This improved RFS was also seen by others [
14],[
15]. Moreover, we could demonstrate a lower CIR in bi
CEBPA-mutated compared to mo
CEBPA- mutated patients. However, to what extent a lower early death rate, a higher CR rate or a longer RFS caused the prolonged survival still remains open and needs to be investigated in larger patient cohorts.
The favorable prognosis of biCEBPA-mutated AML was most evident in patients ≤60 years who displayed a 10-years OS of 81% and a 10-years RFS of 66%.
In contrast to relapse in moCEBPA-mutated patients, occurring in the majority of patients (63%) within the first year after achievement of a CR, only about one third of biCEBPA-mutated patients relapsed within the first year. Almost all moCEBPA-mutated patients and biCEBPA-mutated patients relapsed within the first three years.
Interestingly, one patient with a biCEBPA-mutated AML and one patient with a moCEBPA-mutated AML relapsed more than 7 years after CR. To our knowledge, this is the first case of such a late relapse in biCEBPA-mutated AML. These late AML relapses might be therapy-associated and display different cytogenetics and molecular mutational patterns. Unfortunately, due to lack of material, we cannot confirm the presence of a CEBPA mutation at relapse. Both cases underline the importance of a long-term follow-up of patients with AML and CEBPA mutations.
The long patient follow-up enabled us to detect patients with late deaths (>5 years) unrelated to AML: Two patients with a moCEBPA mutation and one patient with a biCEBPA mutation died in CR after 11.7 years (cause unknown), 11.5 years (cause: development of a cancer of unknown primary) and 9.6 years (cause: cardiac failure). Thus, the inclusion of "AML unrelated death" helps to better estimate prognosis, especially in elderly co-morbide patients.
Schlenk et al. [
35] recently showed in a large cohort of 124 CN-AML patients with bi
CEBPA mutations in first CR, a significantly longer RFS, but similar OS, for those patients receiving allogeneic or autologous transplantation compared to chemotherapy only. We also found a similar OS in our bi
CEBPA-mutated patients (n = 4) receiving allogeneic SCT in first CR, compared to those obtaining chemotherapy (n = 30), although we could not detect an effect of allogeneic SCT on RFS, which might be due to the small number of patients receiving allogeneic SCT (data not shown). In accordance with Schlenk et al. [
35] we found a high second CR rate after reinduction therapy of 78% in bi
CEBPA- mutated patients treated with intensive protocols. Due to transplant-related mortality, infections and a high relapse rate after the second CR, this did not result in a longer OS compared to relapsed mo
CEBPA-mutated patients. These results have to be interpreted with caution since they are limited by small patient numbers. Schlenk et al. [
35] furthermore showed that only relapsed patients treated with allogeneic SCT, but not those treated with chemotherapy alone, survived longer than 2 years. Our analyses - although performed in a smaller patient cohort - are in line with these results: In our cohort, patients with bi
CEBPA mutations that have received allogeneic SCT at the time of relapse are still alive (after 7.0 and 11.3 years) or have died due to treatment related mortality, but not due to AML relapse/refractory AML. In contrast, all 5 patients receiving chemotherapy at the time of relapse died (median OS after relapse: 0.9 years).
Authors' contributions
Conception and design: FP, DK, WH, KS. Collection and assembly of data: FP, DK, AD, NPK, SS, JB, MCS, WEB, TB, BW, WH, KS. Statistical review: EH. Data analysis and interpretation: FP, DK, WH, KS. Manuscript writing: All authors. All authors read and approved the final manuscript.
Competing interests
The authors indicate no potential conflicts of interest.