Análisis epigenético de los genes de la familia pias y detección de mutaciones en el gen tet2 en neoplasias mieloproliferativas

Resumen

BCR-ABL1 negative myeloproliferative neoplasms (MPN) are blood diseases characterized by clonal hematopoiesis, chronic excessive production of differentiated blood cells and increased risk for thrombosis and secondary leukemic transformation. Three major diseases constitute BCR-ABL1 negative MPN: polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). These neoplasms are generally caused by the constitutive activation of tyrosine-kinase signalling proteins or by changes in other molecules involved in those signalling pathways. Over the last years, some genetic events in tyrosine kinase genes have been described as causing events of these diseases (JAK2V617F mutation, JAK2 exon 12 mutations, MPL mutations) but there are patients without these alterations.We will focus on the JAK/STAT pathway, which is frequently altered in this class of tumours. We first analyzed the potential inactivation, through epigenetic mechanisms, of PIAS family genes that is known to be one of the negative regulators of the JAK/STAT pathway, because such inactivation could contribute to the constitutive activation of the pathway and promote transformation. In the four PIAS family genes studied, the only CpG island methylated was detected in PIAS1. But we don¿t observe correlation between its methylation status and transcriptional expression in human cancer cell lines treated with DNA methyltransferase inhibitor (5-aza-2¿-deoxycytidine). Moreover, we showed a similar methylation frequency of PIAS1 CpG island in MPN patients samples and healthy controls. So, by the time we can¿t conclude that the methylation status of these genes may be involved in the development of this disease. Likewise, we analyzed all the coding region of TET2 because it has been recently reported that acquired loss of heterozygosity at chromosome 4q24 (where TET2 is located) is associated with MPN and myelodysplastic syndromes. Moreover, recent research has also detected mutations in TET2 in these syndromes. As it has been previously described by other authors, our results show that TET2 is frequently mutated in NMP patients (23.1%) with higher frequency in V617FJAK2¿negative patients The mutations detected were heterozygous and 17 were potentially inactivating mutations: five nonsense mutations, six missense mutations and six frameshifts. Of these 17 mutations only two changes had been reported previously. Those patients that harboured more than one mutation in TET2 subsequently developed AML. On the other hand we collected 725 TET2 mutations published to date in 3,274 different myeloid neoplasms patients in order to analyze the distribution throughout the gene. The distribution of these mutations showed a higher mutational frequency in the two highly conserved regions of TET2. These two regions have a higher prevalence of missense mutations, whereas nonsense and frameshift mutations predominate in the other two non-conserved regions of the gene. Our results indicate similarities between MPN, MDS and sAML, with a distinct mutational pattern in CMML and especially in nAML. Considering all types of mutations only in the first non-conserved region, we observed that there are higher frequency of missense mutations within this region in CMML and AML, this may indicate a more aggressive disease. Furthermore, the finding that both types of AML have different mutational patterns raises the possibility that the type of TET2 mutation might contribute to specify the sequence of mutational events leading to sAML or to nAML. We detected 10 polymorphisms already described in dbSNP, and four not previously described. All of them were found both in control samples and in patients. None of them showed different distribution between patients and controls or between V617FJAK2¿positive and ¿negative patients. Furthermore, we showed that expression of TET2 is not regulated with STAT5 although this protein binds to the promoter region of TET2.