PANORAMA DE GRUPOS Y CLASES SOCIALES ENTRE CONTINUIDADES Y NOVEDADES

The most frequent single chromosomal translocation associated with paediatric ALL is the t(12;21)(p13;q22) rearrangement (Fig. 4). This translocation results in the fusion of the ETS transcription variant 6 (ETV6), also known as translocation-Ets-leukaemia (TEL) gene, that maps on chromosome 12, and Runt- related transcription factor 1 (RUNX1), also known as Acute myeloid leukaemia 1 (AML1) gene, that maps on chromosome 21 (Romana et al., 1995). This gives rise to the ETV6-RUNX1 (TEL-AML1) fusion gene, which is exclusively associated with pre-B ALL, present in up to 25% of cases (Golub et al., 1995). TEL and AML1 genes both encode transcription factors that play important roles in haematopoiesis. Both genes have been shown to be essential in establishing haematopoiesis of all lineages in the bone marrow and constitutive loss of either gene results in embryonic lethality (Okuda et al., 1996; Wang et al., 1997; Wang et al., 1996). The TEL transcription factor is required for HSC survival in the adult and is an essential regulator of post-natal HSCs. HSCs require TEL in order to migrate to the bone marrow from the foetal liver and for subsequent survival within the bone marrow microenvironment (Hock et al., 2004; Wang et al., 1997; Wang et al., 1998). In contrast, although AML1 is required for haematopoiesis at the embryonic stage, unlike TEL, loss of AML1 in the adult does not result in complete loss of haematopoiesis. Studies using conditional knock out mice have shown that AML1 is required for maturation of megakaryocytes and differentiation of T and B cell lineages, but not for maintenance of HSCs during adult haematopoiesis (Ichikawa et al., 2004). Furthermore, germline variants and somatic mutations in both genes have been linked to predisposition to leukaemia (Osato et al., 1999; Topka et al., 2015;

Zhang et al., 2015). TEL and AML1 are also frequently disrupted by numerous different translocations in lymphoblastic and myeloid leukaemias.

The AML1 gene belongs to the Runt-related transcription factor (RUNX) family of genes, also known as core binding factor (CBF) genes. The highly conserved RUNT domain of AML1 mediates DNA binding and heterodimerisation with CBFβ (Roudaia et al., 2009). Although the protein alone is capable of regulating expression of various different genes, binding to the CBFβ subunit enhances the affinity of the RUNT domain to DNA by 7- to 10- fold. Furthermore, it can inhibit RUNX1 degradation mediated by the ubiquitin proteasome pathway. It has been shown that TEL-AML1 fusion relies on CBFβ for its activity in experimental models, by participating in the formation of protein complexes necessary for its function (Roudaia et al., 2009).

The TEL transcription factor comprises three important domains: The N- terminal pointed domain (PD) required for mediating protein-protein interactions, including TEL oligomerisation; C-terminal DNA-binding domain; and a central repressor domain, which is mainly involved in recruitment of repression complexes. The resulting translocation between the TEL and AML1 genes creates a fusion transcript between the 5’ of the TEL region, excluding the region encoding the DNA-

Figure 4 - Cytogenetic and molecular genetic abnormalities in childhood ALL

The frequencies of the different cytogenetic and molecular genetic abnormalities present in childhood ALL. The TEL-AML1 fusion gene represents the most frequent abnormality. Adapted from (Mullighan, 2012)

binding domain, to almost the entire coding region of AML1 (Poirel et al., 1997; Zelent et al., 2004) (Fig. 5).

In patients the translocation is detected during B cell differentiation prior to the onset of immunoglobulin gene rearrangement, giving rise to leukaemic blasts that appear to be blocked at the pre-B cell stage (Panzer-Grumayer et al., 2005; Romana

C)

Figure 5 - Schematic representation of TEL, AML1 and the TEL-AML1 fusion protein

The figure shows the functional domains present in (A) TEL, (B) AML1 and the (C) TEL-AML1 fusion. TEL has three main domains: the Pointed Domain (PD) is necessary for oligomerisation and protein-protein interactions, the central repression domain (Repression) for binding with transcriptional repressors and the ETS DNA binding domain. AML1 has the RUNT DNA binding domain (RUNT), the mSin3A interaction domain (SID, bracket), the p300 HAT interacting domain (p300 ID) and the transcriptional activation domain (ACTIVATION). The carboxy-terminal VWRPY is a motif that binds Groucho-related co-repressors. The t(12;21) translocation results in the TEL-AML1 fusion with the arrow indicating the fusion points between TEL and AML1. The TEL-AML1 fusion has lost the TEL ETS domain. Adapted from (Zelent et al., 2004)

A)

B)

et al., 1995). The most immature TEL-AML1 population is in fact identified by the aberrant CD34+

CD38–

CD19+

phenotype, that has been associated with a very early stage of lineage commitment (Castor et al., 2005; Hong et al., 2008). The presence of the fusion gene, most likely occurring in utero, is largely detectable years before the clinical onset of the disease (Zelent et al., 2004). However, despite the prevalence of TEL-AML1 in childhood pre-B ALL it is unlikely that the presence of the fusion alone is sufficient for the resulting leukaemia. This is consistent with the variable and protracted latency in the onset of leukaemia in identical twins with concordant TEL- AML1 ALL (Wiemels et al., 1999). Furthermore, analysis of TEL-AML1 frequency in unselected normal cord blood cells, showed that 1% of blood samples had detectable TEL-AML1 transcripts. This was 100 times more frequent than the incidence of disease (Mori et al., 2002). Furthermore, estimation of the frequency of TEL-AML1+

cells in these cord blood samples suggested that acquisition of the fusion was associated with clonal expansion of progenitor cells (Mori et al., 2002). In contrast, another model has shown that the presence of the fusion is as rare as the incidence of disease, suggesting that a high proportion of babies born with the TEL- AML1 fusion go on to develop leukaemia (Lausten-Thomsen et al., 2011). Although the frequency of the fusion is still a matter of debate, whatever the frequency in the normal population, experimental models also indicate that TEL-AML1 alone is insufficient to induce overt leukaemia. Rather, in agreement with the ‘two-hit’ model, it is thought that TEL-AML1 acts as an initiating event resulting in the generation and expansion of pre-leukaemic cells that persist for years before onset of leukaemia, upon acquisition of secondary genetic mutations (Greaves, 1999; Greaves and Wiemels, 2003) (Fig. 6).