**Abstract**

Acute lymphoblastic leukemia (ALL) is more frequent in children than in adults. The ALL is a hematological neoplasia, which is characterized by the hyperproliferation of lymphoid precursors in bone marrow. MicroRNAs (miRNAs) are a class of noncoding RNAs that regulate mRNA expression at posttranscriptional level. miRNAs regulate different biological processes such as development, proliferation, apoptosis, hematopoiesis, drug resistance, and tumorigenesis. It has also been observed that the expression of miRNAs can be used to the classification of the different subtypes of ALL. Likewise, miRNAs can also be used to determine the prognostic value and may represent potential therapeutic target molecules in the treatment of ALL.

**Keywords:** miRNAs, acute lymphoblastic leukemia, diagnosis, prognosis, therapy, biomarkers

## **1. Introduction**

The hematopoiesis is primarily regulated at the transcriptional level by transcription factors that act as master regulators of genes expression. However, the transcriptional process alone does not appear to control all aspects of cellular functioning (cell fate, lineage, etc.), suggesting the participation of other mechanisms. The miRNAs constitute another critical way of hematopoietic regulation. The B- and T-lymphocytes develop from progenitor cells that occur in different organs; B-cell lymphopoiesis is completed in the bone marrow, whereas T-cell lymphopoiesis occurs in the thymus. However, their development and activation are controlled by signaling pathways, which are also regulated by the microRNAs (miRNAs) [1]. miRNA expression profile during the normal and malignant hematopoiesis suggests that miRNAs are regulators of hematopoiesis implicated in regulating and maintenance of the "stemness" of the early progenitors, various stages of cell differentiation, and malignance [2].

Nowadays, there is evidence that miRNAs do not just regulate hematopoietic differentiation and proliferation but also their activity. Deregulation of the expression of miRNAs has been observed in leukemias, and mechanistic studies reveal a role for miRNAs in the pathogenesis of this disorder [3].

Leukemia is a clonal disorder in which the normal hematopoiesis is replaced by a malignant clonal expansion of immature hematopoietic cells (blasts) in the bone marrow or peripheral blood [4]. The first approach between miRNAs and leukemia was carried out by Calin et al. [5]. The author showed that the 13q14 deletion in B-cell chronic lymphocytic leukemias (B-CLLs) causes the loss of the precursor gene of miR-16-1 and miR-15a; therefore, the loss of these miRNAs is observed in approximately 70% of the CLLs [5]. Interestingly, it has been reported that at fragile sites, minimal regions of amplification (minimal amplicons), or common breakpoint regions fragile sites, minimal regions of loss of heterozygosity, and genomic regions related with cancer code for approximately 50% of the miRNAs, hence the aberrant expression of different miRNAs in cancer [6].

The participation of miRNAs in different biological and cellular processes under pathological and normal conditions makes them good candidates in the investigation of functional markers for differential diagnosis, prognosis, and development of new therapeutic regimens, through the investigation of their molecular targets. In this chapter, the role of miRNAs expression profiles in ALL that could be used for classification of the disease establishing specific diagnoses and prognostic values is summarized. Likewise, the relation between the miRNA dysregulation and ALL may be a potential therapeutic target.
