Immunophenotyping and Xist Gene in AML

Acute myeloid leukemia (AML) is a heterogeneous disorder characterized by clonal expansion of myeloid progenitors (blasts) in the bone marrow and peripheral blood. with high mortality and variable prognosis. AML is the most common acute leukemia in adults, accounting for ~ 80 percent of cases in this group. There are approximately 19,520 new cases of AML in the United States (US) each year and 10,670 deaths from AML. In Egypt, the incidence of AML was 0.96% for males and 1.14% for females according to the results of the National Population-Based Registry Program of Egypt (2008-2011). Diagnosis of AML based on morphological diagnosis with proliferation of blast cells ≥ 20% of marrow cells, flow cytomtric immunophenotyping and cytogenetic abnormalities.

Immunophenotyping via flow cytometry comprises an additional fast technique to predict outcome in AML, although only few markers are yet established as prognostic factors in clinical routine diagnosis, despite the fact that new and rapidly available markers are needed to improve the treatment decisions in AML patients. This is even more since therapy in AML patients must be initiated immediately after diagnosis. AML blasts express antigens found also on healthy immature myeloid cells, including common differentiation (CD) markers CD13, CD33 and CD34. Other cells markers are expressed depending on the morphological subtype of AML and stage of differentiation block such as monocytic differentiation markers (CD4, CD14, CD11b, CD11c, CD64, CD36), erythroid (CD36, CD71) and megakaryocytic markers (CD41a and CD61).

Non-random chromosomal abnormalities (e.g., deletions, translocations) are identified in approximately 52% of all adult primary AML patients and have long been recognized as the genetic events that cause and promote this disease. Certain cytogenetic abnormalities, including the t(8;21)(q22;q22), t(15;17)(q22;q12) and inv(16)(p13.1;q22) are associated with longer remission and survival, while alterations of chromosomes 5, 7, complex karyotype (described as >3 chromosomal abnormalities) and 11q23 are associated with poor response to therapy and shorter overall survival.

Several studies have reported correlations of aberrantly expressed markers with clinical outcome in AML. For example, CD7 and CD25 expression has been associated with poor prognosis in normal karyotype (NK) AML. The IL3 receptor alfa (CD123) is overexpressed in 45% of AML patients, and this higher expression has also been associated with poor outcome and correlated with mutations in the fms-like tyrosine kinase receptor (FLT3) gene. A consistent antigenic profile with high CD33 expression has also been associated with AML with mutated nucleophosmin (NPM1). Lo-Coco et al., (2015) results suggest that the CD34/25/123/99+ve leukemia-associated immunophenotypes (LAIPs) is strictly associated with FLT3-ITD positive cells. This identification through multiparametric flow cytometry at diagnosis of an immunophenotypic fingerprint associated with these subclones is a novel and simplified tool with improved sensitivity to unravel these clones and allowing patient stratification and risk adapted treatment with potential impact on outcome of the disease.

At present, the etiological agent and pathogenesis of AML are not entirely clear, only few AML cases can be accurately classified through traditional cellular morphological classification. Thus, it is very difficult to judge the disease condition and predict prognosis. Improper expression of specific genes is a common finding in AML and may induce clinically relevant biological subsets. Consequently, identification of novel biomarkers which could predict outcome or guide treatment choice will make more contribution to the clinical management of AML.

X chromosome aneuploidies have long been associated with human cancers, but causality has not been established. In mammals, X chromosome inactivation (XCI) is triggered by X-inactive specific transcript (Xist) RNA to equalize gene expression between the sexes. In humans, one X chromosome is inactivated (Xi) in every female cell in order to achieve transcriptional balance. An X-linked inactivation centre (XIC) is responsible for the initiation of X inactivation. The exact size of the XIC is unclear but it includes the Xistgene at Xq13.2. This encodes a large non-coding RNA that is initially expressed on both X chromosomes before ceasing expression on the active X and becoming upregulated on the X that is to become inactivated. The Xist RNA product coats the future Xi chromosome, spreading out from the XIC.

X-inactive specific transcript RNA was one of the first long noncoding RNAs (lncRNAs) to be discovered in the early 1990s, a decade before the Human Genome Project (HGP) revealed that the large majority of our genome accounts for noncoding sequences. Xist is a 19 kb, spliced, untranslated regulatory transcript that coats the X chromosome from which it is expressed in cis. Xist RNA is the master regulator of XCI, the epigenetic process that equalizes the dosage of X-linked genes between female (XX) and male (XY) mammals. Deletion of the Xist gene results in skewed inactivation of the wild type X chromosome, indicating that this locus is essential for gene silencing.

The early transgenic studies also unveiled two key features of Xist's function. First, the ability of Xist RNA to trigger gene silencing is strictly dependent on the developmental context. Second, Xist has different tasks, such as cis-localization to the chromosome from which it is expressed and the ability to trigger gene silencing, and these tasks are mediated by genetically independent domains of the RNA. Moreover, inappropriate silencing of human Xist results in qualitatively aberrant stem cells. Whereas Xist has been investigated extensively in cell culture, in vivo studies have been limited, however, none of these studies has been done in human.

In some cases, Xist RNA mislocalisation and sporadic Xi reactivation has been observed. For example, one study on an ovarian cancer cell line, showed a disruption of Xist expression and potential reactivation of the Membrane Palmitoylated Protein-1 (MPP1) (p55) gene. Previous study showed that the inactive X chromosome is genetically unstable in cancer as this study reporting an higher mutations rate on the inactive X compare to rest of the genome.

Xist deletion in the blood compartment of mice demonstrated that mutant females developed a highly aggressive myeloproliferative neoplasm and myelodysplastic syndrome (mixed MPN/MDS) with 100% penetrance. Significant disease components include primary myelofibrosis, leukemia, histiocytic sarcoma, and vasculitis. They found that proliferative and dysplastic changes were present in all hematopoietic cell types. Also, Xist-deficient hematopoietic stem cells (HSCs) showed aberrant maturation and age-dependent loss of long-term HSCs.

Their study implies that human hematologic cancers may result from overdosage of X, either from Xist loss on Xi or from duplication of Xa. And they proposed that carcinogenesis is driven by a series of changes occurring in the HSC and further accumulated in mature hematopoietic cells. These changes are initiated by loss of Xist, which leads to progressive X reactivation, which in turn induces a cascade of unfavorable genome-wide changes that include dysregulation of genes involved in DNA replication, chromosome segregation, cell-cycle checkpoints, and hematopoiesis. A failure of HSC maturation and loss of long-term HSC in the marrow progressively shift hematopoiesis to extramedullary sites resulting in extra medullary hematopoiesis (EMH), thereby causally linking the X chromosome to cancer in mice. Thus, they concluded that Xist RNA not only is required to maintain XCI but also suppresses cancer in vivo.

Indeed, the emerging role of aberrant gene dosage in diseases, whether of the X chromosome or for autosomes, brings with it the possible application of drugs that impact on epigenetic regulators in potential therapeutic strategies.

To date, there are no published studies on human about Xist gene and its relationship with the immunophenotyping in AML patients. So, this will be the first study designed to explain its unexplored pathway in AML and detect its prognostic role and immunophenotypic association.