The significance of molecular diagnosis of leukemia
The purpose of molecular diagnosis of leukemia is to facilitate clinical treatment, drug selection and prognosis, to identify groups of leukemias with different pathogenesis, and to explore the causes of the disease. The MIC typing method still does not fully reflect the type of leukemia. MIC typing methods still do not fully reflect the clinical management of leukemia and its relationship with prognosis. Numerous studies have confirmed that fusion genes formed after chromosomal translocations are closely related to leukemia susceptibility. The combination of morphologic, immunophenotypic, genetic and clinical features is intended to reflect the “true” nature of the disease. Acute myeloid leukemia AML has been classified into five main groups.
1. AML with chromosomal translocations.
2. AML with myelodysplasia.
3. AML associated with treatment and myelodysplastic syndromes
4, AML that cannot be classified (FAB)
5, biphenotypic leukemia
According to the criteria of FAB, the significance of AML is a primitive cell count of 30%. Recent studies have shown that patients with a primitive cell count of 20% to 30% have a prognosis similar to that of patients with a primitive cell count of more than 30%. Therefore, it is agreed that the primocyte count as a criterion for the diagnosis of AML should be 20%, discarding refractory anemia with transformed primocytosis. Recent clinical work has revealed that most leukemias have some sort of chromosomal translocation, and that translocations can produce new fusion genes that encode fusion proteins. Using these markers different types of leukemia can be diagnosed. The World Health Organization (WHO) 2000 leukemia classification scheme has included chromosomal translocations as one of the most important indicators. Detection of fusion genes formed by chromosomal translocations is relatively easy and sensitive and has been used directly in the diagnosis of leukemia. Genetic variants in leukemia are often accompanied by characteristic morphological abnormalities and unique clinical features that are very closely related to clinical treatment. Understanding these features is necessary to accurately diagnose and treat leukemia.
Genetic diagnosis of acute leukemia
Acute promyelocytic leukemia t(15;17) forms PML/RARα and its variants.
Acute promyelocytic leukemia (APL) is aggressive, easily complicated by DIC, and often dies within 1 week without treatment; the APL chromosome t(15;17) (q22;q21) translocation results in the fusion of the promyelocytic leukemia (PML) gene located at 15q22 and and 17q21 retinoic acid receptor alpha (RARα) gene to form PML/RARα [3].APL multi This translocation is detected in both the granulocytic and microgranulocytic phenotypes. Those with t(15;17) and PML/RARα are only effective for retinoic acid and arsenic trioxide treatment. Four rare variant translocations t(11;17)(q13;q21), t(11;17)(q23;q21), t(5;17)(q32;q21) and dup(17q) forming NuMA/ RARα, PLZF/RARα, NPM/RARα and STAT5b/RARα fusion genes have also been identified in recent years [4,5]. APL with t(11;17), t(11;17) is often ineffective with retinoic acid therapy.
AML1-ETO fusion gene with chromosome t(8;21) translocation in acute myeloid leukemia
Chromosome t(8;21) (q22;q22) leads to AML1-ETO fusion gene is the most common abnormal gene in acute granulocytic leukemia (AML) [6].AML1 gene located on chromosome 21q22 is fused with ETO gene on 8q22 to produce AML1-ETO.It represents the type of acute leukemia with better prognosis and easily achieves remission. Approximately 20-40% of AML-M2 patients have t(8;21) and the incidence increases with younger age; in AML, 90% of M2b subtypes have AML1-ETO fusion gene. m2b has typical morphological features such as strong positive myeloperoxidase, significant Auer vesicles, easy to see cytoplasmic vacuoles, and orange-red granules in the cytoplasm of mature neutrophils, which can be clinically diagnosed The clinical diagnosis can be cross-checked with genetic abnormalities.
Acute myeloid leukemia chromosome inv(16) CBFB/MYH11 fusion gene
The CBFB/MYH11 fusion gene formed by chromosome Inv(16)(p13q22) or t(16;16)(p13q22) is only seen in AML-M4EO and not in other types of leukemia [7]. Cytologically this type of leukemia often shows infiltration of leukemic cells of granulocytic and monocytic lineage with characteristic bone marrow eosinophil abnormalities, including increased numbers (>5%) or qualitative abnormalities (large and irregular basophilic staining granules interspersed with eosinophilic granules, and strong positivity for both glycogen and chloroacetate lyase). These leukemias have a better prognosis.
Acute myeloid leukemia with abnormal chromosome 11q23 (MLL)
Mixed lineage leukemia (MLL)[8] The most common types of genetic variants associated with morphology are AML-M5 and AML-M4. Both AML and ALL can have MLL genetic variants, with MLL/AF4 being most common in childhood ALL and MLL/AF9 being most common in AML-M5. Ying Yang et al. found[9] that MLL/AF6-positive cells all had the typical features of primitive monocytic leukemia (AML- M5a): folded nuclei, 1-2 large and well-defined nucleoli, translucent cytoplasm, cytoplasm filled with fine pink granules, visible elongated Auer vesicles, and distinct faintly stained areas next to the nucleus. Peroxidase staining was negative and non-specific lipase staining was positive (could be inhibited by sodium fluoride). MLL/AF6 rearrangement may be an important marker of AML-M5a subtype, which is closely related to monocyte differentiation.
It is believed that those with morphological abnormalities and suspected acute leukemia with any of the above chromosomal abnormalities should be diagnosed with leukemia even if the bone marrow has <20% primitive cells.
Mutations in the NPM1 gene in acute myeloid leukemia
Recently, Gorello et al. found that 25% of AML have mutations in the NPM1 (nucleophosmin) gene [10], resulting in the presence of cytosolic NPM proteins in the cytoplasm.NPM1 mutations are mainly found in leukemias with normal karyotype, which are prone to remission but also to relapse. It can be used as a marker for AML without chromosomal translocations.
Acute lymphoblastic leukemia (ALL) TEL-AML1
TEL-AML1 is a common fusion gene in childhood leukemia formed by t(12;21) (p13;q22), with TEL on chromosome 12 and AML1 on chromosome 21. Seen in 12-28% of B-lineage ALL, but only in B precursor cell ALL (BCP-ALL), not in mature B-ALL or T-ALL.
TEL-AML1 is a better indicator of prognosis. Immunoglobulin and T-cell receptor gene rearrangements in acute lymphoblastic leukemia
The T-cell receptor (TCR) and immunoglobulin (Ig) genes are used to detect lymphocytic leukemia based on the principle that during differentiation of stem cells to lymphocytes, the variable region (V) and binding region (J) genes of the TCR and Ig genes are rearranged, i.e., two distant segments are rearranged to form a new segment. , each lymphocyte has its own sequence of different TCR or and Ig fragments. ALL leukemia cells proliferate monoclonal and leukemia is considered if only one gene rearrangement fragment is detected. The Ig heavy chain (H), TCRγ, and TCRδ genes are often used as markers to detect lymphocytic leukemia.