Childhood B-cell acute lymphoblastic leukemia (B-ALL), also termed B-cell precursor ALL, is a malignant disease characterized by the rapid expansion of clonal blast cells that are phenotypically similar to the normal stages of B-cell differentiation. It is the most common cancer in children, frequently prevalent between the age of 2 and 5 years. Preleukemic clones may already develop during pregnancy, however, these may rarely progress into full-blown cancer without the presence of genetic and/or environmental risk factors. Interestingly, the rising incidence of B-ALL over the past decades seems to correlate with the adaptation to a modern lifestyle.
A recent review published in Nature Reviews Immunology by César Cobaleda et al. discusses the latest discoveries about triggering events for B-ALL development and highlighted possible directions for future research.1
Genetic predisposition to B-ALL
Malfunctioning of key developmental genes can lead to the dysregulation of B-cell development, apoptosis, and cell cycle signalling. These events eventually give rise to a vulnerable progenitor cell population susceptible to a triggering event, leading to malignant transformation. Nonetheless, the most common characteristic is the presence of a clinically silent preleukemic phase that needs secondary stimulus to develop into full-blown leukemia. 5% of children carrying preleukemic B-cell clones develop it due to germline or somatic mutation. However, only 1% of such children would ever develop B-ALL.
Current evidence indicates that a first oncogenic hit takes part in utero and restricts differentiation to a B-cell lineage identity. However, functional B-cell transcription factors can act as metabolic gatekeepers by limiting the amount of cellular adenosine triphosphate (ATP), and thereby keeping preleukemic cells in a latent state. Therefore, a second hit often occurs at the level of these regulating transcription factors (such as PAX5 or IKZF1) allowing for the expansion of leukemic clones.
Another example for the development of genetic vulnerability in preleukemic B cells is explained by the function of recombination activating genes (RAG), RAG1 and RAG2, which are vital for the immunoglobulin gene rearrangement during normal lymphocyte development. At the same time, RAG activity can mediate undesired mutations that then lead to the second hit in B-cell developmental genes, thereby triggering B-ALL development.
Types of molecular aberrations responsible for B-ALL development
Aneuploidies occurs in 30% of childhood B-ALL cases. Hyperdiploid B-ALL is the most common (21%) type of aneuploidy associated with a defective condensin complex, altered Aurora-B kinase activity, and an impaired spindle assembly checkpoint.
Changes in signalling kinase activity such as the ones seen in Philadelphia positive (Ph+) or Ph-like ALL, are less frequent in early childhood (17%) compared with adults (36%), and have a poor prognosis.
Genetic alterations lead to the dysregulation of transcription factors, and such dysregulation can be due to either germline or somatic alterations:
- Germline mutations can be classified into two groups:
- Low-penetrance predisposition to B-ALL with a 1.5-fold to 2-fold increase in relative risk, occurs by common germline polymorphisms, such as the ones in ARID5B, IKZF1, GATA3, CEBPE, ERG, and IKZF3
- High-penetrance susceptibility with 10-fold increase in relative risk is often associated with familial disposition to B-ALL due to germline variations in the IKZF1, PAX5, ETV6, SH2B3, RUNX1, and TYK2 genes
- Somatic alterations affecting transcription factor genes can also be classified into two groups:
- Chromosomal rearrangements in ETV6-RUNX1, TCF3-PBX1, and TCF3-HLF have been reported to cause aberrations in chimeric transcription factors or copy-number alterations, leading to the generation of chimeric fusion proteins; this type is found in 25% of childhood ALL cases
- Sequence mutations in PAX5, EBF1, IKZF1, and BTG1 alter the activity of transcription factors
Known risk factors associated with preleukemic conversion include the following:
- Down syndrome
- Male sex (boys are ~1.2 times more affected than girls)
- Chemotherapeutic drugs
- Severe exposure to ionizing radiation above 100 mSv
- High birthweight
- Exposure to immune stressors or immune stress in susceptible children after exposure to in postnatal life
The role of immune stressors for B-ALL development
There is an early hypothesis correlating the B-ALL peak (2–5 years of age) with a period of increased infections in children. Multiple studies have emphasized the effect of environmental factors on genetic predisposition and, therefore, have proposed a link between infection and B-ALL. Long before B-ALL develops, an increased susceptibility to infection is observed in children aged 2–5 years. Studies suggest that a series of infections, rather than a single infection, may contribute to leukemia development.
Most frequent occurrence of B-ALL has been reported between the age of 2–5 years. This coincides with the period when B-cell activation and proliferation occurs under the influence of RAG enzymes. These RAG enzymes are also responsible for the activation of multiple genes such as PAX5, IKZF1, ERG, CDKN2A, and CDKN2B for which mutations have been correlated with the second hit required for full-blown development of B-ALL.
Immune modulation can influence the progression of precursor B-cells to B-ALL as observed in ex vivo experiments where ETV6-RUNX1+ precursor B cells transformed to B-ALL cells when exposed to bacterial lipopolysaccharides. This shows the potential link of exposure to infection and B-ALL development. Also, in another experimental setting, Pax5+/− mice kept in a specific pathogen-free (SPF) environment never developed B-ALL but when these mice were moved to a contagious facility, 22% of them developed B-ALL. Genetic alterations found in these animal models were similar to genetic mutations observed in human subjects.
It is also proposed that multiple immune stressors and gut microbiota are linked to genetic predisposition irrespective of the exposure to infection. Changes in the gut microbiota environment can trigger the development of multiple health issues which may or may not include B-ALL. In an experiment, antibiotics were used as an immune stressor in Pax5+/− mice kept in a SPF facility. B-ALL development occurred in these mice despite of the infection-free environment. Additionally, a higher frequency in disease was observed when these were moved to a normal environment.
Potential immune evasion routes that could be targeted for B-ALL prevention
Modification of microbiota through dietary methods can be used as a potential therapy target to reduce the immune stress in predisposed carriers. Understanding the regulatory mechanism that influences preleukemic cells to change the structure of bone marrow immune microenvironment will be helpful to avoid triggering B-ALL conversion.
Trained innate immunity (TII) can influence granulopoiesis which is linked to suppressed tumor growth. Therefore, identification of an approach for TII can reduce immune stress and can be helpful in controlling the establishment of B-ALL.
Detection of the fusion protein ETV6-RUNX1 is well known in B-ALL. Presence of these proteins results in the generation of autoreactive T cells. These T cells can potentially be used for targeted therapy and immunosurveillance.
- B-ALL is the most predominant form of pediatric cancer with a peak age of prevalence between 2−5 years. It occurs due to uncontrolled growth of mutant B cells that are phenotypically similar to the normal B cells
- Factors that trigger B-ALL development can be genetic as well as environmental. Known risk factors are the presence of preleukemic clones, modern lifestyle, preexisting health conditions, frequent exposure to pathogens, and infections, etc
- Antibiotics, or changes in the microbiota, can play a major role in triggering leukemia in preleukemic cells. The gut microbial population of genetically predisposed individuals may be different from healthy children. Therefore, an intact gut microbiota can act as a barrier to disease progression
- It is likely that there is an interaction between immune stressors and a preexisting first-hit mutation that channels leukemia development, and influences the nature of the second hit
Further research is required to gain in-depth understanding of leukemogenesis in childhood B-ALL. It is proposed that an enhanced immune response should be balanced to avoid unwanted immune stress.
Also, identification of the regulatory mechanism between the immune stressors and preleukemic cells, will potentially be helpful in childhood B-ALL prevention.