The nucleolus, a prominent sub-compartment of the cell nucleus, is the site of ribosome biogenesis. Ribosomes are molecular machines responsible for the production of proteins. The RNA component of the ribosome, ribosomal RNA (rRNA), is synthesized and modified in the nucleolus, where the assembly of rRNA and ribosomal proteins into ribosomes also occurs. In addition, the nucleolus is involved in other cellular functions such as regulation of the cell cycle, cell proliferation, and sensing of cellular stress. Also pathological conditions such as cancer and viral infections have been linked to the nucleolus. Since the nucleolus is not membrane-bound, molecules are able to enter and exit it freely. The nucleoli are by nature dynamic, and their number and size vary based on the state of the cell. Cancer cells often display enlarged nucleoli, this resulting from an increased need for protein synthesis, which in turn requires efficient production of ribosomes. This seems, however, to be only part of the truth, as the nucleolus has been held to be implicated in the development of cancer.

Leukemia is a cancer which originates in the blood-forming tissues, and includes a wide spectrum of diseases with different manifestations and outcomes. In acute leukemia, abnormal immature blood cells are produced in large quantities to such an extent that normal hematopoiesis is displaced. The leukemic cells do not form a solid tumor but fill the bone marrow and also circulate in the blood. Acute lymphoblastic leukemia (ALL) is the most common form of the disorder in children. With current treatments outcomes have improved, especially in pediatric ALL, with cure rates close to 90 %. Nevertheless, the mechanisms underlying the development of leukemia and relapse are still insufficiently understood. Historically, ALL subtypes displaying abnormal nucleoli have been associated with poorer prognoses.

The aim of this series was to study the role of the nucleoli in acute pediatric leukemia. More specifically, small nucleolar RNAs (snoRNAs) and nucleolar proteins were studied in different acute leukemia subtypes. The differences in the expression of snoRNAs and in the composition of nucleolar proteins between leukemia subtypes were analyzed using leukemic cell lines and patient samples. Moreover, a means of investigating the in vivo function of nucleolar proteins was set up in the zebrafish.

The expression of snoRNAs was analyzed in cell lines representing the main acute leukemia subtypes (AML (acute myeloid leukemia), T-ALL and pre-B-ALL). The results show differential expression of snoRNAs between the subtypes, which suggests that snoRNAs have a subtype-specific expression pattern. Several individual snoRNAs were found to be differentially expressed between T-ALL and pre-B-ALL. The expression of nine snoRNAs was further analyzed by quantitative PCR, and the results show similar changes in expression. Further, the expression of 15 snoRNAs was analyzed in pediatric leukemia and control samples available in a large hematological data set. For most snoRNAs, the expression was relatively uniform in both leukemic and normal samples. For SNORD114-3, the expression was consistent among pediatric samples except for 15 cases with expression approximately fourfold higher. When further examined, 13 of these cases turned out to be APL (acute promyelocytic leukemia), a subtype of AML. Thus, overexpression of SNORD114-3 implies APL.

The nucleolar proteomes were analyzed in leukemic cell lines representing AML, T-ALL and pre-B-ALL. Several differentially expressed proteins were identified among the leukemia subtypes, and the results could be reproduced using another method. The function of the nuclear/nucleolar protein SAP30L, which has been identified in our laboratory, was studied in the zebrafish. The depletion of SAP30L induced marked pericardial edema, deformed cardiac morphology, and impaired cardiac function. Also, reduced levels of hemoglobin and involvement of SAP30L in multiple signaling pathways were observed.

Based on the results, it may be concluded that both nucleolar proteins and snoRNAs are differentially expressed among the acute leukemia subtypes. Furthermore, several tools and methods were applied which proved useful in the study of nucleolar components in leukemia.