In recent years there has been growing interest
In recent years, there has been growing interest regarding the role of spliceosome machinery in neoplastic diseases following evidence of the involvement of splicing factor mutations in diverse cancers . (Serine-Arginine Splicing Factor 2) is a member of the serine/arginine (S/A) rich family pre-mRNA splicing factors. mutations are primarily detected in hematologic diseases, at a reported frequency of 5.5% in refractory anemia with ringed sideroblasts and refractory cytopenia with multilineage dysplasia with ringed sideroblasts, 11.6% in myelodysplastic syndrome (MDS) without ringed sideroblasts, 6.5% in acute myeloblastic leukemia secondary to MDS, 0.7% in de novo acute myeloid leukemia, 1.9% in myeloproliferative neoplasms, 18.8% in secondary acute myeloid leukemia evolving from myeloproliferative neoplasm and 28.4% in chronic myelomonocytic leukemia (CMML) . In MDS and CMML, point mutations mostly occur in exon 1 of the gene, at amino papain position P95 (95% and 93% of cases respectively). In the remaining minority of cases insertions/deletions and frameshifts can be found around amino acid P95 and also point mutations in other positions such as P96, P84, P85 and P86 have been reported. In MDS and CMML, mutated cases show higher transformation rate, shorter time to progression to acute myeloid leukemia and shorter overall survival compared to unmutated cases . mutations are thought to interfere with cellular homeostasis via defective splicing of certain genes such as (runt-related transcription factor 1 gene) with a loss-of-function effect seen in MDS and CMML . Recent publications have liked mutations with older male patients with MDS whose clinical course remains stable with no co-occurrence of other mutations .
The frequency of other splicing mutations in CLL besides such as has been studied recently and it is known that mutations in this gene are uncommon . In our study, we found one case of mutation in a patient with CLL with a very variable clinical course and many relapses. Little can be assumed in this case due to the lack of similar reports in literature, however it is possible that the mutation could have contributed to the worsening of her disease.
Another assumption that arises from this case is that the presence of myelodysplasia secondary to treatment could explain the presence of mutation in this patient. This brings us to think on many relapsed CLL cases with residual cytopenias where no other molecular abnormalities or disease markers can explain poor clinical outcomes.
Introduction Ikaros, a transcription factor encoded by IKZF1 gene, plays essential role in lymphocyte development . Alternative splicing regulates the function of Ikaros, resulting in the production of at least 13 different isoforms. Long isoforms (Ik1–3) are able to bind DNA and considered to be functional. Short isoforms (Ik4–10) are not efficient to bind DNA, and function in a dominant-negative (DN) manner [2,3]. Several studies have shown that focal IKZF1 deletions are frequent genetic alterations in acute lymphoblastic leukemia (ALL) [4–6]. All IKZF1 mutations in ALL are loss-of-function, resulting in null Ikaros, haplosufficient or DN phenotype . Focal IKZF1 deletions in ALL are supposed to be a reason for expression of short, non-DNA-binding Ikaros isoforms which switch their localization in the cell from nucleus to cytoplasm. IKZF1 deletions were first discovered as frequent genetic lesion in BCR-ABL1 positive ALL , proved to be a marker of poor prognosis in general ALL group [9,10] and BCR-ABL1 negative ALL . IKZF1 deletions are different from most oncogenes that may occur in leukemic subclones [12,13].
Discussion Generally, three different intragenic IKZF1 deletions were found in the patient by PCR: ΔEx3–6d and ΔEx1–6 at diagnosis and ∆Ex3-Ex6r at relapse. Ex3-Ex6r deletion was also revealed in minor leukemic subclone at diagnosis by means of RQ-PCR with patient-specific primer at a lower level (0.087). FISH analysis indicated all deletions as heterozygous. These results can be explained by the presence of three clones of leukemic cells, differ in IKZF1 genotype. Two major clones caring ΔEx3–6d and ΔEx1–6 finally were eliminated during treatment, however minor ∆Ex3-Ex6r clone retained and led to early relapse. This case clearly demonstrated that clonal heterogeneity by IKZF1 gene status and clonal selection during the treatment took place. It is important to note that leukemic cells at diagnosis and relapse were positive for the same IgH and TCRG rearrangements, which proves that all leukemic clones were of common origin, and IZKF1 deletions are not initial, but secondary events. IKZF1 deletion is definitely bound to disease progression, as far as large majority of the cells were IKZF1 deleted at diagnosis, and 26 months later at relapse. So, this or that deletion required for retaining leukemic phenotype and progression. It might appear that mutant ΔEx3-Ex6r IKZF1 deletion minor at the diagnosis itself provide to the cells sufficient proliferative or drug resistance advantage. However, to our mind, Ex3-Ex6r IKZF1 deletion itself does not provide more aggressive phenotype, than Ex3-Ex6d or Ex1-Ex6 deletions, because all of them resulting in the expression of short Ikaros isoforms with identical dominant-negative phenotype. We assume that minor ∆Ex3-Ex6r clone acquired additional genetic lesion during the treatment.