Numerical Aberrations Involving The X Chromosome As a New Recurrent Aberration In Patients With Chronic Lymphocytic Leukemia

Genomic aberrations in CLL are important independent predictors of disease progression and survival in chronic lymphocytic leukemia (CLL). The most frequent changes are a deletion in 13q with a median survival of 133 months, a deletion in 11q (median survival 79 months), trisomy of chromosome 12 (114 months) and a deletion in 17p (32 months) (Döhner et al., NEJM, 2000; Hallek et al., Lancet, 2010).

Between 2001 and 2012 we performed cytogenetic analyses of 2360 patients with CLL. We detected clonal chromosome aberrations in 1298/2360 (55%). Besides typical recurrent aberrations associated with CLL, we observed in 72/2360 (3%) clonal numerical aberrations involving the X chromosome. In 50/72 patients (69%) a loss of chromosome X and in 22/72 patients (31%) a gain of an X chromosome was detected.

So far, numerical alterations of the X chromosome have not been described as recurrent aberrations detected in CLL. In order to understand their significance, we analysed those cases in greater detail.

Median age of patients was 69 years (33-89 years) and the sex distribution was 64 female and 8 male (ratio 8:1). The patient cohort with the loss of the X chromosome consisted of 48 female and two male patients between 35 and 89 years of age (median age 69 years). In 52% (26/50) loss of chromosome X was detected as a sole abnormality and 22% (11/50) showed the loss of X within a complex karyotype. There was an association between loss of the X chromosome and trisomy 12 in 10% (5/50) and with deletion in 13q in 22% (11/50). In two cases, a clonal evolution was observed with the loss of the X chromosome in the main clone. Four independent clones (8%) were identified, three with trisomy 12 (6%) and one with a deletion in 11q (2%).

Further, we analysed cases with a gain of the X chromosome (22 patients with CLL (16 female,6 male) between 33 and 81 years of age (median age 63 years)). In 15/22 patients (68%) a gain of the X chromosome was identified as a sole abnormality and in 6 of 22 within a complex aberrant karyotype. In 3/22 patients (14%) with an isolated gain of the X chromosome, we detected an independent clone. The independent clone was a trisomy 12 (two patients) and a monosomy X (one patient). In two cases, a clonal evolution was identified with gain of chromosome X in the subclone as a secondary event.

Interestingly, the clone size was rather small. The median size of clones with loss of the X chromosome was 6/20 metaphases (30%) and with gain of the X chromosome was 4/20 metaphases (20%). In 8 patients, a loss of chromosome X could be detected in two metaphases only or a gain in one metaphase only, not fulfilling the criteria for clonality. In these cases, fluorescence in situ hybridization never confirmed the presence of a clone with X chromosome abnormalities.

Even though a monosomy X has been reported as a recurrent somatic sole abnormality in MDS, it is not a characteristic aberration specific for any subtype of leukemia, lymphoma or a solid tumor (Abruzzese et al., Cancer Genet Cytogenet, 1997). The constitutional loss of the X chromosome is associated with Turner syndrome, a genetic disorder affecting 1/2500-3000 live-born. Notably, in a mosaic Turner syndrome patient with CLL, abnormal clones were restricted to the monosomic cells, indicating that loss of the X chromosome provides an advantage for leukemia cells (Manola et al., Leuk Res., 2008). However, CLL is not associated with Turner syndrome.

A gain of chromosome X was observed by Aamot et al. (J Cancer Res Clin Oncol., 2007) in 17% of patients with follicular lymphomas as a secondary aberration with the characteristic translocation t(14;18) not affecting the overall survival. This is in accordance with our findings that a gain of the X chromosome could be a characteristic aberration in lymphatic neoplasms. Since we detected the aberrations involving chromosome X mostly in CLL, a constitutional mosaicism is unlikely. Otherwise, a constitutional mosaic would possibly be a predisposing factor for CLL. Putative candidate genes on chromosome X are not known. BRWD3, located on Xq13, encodes a bromodomain and WD repeat domain-containing protein and could be a putative novel transcription factor, since it is involved in translocations and is slightly down-regulated in CLL patients (Kalla et al., Genes Chromosomes Cancer, 2005).

In summary, we identified numerical aberrations of the X chromosomes as new recurrent aberrations in CLL. The impact is still unknown and has to be analysed in further studies.