Clonality and karyotype studies in polycythemia vera

The molecular determination of the monoclonal or non-clonal nature of polycythemia cases is now possible in all or almost all females by study of the methylation pattern of several polymorphic X-linked loci. However, the techniques used are sophisticated and costly and the results do not clarify all cases. Karyotypic studies of PV, despite the number of cases reported thus far, remain insufficient. The natural history of cytogenetic events and their mechanisms of occurrence are still poorly known. Unsuccessful examinations are around 10-15%. The proportion of patients with clonal anomalies, 10-15% at diagnosis, increases with time, myelo-ablative treatments and evolution to myelofibrosis/myeloid metaplasia, so as to be close to 100% in cases with myelodysplasia/acute transformation. A few of the most frequently found non-random anomalies among which in particular trisomy 8 and 9, double trisomy 8+9, 20q- and trisomy of part or totality of 1q have some degree of specificity. Other recurring aberrations, such as 13q- mainly occur in late stages of PV, in which complex, unstable karyotypes are found. Finally none of those anomalies can be considered as a primary lesion. The frequency of clonal evolution is a matter of discussion. Despite their theoretical interest, PV cytogenetic results are of little diagnostic value, and as regards prognosis, they are statistically significant, but are of poor value in most individual patients. In conclusion, it is felt that clonality and karyotype studies should be pursued in PV, completed by "interphase cytogenetics" and molecular biology investigations, with theoretical aims rather than for immediate practical reasons.