Clonal origin of abnormal granulocytes in paroxysmal nocturnal hemoglobinuria.

Two studies recently published in Blood have used the pattern of X-chromosome inactivation to test whether white blood cells of patients with paroxysmal nocturnal hemoglobinuria (PNH) and aplastic anemia (AA) belong to a single The investigators’ conclusions were in keeping with data previously obtained by the same approach on red blood cells? However, we find the interpretation of their findings rather difficult, because the PNH cells were not separated from the normal cells, which usually coexist in the patients’ peripheral blood. In our view, the finding of a homogeneous (or unbalanced) pattern of X-chromosome inactivation can be regarded as conclusive proof of the clonal origin of a certain cell population only when it is demonstrated in the context of a dimorphic (or balanced) pattern in normal cells from the same lineage. We have performed similar tests on granulocytes in five patients with PNH, but we have analyzed separately the normal and the PNH cells (see Table 1). All patients except patient 3 suffered from AA before the diagnosis of PNH. In patients 1 and 2, an unbalanced pattern is seen in both the PNH and the normal granulocytes. In patient 3 the PNH granulocytes show an unbalanced pattern, whereas normal granulocytes do not. In patients 4 and 5 the PNH granulocytes show an unbalanced pattern, and there are not enough normal granulocytes to analyze. However, in patient 4 analysis of lymphocytes (85% of which were normal) showed an unbalanced pattern; in patient 5 analysis of lymphocytes (60% of which were normal) showed a balanced pattern. Thus, for patients 3 and 5 we conclude that the PNH granulocytes are monoclonal, because the granulocytes in patient 3 and the lymphocytes in patient 5 serve as controls. For the other patients (who are similar to the majority of those in ref 1 and to some of those in ref 2), we cannot draw any unambiguous conclusion, because both the PNH and the “normal” granulocytes show an unbalanced pattern. Table 1. Proportion of Cells With the PNH-Phenotype and the Corresponding X-Chromosome Inactivation Pattern