Change in cellular phenotype from lymphoid to erythroid in a case of ALL

Acute leukaemia was diagnosed in a 3-month-old boy. Initial blood leucocyte count was 97 x 10(9)/1, 92% of the cells were morphologically small lymphoblasts which according to surface marker analysis were of the common ALL (non-T non-B) type. Remission was achieved with initial treatment. During relapse 5 months later a morphologically different leukaemic cell appeared in the bone marrow, blood and liquor. This cell type which dominated during the terminal stage of the disease was larger and had an abundant bosophilic cytoplasm which contained PAS-positive granulae. Sudan, myeloperoxidase and benzidine staining gave negative results. Neither T nor B lymphocyte markers were seen but a strong surface expression of glycophorin A was found by indirect immunofluorescence with specific rabbit anti-glycophorin A antiserum. The cells showed a unique surface glycoprotein pattern. Bone marrow karyotype analysis gave in about 80%: 47,XY,+8,t(4:11) indicating that this blast cell represented a malignant clone. As glycophorin A is expressed exclusively on erythroid cells and their precursors, this finding indicates a change from a 'lymphoid' to an erythroid phenotype of the leukaemic cells during the course of the disease.