Genetic heterogeneity in human astrocytomas: Spatial distribution of P16 and TP53 deletions in biopsies

Smear preparations of 23 fresh astrocytoma biopsies were analyzed by two-co lor fluorescence in situ hybridization with cosmids specific for the P16 an d the TP53 genes. Additionally, tissue sections of the same tumors were imm unostained with the use of a monoclonal antibody that recognizes both wild- type and mutant TP53 protein. In 21 astrocytomas, loss of P16 was observed in a significant proportion of cells. Cells with homozygous P16 loss were p resent in 23 astrocytomas; 14 astrocytomas showed cells with heterozygous l oss of P16. Remarkably, 5 astrocytomas showed a scattered mosaic pattern of cells with homozygous and, respectively, heterozygous p16 loss. Homozygous deletion of TP53 was not observed. Cells with heterozygous TP53 loss were defected in 12 tumors, in 7 of them in association with P16 loss. One tumor showed aberrant cells for neither TP53 nor P16 but strong immunostaining f or TP53. Positive TP53 immunostaining was found in 16 astrocytomas. Heteroz ygous loss of TP53 tvas significantly correlated with TP53 protein expressi on. We conclude that, unlike typical tumor suppressor genes, P16 might enha nce cellular proliferation after heterozygous loss through a dosage effect and that the distribution of cells with homozygous loss of P16 speaks in fa vor of a polyclonal loss of the second copy of this gene. (C) Elsevier Scie nce Inc., 1999. All rights reserved.