Clonal origins of human breast cancer

Tumours are usually considered as the clonal progeny of single transformed cells. An X-chromosome inactivation assay has been applied to exploring clo nal relationships in human breast cancer. Analysis of X-inactivation in DNA extracted from microdissected in situ and invasive breast carcinoma by Hpa It restriction and polymerase chain reaction (PCR) of the androgen recepto r exon I CAG polymorphism confirmed monoclonality in 105/133 samples of car cinoma cells from 31/32 informative breast cancers. Clonality was identical in seven cases between in situ and invasive carcinoma. Unexpectedly, 4 of 12 cancers (33%) with two or more monoclonal samples available were mosaic (polyclonal) in respect of X-chromosome inactivation between separate morph ologically homogeneous tumour cell samples. Concordant clonality supports a common clonal origin of in situ and invasive breast cancers, but frequent apparently mosaic X-inactivation in breast cancer cannot be explained by no n-tumour cell contamination. It is concluded that these carcinomas may be g enuinely multiclonal. Possible mechanisms of multiclonality include simulta neous transformation of cell groups straddling X-chromosome inactivation pa tch boundaries, tumour-initiating mutations prior to X-inactivation, or rec ruitment of bystander stem cells by DNA transfer from necrotic or apoptotic tumour cells. Collision of independent cancers appears implausible at this frequency. Further studies using independent analytical techniques are req uired to test the important possibility that a significant proportion of ma mmary carcinomas are not monoclonal. Copyright (C) 2001 John Wiley & Sons, Ltd.