Immunohistochemical demonstration of histone H10 in human breast carcinoma

Histone H10 is a linker histone subvariant present in tissues of low proliferation rate. It is supposed to participate in the expression and maintenance of the terminal differentiation phenotype. The aim of this work was to study histone H10 distribution in human breast carcinoma and its relationship with the processes of proliferation and differentiation. Most of the cells in carcinomas of moderate and high level of differentiation expressed histone H10 including cells invading connective and adipose tissues. In low differentiated tumours, the number of H10 expressing cells was considerably lower. Staining of myoepithelial cells, when seen, and of stromal fibroblasts was variable. The metastatic malignant cells in the lymph nodes also accumulated H10 but lymphocytes were always negative. All immunopositive malignant cells exhibited signs of polymorphism. Double H10/Ki-67 staining showed that the growth fraction in more differentiated tumours belonged to the H10-positive cells, while in poorly differentiated carcinomas it also included a cell subpopulation not expressing H10. If expressed, p27Kip1 was always found in H10-positive cells. These findings are inconsistent with the widespread view that histone H10 is expressed only in terminally differentiated cells. Rather, they suggest that the protein is expressed in cells in a prolonged intermitotic period irrespective of their level of differentiation. Double H10/Ki-67 immunostaining could be a useful tool in studying the growth fraction in tumours.