Heterochromatinization has been implicated in fundamental biological and
pathological processes including differentiation, senescence, ageing and tumourigenesis; however, little is known about its regulation and roles in human cells and tissues in vivo. Here, we show distinct cell-type- and
cancer-stage-associated patterns of key
heterochromatin marks: histone H3 trimethylated at
lysine 9 (H3K9me3) and heterochromatic adaptor
proteins HP1α and HP1γ, compared with the γH2AX marker of endogenously activated DNA damage response (DDR) and proliferation markers in normal human foetal (n=4) and adult (n=29) testes, pre-invasive
carcinoma in situ (CIS; n=26) lesions and a series of overt germ cell tumours, including
seminomas (n=26),
embryonal carcinomas (n=18) and
teratomas (n=11). Among striking findings were high levels of HP1γ in foetal gonocytes, CIS and
seminomas; enhanced multimarker heterochromatinization without DDR activation in CIS; and enhanced HP1α in
teratoma structures with epithelial and neuronal differentiation. Differential expression of the three
heterochromatin markers suggests their partly non-overlapping roles, and separation of heterochromatinization from DDR activation highlights distinct responses of germ cells vs. somatic tissues in early tumourigenesis. Conceptually interesting findings were that subsets of human cells in vivo proliferate despite enhanced heterochromatinization, and that cells can strongly express even multiple
heterochromatin features in the absence of functional
retinoblastoma protein and without DDR activation. Overall, these results provide novel insights into cell-related and tumour-related diversity of
heterochromatin in human tissues in vivo, relevant for andrology and intrinsic anti-tumour defence roles attributed to activated DDR and cellular senescence.