Trop-2 is a calcium signal transducer that is associated with transformed cell growth in experimental systems. However, its role in human cancer remains essentially unknown. In this study, we profiled Trop-2 expression in normal human tissues at the mRNA and protein levels. We then systematically compared Trop-2 mRNA and protein levels in tumours with their tissues of origin. We find that Trop-2 expression is invariably upregulated in tumours, regardless of baseline expression in normal tissues, which suggests a corresponding selective advantage. Thus, we investigated the outcome of Trop-2 upregulation on tumour growth. Overexpression of wild-type Trop-2 was shown to be necessary and sufficient to drive cancer growth in a widely invariant manner across cell type and species. Upregulation of Trop-2 was shown to quantitatively stimulate tumour growth, as proportional to expression levels in vivo, and tumour cell growth was abrogated by somatic knockdown of Trop-2 expression. On the other hand, we found no evidence of tumour-associated TROP2 mutations, nor of TROP2 induction of oncogenic transformation per se. Our data support a model where above-baseline expression of wild-type Trop-2 is a key driver of human cancer growth.

Genome-wide association studies of complex physiological traits and diseases consistently found that associated genetic factors, such as allelic polymorphisms or DNA mutations, only explained a minority of the expected heritable fraction. This discrepancy is known as “missing heritability”, and its underlying factors and molecular mechanisms are not established. Epigenetic programs may account for a significant fraction of the “missing heritability.” Epigenetic modifications, such as DNA methylation and chromatin assembly states, reflect the high plasticity of the genome and contribute to stably alter gene expression without modifying genomic DNA sequences. Consistent components of complex traits, such as those linked to human stature/height, fertility, and food metabolism or to hereditary defects, have been shown to respond to environmental or nutritional condition and to be epigenetically inherited. The knowledge acquired from epigenetic genome reprogramming during development, stem cell differentiation/de-differentiation, and model organisms is today shedding light on the mechanisms of (a) mitotic inheritance of epigenetic traits from cell to cell, (b) meiotic epigenetic inheritance from generation to generation, and (c) true transgenerational inheritance. Such mechanisms have been shown to include incomplete erasure of DNA methylation, parental effects, transmission of distinct RNA types (mRNA, non-coding RNA, miRNA, siRNA, piRNA), and persistence of subsets of histone marks.

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