GDNF and pachyonychia congenita: MMP-9, positively correlated to nerve growth factor (NGF) and glial cell-derived neurotrophic factor (GDNF) in PC patients with lymph-vascular, extra-prostatic and perineuronal cell invasion, suggesting that the interactions between NGF, GDNF and MMP-9 during the transition to malignancy may play a role in PC aggressiveness [32].