Mutations in the XPD subunit of the DNA repair/transcription factor TFIIH result in distinct clinical entities, including the cancerprone xeroderma pigmentosum (XP) and the multisystem disorder trichothiodystrophy (TTD), which share only cutaneous photosensitivity. Gene-expression profiles of primary dermal fibroblasts revealed overexpression of matrix metalloproteinase 1 (MMP-1), the gene encoding the metalloproteinase that degrades the interstitial collagens of the extracellular matrix (ECM), in TTD patients mutated in XPD compared with their healthy parents. The defect is observed in TTD and not in XP and is specific for fibroblasts, which are the main producers of dermal ECM. MMP-1 transcriptional up-regulation in TTD is caused by an erroneous signaling mediated by retinoic acid receptors on the MMP-1 promoter and leads to hypersecretion of active MMP-1 enzyme and degradation of collagen type I in the ECM of cell/tissue systems and TTD patient skin. In agreement with the well-known role of ECM in eliciting signaling events controlling cell behavior and tissue homeostasis, ECM alterations in TTD were shown to impact on the migration and wound-healing properties of patient dermal fibroblasts. The presence of a specific inhibitor of MMP activity was sufficient to restore normal cell migration, thus providing a potential approach for therapeutic strategies. This study highlights the relevance of ECM anomalies in TTD pathogenesis and in the phenotypic differences between TTD and XP.T he extracellular matrix (ECM) is a complex structural network that surrounds and supports cells within connective tissues. It generally includes three major types of macromolecules: fibrous proteins (collagens and elastic fibers), glycoproteins (such as fibronectins and laminins), and glycosaminoglycans/ proteoglycans. The type, amount and composition of the ECM give tissues their unique physical and biological properties (1). Notably, the ECM is not only a structural scaffold but also exhibits important functional roles in controlling key cellular events (e.g., adhesion, migration, proliferation, differentiation, and survival) involved in tissue homeostasis, development, inflammation, and tissue repair (2, 3). Thus, inherited or acquired structural alterations as well as metabolic disturbances of the ECM may cause cellular and tissue changes leading to the onset and progression of specific disorders, such as those affecting the connective tissues (osteogenesis imperfecta, Ehlers-Danlos syndrome, and Marfan's syndrome) or demanding ECM degradation (tumor invasion and metastasis) (4, 5). Recently, a reduced synthesis of the ECM component collagen type VI (COL6) was shown in confluent fibroblast cultures from patients with trichothiodystrophy [TTD; Online Mendelian Inheritance in Man (OMIM) #601675] (6), an autosomal recessive disorder characterized by symptoms affecting several tissues and organs (7). The most relevant features of TTD are hair abnormalities, ichthyotic skin, physical and mental retardation, neurodegeneration, and sign...