G.P.156: Mutations in FAM111B cause Hereditary Fibrosing Poikiloderma with tendon contracture, myopathy and pulmonary fibrosis

Abstract

Hereditary Fibrosing Poikiloderma (HFP) with tendon contracture and pulmonary fibrosis has been described in a South-African family with autosomal dominant inheritance by Khumalo et al. in 2006. Recently, we confirmed the existence of this new syndrome by reporting four additional cases and identified the causative gene called FAM111B (NM_198947.3) by whole exome sequencing. Here we report the clinical and molecular data of seven independent patients: four males and three females (ages 3–38). Key features consist of: (i) congenital poikiloderma, hypotrichosis, hypohidrosis; (ii) muscle contractures in particular triceps surae and/or biceps brachii muscle contractures; (iii) progressive muscular weakness with a proximal and distal pattern and (iv) progressive pulmonary fibrosis. Muscle MRI showed extensive fatty infiltration confirmed by muscle biopsy. Histological examination of skeletal muscle revealed a partial loss of muscle tissue associated with extensive fibrofatty tissue infiltration regardless of age. There was no indication of denervation, necrosis, or inflammation. Microscopy of the skin showed a sclerodermiform aspect with fibrosis and alterations of the elastic network. FAM111B gene analysis identified five different missense variants (two mutations were shared respectively by three and two patients). All the mutations were predicted to localize in the trypsin-like cysteine/serine peptidase domain of the protein. We suggest gain of function or dominant negative mutations resulting in FAM111B enzymatic activity changes. Functional studies are ongoing to better understand the pathophysiology of this entity. In conclusion, HFP with tendon contracture, myopathy and pulmonary fibrosis, a phenotypically recognisable syndrome, is caused by autosomal dominant mutations in FAM111B gene. These findings provide genetic evidence for a new pathogenetic pathway for muscle impairment.