Mutations in the TTDN1 gene are associated with a distinct trichothiodystrophy phenotype

Trichothiodystrophy (TTD) is a rare multisystem disorder, characterized by sulfur-deficient hair with alternating dark and light "tiger tail" banding on polarized light microscopy. TTD is caused by mutations in DNA repair/transcription genes XPD, XPB or TTDA, and in TTDN1, a gene of unknown function. Although most of the TTD patients are photosensitive, patients with TTDN1 mutations were reported to be nonphotosensitive. We followed a cohort of 36 TTD patients from 2001 to 2013. We describe five patients from four families with defects in the TTDN1 gene: four had no photosensitivity, and one patient exhibited cutaneous burning. Deep phenotyping of our cohort revealed differences between the patients with and without TTDN1 mutations. Delayed bone age and seizure disorders were overrepresented in the TTDN1 group (P=0.009 and P=0.024, respectively), whereas some characteristic TTD clinical, laboratory, and imaging findings were absent. The three oldest TTDN1 patients displayed autistic behaviors in contrast to the characteristic friendly, socially interactive personality in the other patients. DNA sequencing revealed deletion mutations in TTDN1 ranging in size from a single base pair to over 120 kb. These data identify a distinct phenotype relationship in TTD caused by TTDN1 mutations and suggest a different mechanism of disease.