Linear localized scleroderma (LLS) is a rare connective tissue disorder characterised by chronic inflammation and massive accumulation of collagen. This then results in both hardening and thickening of the lesion leading to the affected areas to cave in from atrophy. The sharply delimited and linear lesions, can affect patients in areas throughout the body and more rarely the face in the clinical subtypes en coup de sabre and Parry-Romberg syndrome. This leads to terrible disfigurement that cannot be concealed. The disease affects mostly children and is limited in treatment options, which are most often unsatisfactory. The incidence of LLS in adults and children is 2.7/100’000 population per year.

Very little is understood about the condition in terms of both genetic and clinical aetiology. There is increasing evidence that LLS might be based on genetic alterations in affected tissues. Blaschko’s lines are the patterns of cell migration and proliferation during embryological development. Multiple skin conditions have been shown to follow Blaschko’s lines including LLS. Several of these diseases have been demonstrated to be caused by genetic factors such as a de novo somatic mutation causing a cutaneous mosaicism. Here we tested the hypothesis that LLS is caused by a somatic genetic mutation.

Blood and affected skin taken from 19 histologically confirmed LLS patients was Whole Exome Sequenced (WES). Library preparation and hybridization was performed using the SureSelectXT Reagent kits from Agilent Technologies. Sequence analysis was performed as per GATKv3.5. Somatic mutations were called using 3 somatic callers. All SNPs and indels were analysed and no rare and damaging mutation was found in common. 5 patients had damaging germline SNP mutations in PRSS3, a gene involved in metabolism and the immune system. No minor allelic frequency data is available for any of the PRSS3 mutations found so comparison with internal controls has shown 2 of 5 to be rare and 3 of 5 were present in some controls.

An analysis of the 3 somatic callers revealed that an increase in read depth (DP) allowed for a more sensitive detection of mutations at low allelic fraction. To this end 4 patients with similar lesions were chosen for deep sequencing (DP of >300). However, somatic analysis of these samples revealed no suitable causative mutation. CGH was performed on 3 patients to find large scale chromosomal aberrations too large to be detected by WES. Samples were compared with 3000 control karyotypes. No rare aberrations were found in common between patients. A related work recently analysed three samples of LLS on the transcription level. No signal survived after correction for multiple testing. Taken together, our analysis revealed the absence of genetic mosaicism in LLS.

Absence of Somatic Mutations in Linear Localized Scleroderma