Molecular analysis of lipoid proteinosis: identification of a novel nonsense mutation in the ECM1 gene in a Pakistani family

Lipoid proteinosis (LP; MIM 247100) also known as Urbach-Wiethe disease or hyalinosis cutis et mucosae, was first reported by Urbach and Wiethe, in 1929 [1]. It is a rare genetic disease, which is inherited in an autosomal recessive fashion. The disease occurs worldwide but is more common in certain geographical areas such as the Northern Cape province of South Africa, including Namaqualand. Clinical heterogeneity is reported in LP [2], although it usually presents in early childhood with hoarseness, caused by infiltration of the laryngeal mucosa [3]. Skin lesions or pox-like scars usually appear simultaneously or shortly afterwards. Other characteristic findings include the arrangement of 'beaded' waxy papules, known as moniliform blepharosis, which may be present along the margins of both eyelids [4, 5]. Histological and ultra structural examination has revealed the widespread deposition of hyaline-like material and disruption/reduplication of basement membrane around blood vessels and at the dermal-epidermal junction, mouth and upper respiratory tract, and other internal organs [2, 6]. Overproduction of normally expressed non-collagenous protein in the hyaline material has also been reported [7]. Tongue is often firm and its mobility may be limited. Other symptoms may include thickening of frenulum, scarring, warty skin papules, nail dystrophy, dental anomalies and some neuropsychiatric symptoms [8, 9]. Molecular genetic studies of LP linked the disorder to chromosome 1q21.1 [2]. The responsible gene was identified as ECM1, which encodes for the glycoprotein extracellular matrix protein 1. To date, several mutations in the ECM1 gene have been reported in unrelated LP families from different geographical areas. In this study we report a novel nonsense mutation in a consanguineous Pakistani family affected with lipoid proteinosis; and an update of ECM1 gene mutation data base.

The ECM1 gene comprises of 10 exons and encodes for the extracellular matrix protein 1. There are four splice variants including ECM1a, ECM1b and ECM1c, encoding proteins of 540, 415 and 559 amino acids, respectively. The recently described fourth splice variant comprises transcription of 71 bp at the 3' end of intron 1 and part of exon 2 to give a truncated 57 amino acid protein [10]. ECM1 is a negative regulator of endochondral bone formation, inhibiting alkaline phosphatase activity and mineralization [11]. It promotes angiogenesis, and shows over expression in certain malignancies. It interacts with a variety of extracellular and structural proteins, contributing to the maintenance of skin integrity and homeostasis [12‐14]. The expression studies have demonstrated presence of ECM1a within basal keratinocytes and ECM1b in suprabasal cells, suggesting a role for ECM1 in terminal keratinocyte differentiation [15].

Forty six mutations in the ECM1 gene have been described so far in unrelated patients affected with lipoid proteinosis (Table 1). These include 19 insertions/deletions, 15 nonsense, 8 missense and 4 splice site mutations. Both homozygous and compound heterozygous genotypes have been reported. About half of all mutations (22 of 46) are located within exon 6 or 7 (including adjacent splice sites). Therefore, sequencing of these two exons has been suggested as the initial step in efficiently determining the molecular pathology in new cases of LP. Furthermore, as previously reviewed by Chan et al 2007, there is no clear paradigm for genotype phenotype correlation, considering the type and position of the ECM1 gene mutations reported so far. Most of the mutations in the ECM1 gene are specific to individual families and only few are recurrent. In the present study, we identified a novel nonsense mutation (Q206X) in exon 6 of the ECM1 gene in a consanguineous Pakistani family. Five ECM1 mutations have been previously reported in unrelated Pakistani LP families (Table 1) and to our knowledge there is no report of recurrent mutation. The Q206X mutation is predicted to result in the increased degradation of both full-length ECM1a and ECM1b transcripts due to nonsense mediated mRNA decay mechanism. Exceptionally, ECM1 transcripts might be expressed stably leading to production of truncated ECM1 protein. Although, functional consequences of the premature termination codon mutations must be established by northern blotting or quantitative reverse transcriptase PCR, previous reports have not suggested any difference in phenotype related to different mutation genotypes. Premature stop codons in the last exon lead to the presence of ECM1 truncated protein because the non sense mediated mRNA decay is dependent on an upstream exon-exon junction [16]. However, such patients did not show a different or milder phenotype in comparison with cases with more upstream mutations [17, 18].

We have identified a novel nonsense mutation in exon 6 of the ECM1 gene in a Pakistani family extending the mutation spectrum of the gene. The study extends the body of evidence that supports the role of ECM 1 gene in the development of lipoid proteinosis. Identification of pathogenic mutations in the ECM1 gene should be helpful to improve genetic counseling and DNA based prenatal diagnosis.