Clinical and Molecular Study of the Extracellular Matrix Protein 1 Gene in a Spanish Family with Lipoid Proteinosis

Lipoid proteinosis (LP; OMIM 247100), also known as Urbach-Wiethe disease, is a rare autosomal recessive disorder characterized by variable scarring and infiltration of the skin and mucosa. Clinical features include warty skin infiltration, papules on the eyelids, and skin scarring, in addition to extracutaneous abnormalities such as hoarseness of the voice, epilepsy, and neuropsychiatric abnormalities.1 In severe cases this infiltration can lead to respiratory obstruction. Histological examinations reveal the widespread deposition of hyaline material and disruption of the basement membrane around blood vessels and at the dermal-epidermal junction.

The molecular basis of LP has been elucidated: it appears to result from mutations in the gene encoding extracellular matrix protein 1 (Ecm1, ECM1; OMIM 602201).2 Ecm1 is a key protein in epidermal differentiation, binding of dermal collagens and proteoglycans, and regulation of angiogenesis. Homozygous and compound heterozygous mutations in ECM1 have previously been described in patients with LP. ECM1 maps to chromosome 1q21.2, and there are three known splice variants (Ecm1a, Ecm1b, and Ecm1c) and a putative fourth variant.2, 3 Ecm1a is the most widely expressed splice variant, while Ecm1b has a much more restricted expression pattern.4 Ecm1 contains numerous cysteine residues that are arranged in a specific manner. The cysteine-containing domains all have the typical CC-(X_7-10)-C arrangement that is capable of the forming protein double loops involved in protein-protein interactions.5 Therefore, Ecm1 and its alternative splicing isoforms may serve as binding and transport proteins in epidermal differentiation.6 Ecm1 was found to negatively regulate endochondral bone formation by inhibiting alkaline phosphatase activity and mineralization.7, 8

This report describes the case of an LP patient who carries a homozygous, undescribed nucleotide transition in exon 7 of ECM1 causing a homozygous nonsense mutation. Her clinical, histological, and molecular features indicate that the truncating mutation of exon 7 causes a severe phenotype.

Lipoid proteinosis was first described in 1929, but the current literature still contains only around 300 published cases. A Spanish patient carrier of a homozygous mutation (c.1076G>A) ECM1 has been described herein. A previous report5 described a different mutation in two Caucasian brothers (c.1077G>A). Nonetheless, the same stop codon, W359X, predicts a truncated protein in both cases. Reports on the genotype-phenotype relationship point to a milder phenotype in carriers of missense mutations in the Ecm1a isoform, whereas mutations in the Ecm1b isoform (i.e., with exon-7 skipping) have been associated with more severe phenotypes.9 This is in agreement with the present finding in a Spanish patient carrying a truncating mutation in exon 7 and exhibiting a complete dermatological and neurological manifestation. It is difficult to study the genotype-phenotype correlation because of the different ages of patients, since many clinical features vary with age. The patient studied herein reported spontaneous hemorrhagic blisters under both humid and very dry environmental conditions, which suggests that both genetic and environmental factors can modify the disease severity. Therefore, genotype-phenotype differences among reports can be at least partially attributed to age and nongenetic factors that differentially affect the development of neurologic symptoms and the severity of this disease.

Extracellular matrix protein 1 has specific binding sites for several extracellular matrix proteins and polysaccharides: 1) the NH2-terminal free cysteine domain can bind collagen IV [amino acids (aa) 14-207]; 2) the tandem repeat can bind fibulin 1C/1D, matrix metalloproteinase 9 (MMP-9), and laminin (aa 207-361); 3) the C-terminal region can interact with domain V of perlecan (aa 424-540); and 4) Ecm1 promotes the link between collagen IV and laminin.6, 10-12 Moreover, Sercu et al.12 suggested that Ecm1 can act as a "biological glue" in the skin's basement membrane to maintain the structural and functional integrity of this organ. If translated, the stop codon described in the present Spanish patient predicts a truncated protein that would impair the interaction with perlecan, MMP-9, fibulin, and laminin. Given the lack of symptoms in the carrier parents, haploinsufficiency or toxic effects could be discarded as pathogenic mechanisms. Extracutaneous manifestations such as epilepsy and neuropsychiatric disorders13, 14 can be related to the inhibition by Ecm1 of MMP-9 activity,10 a protein that is strongly expressed in the brain.15 The truncated 358 polypeptide lacks regulatory MMP-9 activity and the aforementioned binding domains to several other matrix components, and therefore causes impairment of the extracellular matrix structure. The histological images obtained in this study clearly showed material deposition around sweat coils and capillaries.

A new mutation in exon 7 of ECM1 is described herein that leads to a premature termination codon as the cause of a severe phenotype of LP. The lack of coincidence in the parents' haplotypes is noteworthy, although a recombination among the microsatellite markers cannot be ruled out. In conclusion, the identification of pathogenic mutations in ECM1 is significant for genetic counseling with respect to clinical severity and prognosis.