Clinical and molecular characterization of lipoid proteinosis in Namaqualand, South Africa

Lipoid proteinosis (LiP) is a rare autosomal recessive disorder characterized by a hoarse voice, warty skin infiltration and scarring. Mutations within the extracellular matrix protein 1 (ECM1) gene cause LiP. Since the early 1970s it has been recognized that South Africa has one of the largest groups of LiP patients worldwide, suggesting a probable founder effect. As LiP patients present with considerable clinical variability, this group of patients offers a unique opportunity for genotype-phenotype correlation.

To assess the clinical features and the molecular basis of LiP in patients from the Namaqualand area of the Northern Cape province of South Africa and to examine molecular evidence for a founder effect.

The LiP patient cohort consisted of 29 Coloured patients from Namaqualand and a further seven Caucasoid patients from other areas of South Africa. The control group included 100 healthy geographically and ethnically matched individuals from Namaqualand. Samples were collected after informed consent and with ethics committee approval from the University of the Witwatersrand. LiP patients were examined clinically and a structured recording sheet was completed. A brief neurological evaluation was also performed. The LiP founder effect was investigated at the molecular level by ECM1 mutation detection and haplotype analysis.

The most consistent clinical signs for a diagnosis of LiP in this group were a hoarse voice and thickened sublingual frenulum leading to restricted tongue movement. Homozygosity for a nonsense mutation in exon 7 of the ECM1 gene, Q276X, was identified in all patients (Coloured and Caucasoid). Despite this genetic homogeneity, considerable clinical variability in skin presentation and psychiatric involvement was observed. Haplotype analysis using markers from a 9.98-Mb region around the ECM1 locus confirmed the founder effect with a founder core haplotype, 19-Q276X-12 (ND1-ECM1-D1S2343), in all but four LiP-associated alleles (n = 58). A LiP carrier rate of 1 in 9 was observed among the 100 Namaqualand controls, predicting a LiP incidence of 1 in 324 in this community.

Although several consistent clinical features in LiP patients homozygous for the Q276X mutation in the ECM1 gene were observed, there remains considerable clinical variability. This suggests the action of genetic and environmental modifiers of disease severity. Strong molecular evidence supports a single founder effect for the high prevalence of LiP in South Africans, both Coloured and Caucasoid.