COL4A4 gene study of a European population: description of new mutations causing autosomal dominant Alport syndrome

Abstract

Background: Autosomal forms of Alport syndrome represent 20% of all patients (15% recessive and 5% dominant). They are caused by mutations in the COL4A3 and COL4A4 genes, which encode a-3 and a-4 collagen IV chains of the glomerular basement membrane, cochlea and eye. Thin basement membrane nephropathy may affect up to 1% of the population. The pattern of inheritance in the 40% of cases is the same as autosomal dominant Alport syndrome: heterozygous mutations in these genes. The aim of this study is to detect new pathogenic mutations in the COL4A4 gene in the patients previously diagnosed with autosomal Alport syndrome and thin basement membrane nephropathy in our hospital.

Methods: We conducted a clinical and genetic study in eleven patients belonging to six unrelated families with aforementioned clinical symptoms and a negative study of COL4A3 gene. The molecular study was made by conformation of sensitive gel electrophoresis (CSGE) and direct sequencing of the fragments that show an altered electrophoretic migration pattern.

Results: We found two pathogenic mutations, not yet described: IVS3 + 1G > C is a replacement of Guanine to Cytosine in position +1 of intron 3, in the splicing region, which leads to a pathogenic mutation. c.4267C > T; p.P1423S is a missense mutation, also considered pathogenic. We also found seven new polymorphisms.

Conclusions: We describe two new pathogenic mutations, responsible for autosomal dominant Alport syndrome. The other families of the study were undiagnosed owing to problems in the method employed and the possibility of mutations in other genes, giving rise to other diseases with similar symptoms.

Keywords: Autosomal alport syndrome; COL4A4 gene; missense; spicing.

Figure 1

Genealogic trees of the studied families. Family 2 shows an autosomal recessive pattern of inheritance. The rest of them have an autosomal dominant one.

Figure 2

Genetic sequence of the c.4267C > T; p.P1423S mutation and the polypheny multiple sequence alignment, in which we can see the high degree of conservation of proline among different species. This fact leads us to think that the change of proline to serine could have a critical effect on the resulting protein.

Figure 3

Genetic sequence of the IVS3 + 1G > C mutation and possible changes in the resultant protein: A. Initial sequence of the protein COL4A4 in normal conditions. B. COL4A4 protein sequence without exon 3. C. COL4A4 protein secuence with intron 3.