X-Linked and Autosomal Recessive Alport Syndrome: Pathogenic Variant Features and Further Genotype-Phenotype Correlations

PLoS One. 2016 Sep 14;11(9):e0161802. doi: 10.1371/journal.pone.0161802. eCollection 2016.

Abstract

Alport syndrome results from mutations in the COL4A5 (X-linked) or COL4A3/COL4A4 (recessive) genes. This study examined 754 previously- unpublished variants in these genes from individuals referred for genetic testing in 12 accredited diagnostic laboratories worldwide, in addition to all published COL4A5, COL4A3 and COL4A4 variants in the LOVD databases. It also determined genotype-phenotype correlations for variants where clinical data were available. Individuals were referred for genetic testing where Alport syndrome was suspected clinically or on biopsy (renal failure, hearing loss, retinopathy, lamellated glomerular basement membrane), variant pathogenicity was assessed using currently-accepted criteria, and variants were examined for gene location, and age at renal failure onset. Results were compared using Fisher's exact test (DNA Stata). Altogether 754 new DNA variants were identified, an increase of 25%, predominantly in people of European background. Of the 1168 COL4A5 variants, 504 (43%) were missense mutations, 273 (23%) splicing variants, 73 (6%) nonsense mutations, 169 (14%) short deletions and 76 (7%) complex or large deletions. Only 135 of the 432 Gly residues in the collagenous sequence were substituted (31%), which means that fewer than 10% of all possible variants have been identified. Both missense and nonsense mutations in COL4A5 were not randomly distributed but more common at the 70 CpG sequences (p<10-41 and p<0.001 respectively). Gly>Ala substitutions were underrepresented in all three genes (p< 0.0001) probably because of an association with a milder phenotype. The average age at end-stage renal failure was the same for all mutations in COL4A5 (24.4 ±7.8 years), COL4A3 (23.3 ± 9.3) and COL4A4 (25.4 ± 10.3) (COL4A5 and COL4A3, p = 0.45; COL4A5 and COL4A4, p = 0.55; COL4A3 and COL4A4, p = 0.41). For COL4A5, renal failure occurred sooner with non-missense than missense variants (p<0.01). For the COL4A3 and COL4A4 genes, age at renal failure occurred sooner with two non-missense variants (p = 0.08, and p = 0.01 respectively). Thus DNA variant characteristics that predict age at renal failure appeared to be the same for all three Alport genes. Founder mutations (with the pathogenic variant in at least 5 apparently- unrelated individuals) were not necessarily associated with a milder phenotype. This study illustrates the benefits when routine diagnostic laboratories share and analyse their data.

MeSH terms

  • Adult
  • Age of Onset
  • Alternative Splicing / genetics
  • Autoantigens / genetics
  • Codon, Nonsense / genetics
  • Collagen Type IV / genetics
  • Female
  • Gene Deletion
  • Genetic Association Studies / statistics & numerical data
  • Humans
  • Male
  • Mutation, Missense / genetics
  • Nephritis, Hereditary / genetics
  • Nephritis, Hereditary / pathology*
  • Young Adult

Substances

  • Autoantigens
  • COL4A4 protein, human
  • COL4A5 protein, human
  • Codon, Nonsense
  • Collagen Type IV
  • type IV collagen alpha3 chain

Grant support

JS received seed funding to establish the Alport mutation databases from the US Alport Foundation and the Australian Alport Foundation ($USD 10,000 and $AUD 20,000 respectively). HIC, HGK and EP received funding from the Korean government through the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea ($USD142,000) (HI12C0014).The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.