Mutations in the codon for a conserved arginine-1563 in the COL4A5 collagen gene in Alport syndrome

Aberrant splicing caused by exonic single nucleotide variants positioned 2nd or 3rd to the last nucleotide in the COL4A5 gene

BACKGROUND AND OBJECTIVES

The evident genotype-phenotype correlation shown by the X-linked Alport syndrome warrants the assessment of the impact of identified gene variants on aberrant splicing. We previously reported that single nucleotide variants (SNVs) in the last nucleotide of exons in COL4A5 cause aberrant splicing. It is known that the nucleotides located 2nd and 3rd to the last nucleotides of exons can also play an essential role in the first step of the splicing process. In this study, we aimed to investigate whether SNVs positioned 2nd or 3rd to the last nucleotide of exons in COL4A5 resulted in aberrant splicing.

METHODS

We selected eight candidate variants: six from the Human Gene Variant Database Professional and two from our cohort. We performed an in-vitro splicing assay and reverse transcription-polymerase chain reaction (RT-PCR) for messenger RNA obtained from patients, if available.

RESULTS

The candidate variants were initially classified into the following groups: three nonsense, two missense, and three synonymous variants. Splicing assays and RT-PCR for messenger RNA revealed that six of the eight variants caused aberrant splicing. Four variants, initially classified as non-truncating variants, were found to be truncating ones, which usually show relatively more severe phenotypes.

CONCLUSION

We revealed that exonic SNVs positioned 2nd or 3rd to the last nucleotide of exons in the COL4A5 were responsible for aberrant splicing. The results of our study suggest that attention should be paid when interpreting the pathogenicity of exonic SNVs near the 5' splice site.

Last Nucleotide Substitutions of COL4A5 Exons Cause Aberrant Splicing

Introduction

COL4A5 is a causative gene of X-linked Alport syndrome (XLAS). Male patients with XLAS with nonsense variants have the most severe phenotypes of early onset end-stage kidney disease (ESKD); those with splicing variants have middle phenotypes and those with missense variants have the mildest phenotypes. Therefore, genotyping for male patients with XLAS can be used to predict kidney prognosis. Single-base substitutions at the last nucleotide position in each exon are known to affect splicing patterns and could be splicing variants. Nevertheless, in XLAS, these variants are generally considered to be missense variants, without conducting a transcript analysis, which underestimates some patients as having mild phenotypes. This study aimed to investigate whether single-base substitutions at the last nucleotide position of COL4A5 exons cause aberrant splicing.

Methods

In total, 20 variants were found in the Human Gene Mutation Database (n = 14) and our cohort (n = 6). We performed functional splicing assays using a hybrid minigene analysis and in vivo transcript analyses of patients’ samples when available. Then, we investigated genotype–phenotype correlations for patients with splicing variants detected in this study by comparing data from our previous studies.

Results

Among the 20 variants, 17 (85%) caused aberrant splicing. Male patients with splicing variants had more severe phenotypes when compared with those with missense variants. Findings from the in vivo analyses for 3 variants were identical to those from the minigene assay.

Conclusion

Our study revealed that most single-base substitutions at the last nucleotide position of COL4A5 exons result in splicing variants, rather than missense variants, thereby leading to more severe phenotypes.

Novel mutations of COL4A3, COL4A4, and COL4A5 genes in Chinese patients with Alport Syndrome using next generation sequence technique

Background

Alport syndrome (AS) is an inherited progressive renal disease caused by mutations in COL4A3, COL4A4, and COL4A5 genes. The large sizes of these genes and the absence of mutation hot spots have complicated mutational analysis by routine PCR‐based approaches. In recent years, the development of next‐generation sequencing (NGS) has made possible the time‐ and cost‐effective and accurate analysis of the three genes in a single step.

Methods

Here, we analyze COL4A3, COL4A4, and COL4A5 simultaneously in 29 AS patients using NGS. Candidate mutations were validated by classic Sanger sequencing and Real‐time PCR.

Results

Twenty two new mutations and 10 known mutations were detected. Of those novel mutations, 18, 3, and 1 mutations were detected in COL4A5, COL4A4, and COL4A3, respectively. Twenty six patients showed X‐linked inheritance, one showed autosomal recessive inheritance and two showed digenic inheritance (DI).

Conclusion

A comparison of the clinical manifestations caused by different types of mutations in COL4A5 suggested that large fragment mutations are relatively more severe than the other missense mutations and AS by some mutations may show inter‐ and intra‐familial phenotypic variability. It is important to consider these transmission patterns in the clinical evaluation according to the results of genetic testing, especially for DI. Twenty two new mutations can expand the genotypic spectrum of AS.

De novo mutations in COL4A5 identified by whole exome sequencing in 2 girls with Alport syndrome in Korea

Alport syndrome (ATS) is an inherited glomerular disease caused by mutations in one of the type IV collagen novel chains (α3, α4, and α5). ATS is characterized by persistent microscopic hematuria that starts during infancy, eventually leading to either progressive nephritis or end-stage renal disease. There are 3 known genetic forms of ATS, namely X-linked ATS, autosomal recessive ATS, and autosomal dominant ATS. About 80% of patients with ATS have X-linked ATS, which is caused by mutations in the type IV collagen α5 chain gene, COL4A5. Although an 80% mutation detection rate is observed in men with X-linked ATS, some difficulties do exist in the genetic diagnosis of ATS. Most mutations are point mutations without hotspots in the COL4A3, COL4A4, and COL4A5 genes. Further, there are insufficient data on the detection of COL4A3 and COL4A4 mutations for their comparison between patients with autosomal recessive or dominant ATS. Therefore, diagnosis of ATS in female patients with no apparent family history can be challenging. Therefore, in this study, we used whole-exome sequencing (WES) to identify mutations in type IV collagen in 2 girls with glomerular basement membrane structural changes suspected to be associated with ATS; these patients had no relevant family history. Our results revealed de novo c.4688G>A (p.Arg1563Gln) and c.2714G>A (p.Gly905Asp) mutations in COL4A5. Therefore, we suggest that WES is an effective approach to obtain genetic information in ATS, particularly in female patients without a relevant family history, to detect unexpected DNA variations.

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

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.

Two families with novel missense mutations in COL4A1: When diagnosis can be missed

Mutations in COL4A1, encoding one of the six collagen type IV proteins, cover a wide spectrum of autosomal dominant overlapping phenotypes including porencephaly, small-vessel disease and hemorrhagic stroke, leukoencephalopathy, hereditary angiopathy with nephropathy, aneurysms and muscle cramp (HANAC) syndrome, and Walker-Warburg syndrome. Over 50 mutations are known, mainly being missense changes. Intra- and inter-familial variability has been reported. We studied two Italian families in which the proband had a clinical diagnosis of COL4A1-related disorder. We found two novel mutations (c.1249G>C; p.Gly417Arg and c.2662G>C; p.Gly888Arg). Both involved highly conserved amino acids and were predicted as being deleterious by bioinformatics tools. The c.1249G>C (p.Gly417Arg) segregated in four subjects with variable neurological phenotypes, namely leukoencephalopathy with muscle symptoms, brain small-vessel disease, and mild infantile encephalopathy. A fourth case was a carrier of the mutation without any neurological symptoms and an MRI with a specific white matter anomaly. The c.2662G>C (p.Gly888Arg) mutation was de novo in the proband. After a temporary motor impairment at age 14, the subject complained of mild imbalance at age 30, during the third trimester of her twin pregnancy, when an anomaly of the left brain hemisphere was documented in one fetus. Both her male dizygotic twins presented a severe motor delay, early convulsions, and leukoencephalopathy, and were carriers of the mutation. In summary, we confirm that high intra-familial variability of COL4A1 mutations with very mild phenotypes, the apparent incomplete penetrance, and de novo changes may become a "dilemma" for clinicians and genetic counselors.

Alport syndrome and leiomyomatosis: the first deletion extending beyond COL4A6 intron 2

Alport syndrome (ATS) is a nephropathy characterized by the association of progressive hematuric nephritis with ultrastructural changes of the glomerular basement membrane (thinning, thickening, and splitting), sensorineural deafness, and variable ocular abnormalities (anterior lenticonus, macular flecks, and cataracts). The most common mode of transmission is X-linked inheritance, due to COL4A5 mutations. X-linked ATS is rarely associated with diffuse leiomyomatosis (DL), a benign hypertrophy of the visceral smooth muscle in gastrointestinal, respiratory, and female reproductive tracts. The ATS-DL complex is due to deletions that encompass the 5' ends of the COL4A5 and COL4A6 genes and include the bidirectional promoter. In this paper, we described 3 ATS-DL cases, 2 familial and 1 sporadic bearing a deletion encompassing the 5'-end of both the COL4A5 and COL4A6 genes, as identified by multiplex ligation-dependent probe amplification (MLPA) analysis. The array-CGH technique allowed a better definition of deletion size, confirming that the proximal breakpoint was within COL4A6 intron 2 in 2 cases. Surprisingly, 1 case had a deletion extending proximally beyond exon 3 of COL4A6, as confirmed by qPCR analysis. This is the largest deletion reported to date that has been associated with ATS-DL and this case should lead us to reconsider the mechanisms that might be involved in the development of diffuse leiomyomatosis.

Mutational analysis of type IV collagen alpha5 chain, with respect to heterotrimer formation

Alport syndrome (AS) is caused by mutations in type IV collagen alpha3, alpha4, and alpha5 chains. The three chains form a heterotrimer. In this study, we introduced 12 kinds of missense and three kinds of nonsense mutations, corresponding to AS mutations, into the NC1 domain of alpha5(IV) and characterized the mutant chains. Nine alpha5(IV) chains with amino acid substitutions and all three truncated alpha5(IV) chains did not form a heterotrimer and were not secreted from cells. Three alpha5(IV) chains with amino acid substitutions did, however, form heterotrimers in cells, but these were not secreted from cells. These findings indicate that a defect in heterotrimer formation is the main molecular mechanism underlying the pathogenesis of AS caused by mutation in the NC1 domain. We also showed that even a single amino acid deletion in the carboxyl-terminal region markedly affected the heterotrimerization, indicating that the carboxyl-terminal end is indispensable for heterotrimer formation.

Genetic testing for X-linked Alport syndrome by direct sequencing of COL4A5 cDNA from hair root RNA samples

BACKGROUND

Alport syndrome (AS) is a genetically heterogeneous hereditary renal disease. X-Linked AS (XLAS) is responsible for 80% to 85% of familial cases and is caused by mutations in the COL4A5 collagen gene. To date, indirect molecular diagnosis for XLAS is not well defined, and mutation screening of the COL4A5 gene is time consuming and complicated because of its large size and high allelic heterogeneity. Our aim is to facilitate XLAS genetic testing.

METHODS

For linkage analysis, we tested the applicability of 4 microsatellite markers defining a 1.2-megabase region flanking the COL4A5 gene. For mutation screening of the COL4A5 gene, we describe a new strategy based on direct sequencing of hair root COL4A5 messenger RNA (mRNA).

RESULTS

Three microsatellite markers proved accurate (DXS1120, DXS6802, and DXS1210) and 1 was discarded (DXS6797) because it was difficult to interpret. The mutation screening method provides results in 4 days, and when applied to 29 patients suspected of having XLAS, it identified mutations in 76% (22 of 29 patients). This study correlates COL4A5 mutations with effects at the mRNA level and suggests that mutations affecting mRNA splicing of the COL4A5 gene (41%; 9 of 22 patients) are more common than previously described. Many splicing mutations did not alter the canonical 5' and 3' splice sites.

CONCLUSIONS

A more reliable linkage analysis and a simple, fast, and efficient mutation screening are now available for the genetic testing of patients with XLAS.

Male-to-male transmission of X-linked Alport syndrome in a boy with a 47,XXY karyotype

Alport syndrome (AS) is a genetically heterogeneous renal hereditary disease. Male-to-male transmission has been considered fully indicative of autosomal dominant AS. We report a family with male-to-male transmission of X-linked AS due to an extra X chromosome of paternal origin in the proband. Linkage analysis excluded the autosomal loci and demonstrated segregation with the COL4A5 locus (Xq22.3). Sperm FISH analysis from his father detected an increased XY disomy. Mutation screening of the COL4A5 gene identified a splicing mutation, c.4688G>A. The proband and his paternal grandmother showed random X chromosome inactivation. However, a preferential expression of the aberrantly spliced transcript was detected in the proband when compared to his grandmother. This finding could explain why the AS phenotype of this 47,XXY boy resembles more an affected male than a female carrier. This is the first reported case of concurrence of Alport and Klinefelter syndromes.

Detection of mutations in the COL4A5 gene by SSCP in X-linked Alport syndrome

Alport syndrome is a progressive renal disease leading to chronic renal failure, which often is accompanied by sensorineural deafness and ophthalmological signs in the form of anterior lenticonus. The X-linked form of the disease is caused by mutations in the COL4A5 gene encoding the alpha5-chain of type IV-collagen. We performed mutation analysis of the COL4A5 gene by PCR-SSCP analysis of each of the 51 exons with flanking intronic sequences in 81 patients suspected of X-linked Alport syndrome including 29 clear X-linked cases, 37 cases from families with a pedigree compatible with X-linked inheritance, and 15 isolated cases. We found a mutation detection rate of 52% (42/81) (58% in males and 21% in females), and 69% (20/29) in families who clearly demonstrated X-linked inheritance. Thirty-six different mutations were found in 42 patients comprising 16 missense mutations, seven frameshifts, three in-frame deletions, four nonsense mutations, and six splice site mutations. Twenty-two of the mutations have not previously been reported. Furthermore, we found one non-pathogenic amino acid substitution, one rare variant in a non-coding region, and one polymorphism with a heterozygosity of 28%. Three de novo mutations were found, two of which were paternal and one of maternal origin.

The clinical spectrum of type IV collagen mutations

Clinical manifestations of type IV collagen mutations can vary from the severe, clinically and genetically heterogeneous renal disorder, Alport syndrome, to autosomal dominant familial benign hematuria. The predominant form of Alport syndrome is X-linked; more than 160 different mutations have yet been identified in the type IV collagen alpha 5 chain (COL4A5) gene, located at Xq22-24 head to head to the COL4A6 gene. The autosomal recessive form of Alport syndrome is caused by mutations in the COL4A3 and COL4A4 genes, located at 2q35-37. Recently, the first mutation in the COL4A4 gene was identified in familial benign hematuria. This paper presents an overview of type IV collagen mutations, including eight novel COL4A5 mutations from our own group in patients with Alport syndrome. The spectrum of mutations is broad and provides insight into the clinical heterogeneity of Alport syndrome with respect to age at renal failure and accompanying features such as deafness, leiomyomatosis, and anti-GBM nephritis.

Comparative analysis of the noncollagenous NC1 domain of type IV collagen: identification of structural features important for assembly, function, and pathogenesis

Type IV collagen alpha1-alpha6 chains have important roles in the assembly of basement membranes and are implicated in the pathogenesis of Goodpasture syndrome, an autoimmune disorder, and Alport syndrome, a hereditary renal disease. We report comparative sequence analyses and structural predictions of the noncollagenous C-terminal globular NC1 domain (28 sequences). The inferred tree verified that type IV collagen sequences fall into two groups, alpha1-like and alpha2-like, and suggested that vertebrate alpha3/alpha4 sequences evolved before alpha1/alpha2 and alpha5/alpha6. About one fifth of NC1 residues were identified to confer either the alpha1 or alpha2 group-specificity. These residues accumulate opposite charge in subdomain B of alpha1 (positive) and alpha2 (negative) sequences and may play a role in the stoichiometric chain selection upon type IV collagen assembly. Neural network secondary structure prediction on multiple aligned sequences revealed a subdomain core structure consisting of six hydrophobic beta-strands and one short alpha-helix with a significant hydrophobic moment. The existence of opposite charges in the alpha-helices may carry implications for intersubdomain interactions. The results provide a rationale for defining the epitope that binds Goodpasture autoantibodies and a framework for understanding how certain NC1 mutations may lead to Alport syndrome. A search algorithm, based entirely on amino acid properties, yielded a possible similarity of NC1 to tissue inhibitor of metalloproteinases (TIMP) and prompted an investigation of a possible functional relationship. The results indicate that NC1 preparations decrease the activity of matrix metalloproteinases 2 and 3 (MMP-2, MMP-3) toward a peptide substrate, though not to [14C]-gelatin. We suggest that an ancestral NC1 may have been incorporated into type IV collagen as an evolutionarily mobile domain carrying proteinase inhibitor function.

Coordinate gene expression of the alpha3, alpha4, and alpha5 chains of collagen type IV. Evidence from a canine model of X-linked nephritis with a COL4A5 gene mutation

Canine X-linked hereditary nephritis is an animal model for human X-linked hereditary nephritis with a premature stop codon in the alpha5(IV) gene of collagen type IV. We used this model to examine the other alpha(IV) chains at the mRNA and protein level in the kidney, since in human X-linked hereditary nephritis, the alpha3(IV) and alpha4(IV) chains are often absent from the glomerular basement membrane, although both are encoded by autosomal genes. cDNA probes for the alpha1(IV)-alpha6(IV) chains were generated from normal dog kidney using the polymerase chain reaction. Sequences were >/=88% identical at the DNA level and >/=92% identical at the protein level to the respective human alpha(IV) chains. By Northern analysis, transcripts for the alpha1(IV), alpha2(IV), and alpha6(IV) chains were detected at comparable levels in both normal and affected male dog kidney RNA. As previously shown, the transcript for the alpha5(IV) chain was reduced to approximately 10% of normal. Unexpectedly, the alpha3(IV) and alpha4(IV) transcripts were both decreased >/=77% in affected male dog kidney, suggesting a mechanism coordinating the expression of these three basement membrane components. The NC1 domain of collagen type IV isolated from normal dog glomeruli was positive for the alpha3(IV), alpha4(IV), and alpha5(IV) chains by Western blotting. In contrast, in the NC1 domain isolated from affected dog glomeruli, these three chains were not detectable, except for a trace of alpha3(IV) dimer. In X-linked hereditary nephritis, the absence of the alpha3(IV) and alpha4(IV) chains from glomerular basement membrane may reflect factors acting at the transcriptional and/or translational level in addition to the protein assembly level.

A nonsense mutation in the COL4A5 collagen gene in a family with X-linked juvenile Alport syndrome

The X-linked form of Alport syndrome is associated with mutations in the COL4A5 gene encoding the alpha 5-chain of type IV collagen. By using PCR-amplification and direct sequencing we identified a novel mutation involving a deletion of the last two bases in the codon GGA for Glycine-1479 in exon 47 of the COL4A5 gene in a patient with a juvenile form of X-linked Alport syndrome with deafness. This two base deletion caused a shift in the reading frame and introduced a premature stop codon which resulted in an alpha 5(IV)-chain shortened by 202 residues and lacking almost the entire NC1 domain. The mutation was found to co-segregate with the disease in the family. The information of the sequence variation in this family was used to perform carrier detection and prenatal diagnosis by allele-specific oligonucleotide hybridization analysis and direct sequencing of PCR amplified exon 47. Prenatal diagnosis on chorionic villi tissue, obtained from one of the female carriers in the family, revealed a male fetus hemizygous for the mutated allele. A subsequent prenatal test in her next pregnancy revealed a normal male fetus. Prenatal diagnosis of Alport syndrome has not previously been reported.

Deletions in the COL4A5 collagen gene in X-linked Alport syndrome. Characterization of the pathological transcripts in nonrenal cells and correlation with disease expression

The type IV collagen alpha 5 chain (COL4A5) gene of 88 unrelated male patients with X-linked Alport syndrome was tested for major gene rearrangements by Southern blot analysis, using COL4A5 cDNA probes. 14 different deletions were detected, providing a 16% deletion rate in the COL4A5 gene in the patient population. The deletions are dispersed all over the gene with different sizes, ranging from 1 kb to the complete absence of the gene (> 250 kb) in one patient. In four patients with intragenic deletions, absence of the alpha 3 (IV) chain in the glomerular basement membrane was demonstrated by immunohistochemical studies. This finding supports the hypothesis that abnormalities in the alpha 5 (IV) chain may prevent normal incorporation of the alpha 3 (IV) chain into the glomerular basement membrane. Direct sequencing of cDNA amplified from lymphoblast mRNA of four patients with internal gene deletions, using appropriate combinations of primers amplifying across the predicted boundaries of the deletions, allowed us to determine the effect of the genomic rearrangements on the transcripts and, by inference, on the alpha 5 (IV) chain. Regardless of the extent of deletion and of the putative protein product, the 14 deletions occur in patients with juvenile-type Alport syndrome.