X-Linked Alport Syndrome in Women: Genotype and Clinical Course in 24 Cases

Objectives: X-linked Alport syndrome (XLAS) females are at risk of developing proteinuria and chronic kidney damage (CKD). The aim of this study is to evaluate the genotype-phenotype correlation in this rare population.

Materials and Methods: This is a prospective, observational study of XLAS females, confirmed by a pathogenic mutation in COL4A5 and renal ultrastructural evaluation. Proteinuria, renal function and extrarenal involvement were monitored during follow-up. Patients were divided in 2 groups, according to mutations in COL4A5: missense (Group 1) and non-missense variants (Group 2).

Results: Twenty-four XLAS females, aged 10.6 ± 10.4 years at clinical onset (mean follow-up: 13.1 ± 12.6 years) were recruited between 2000 and 2017 at a single center. In group 1 there were 10 patients and in group 2, 14 (mean age at the end of follow-up: 24.9 ± 13.6 and 23.2 ± 13.8 years, respectively). One patient in Group 1 and 9 in Group 2 (p = 0.013) developed proteinuria during follow-up. Mean eGFR at last follow-up was lower in Group 2 (p = 0.027), where two patients developed CKD. No differences in hearing loss were documented among the two groups. Two patients in Group 2 carried one mutation in both COL4A5 and COL4A3 (digenic inheritance) and were proteinuric. In one family, the mother presented only hematuria while the daughter was proteinuric and presented a greater inactivation of the X chromosome carrying the wild-type allele.

Conclusions: The appearance of proteinuria and CKD is more frequent in patients with severe variants. Carrying digenic inheritance and skewed XCI seem to be additional risk factors for proteinuria in XLAS females.

Exome Sequencing and Identification of Phenocopies in Patients With Clinically Presumed Hereditary Nephropathies

RATIONALE & OBJECTIVE

Hereditary nephropathies are clinically and genetically heterogeneous disorders. For some patients, the clinical phenotype corresponds to a specific hereditary disease but genetic testing reveals that the expected genotype is not present (phenocopy). The aim of this study was to evaluate the spectrum and frequency of phenocopies identified by using exome sequencing in a cohort of patients who were clinically suspected to have hereditary kidney disorders.

STUDY DESIGN

Cross-sectional cohort study.

SETTING & PARTICIPANTS

174 unrelated patients were recruited for exome sequencing and categorized into 7 disease groups according to their clinical presentation. They included autosomal dominant tubulointerstitial kidney disease, Alport syndrome, congenital anomalies of the kidney and urinary tract, ciliopathy, focal segmental glomerulosclerosis/steroid-resistant nephrotic syndrome, VACTERL association, and "other."

RESULTS

A genetic diagnosis (either likely pathogenic or pathogenic variant according to the guidelines of the American College of Medical Genetics) was established using exome sequencing in 52 of 174 (30%) cases. A phenocopy was identified for 10 of the 52 exome sequencing-solved cases (19%), representing 6% of the total cohort. The most frequent phenocopies (n=5) were associated with genetic Alport syndrome presenting clinically as focal segmental glomerulosclerosis/steroid-resistant nephrotic syndrome. Strictly targeted gene panels (<25 kilobases) did not identify any of the phenocopy cases.

LIMITATIONS

The spectrum of described phenocopies is small. Selection bias may have altered the diagnostic yield within disease groups in our study population. The study cohort was predominantly of non-Finnish European descent, limiting generalizability. Certain hereditary kidney diseases cannot be diagnosed by using exome sequencing (eg, MUC1-autosomal dominant tubulointerstitial kidney disease).

CONCLUSIONS

Phenocopies led to the recategorization of disease and altered clinical management. This study highlights that exome sequencing can detect otherwise occult genetic heterogeneity of kidney diseases.

A family case of X-linked Alport syndrome patients with a novel variant in COL4A5

We herein report 2 Japanese patients with X-linked Alport syndrome (XLAS), with a novel variant in COL4A5. Patient 1 was a 16-year-old Japanese girl with a history of microscopic hematuria, without proteinuria, renal dysfunction, deafness, or ocular abnormalities. At 13 years of age, renal biopsy was performed; however, a diagnosis of AS was not considered. When her mother (patient 2) was 40 years of age (3 years after patient 1 underwent a renal biopsy), patient 2 was found to have asymptomatic hematuria, proteinuria, and an increased serum creatinine level, without deafness and ocular abnormalities. Subsequently, immunofluorescence staining for alpha 5 chains of type IV collagen was performed in patient 1. Pathological findings were consistent with AS, and genetic analysis demonstrated that both patients had a heterozygous mutation in COL4A5 (NM_000495.4: exon41:c.C3769T: p.Q1257X). To date, more than 900 different COL4A5 mutations have been identified; however, this variant has not been previously described. Physicians have to consider AS when they perform a renal biopsy in all patients with hematuria despite absent/present of family history, hearing loss, and ocular abnormality. Especially, when findings of light microscopy and immunofluorescence microscope are unclear, it should be considered carefully. Electron microscopy findings are very important.