Could This Be Alport Syndrome?

Introduction

Alport syndrome is characterized by hematuria, sensorineural hearing loss, and ocular abnormalities. It was first described by Cecil Alport in 1927, who connected kidney disease and deafness as coexpressed hereditary traits. The molecular basis of Alport syndrome was identified in the 1990s with the discovery of variants in COL4A5, which cause X-linked Alport Syndrome (1), and the genes COL4A3 and COL4A4 (2), which cause autosomal Alport syndrome. These three Alport genes are required for assembly of the alpha-3,4,5 network of type IV collagen, which is a fundamental component of basement membranes in the glomerulus, cochlea, and eye (3). With the advent of next-generation sequencing, variants in Alport genes have been identified in cohorts of individuals with a much broader spectrum of kidney disease (4). Furthermore, there is a strong association between the histologic pattern of FSGS and Alport gene variants. In fact, a large proportion of patients with hereditary FSGS have COL4A3 and COL4A4 variants (5). Such studies highlight the need to consider Alport syndrome or evaluate the contribution of Alport gene variants in a wider spectrum of kidney disease.

Clinical Patient

A 15-year-old boy was referred to Pediatric Nephrology for investigation of proteinuria and asymptomatic hypertension detected by his general pediatrician. At presentation, his BP was 150/80, he had microscopic hematuria (20–50×10⁶/L) and proteinuria (protein-creatinine ratio 286 mg/mmol), and his serum creatinine was 1.04 mg/dl (Schwartz eGFR 78 ml/min per 1.73 m²). His past medical history revealed a term birth weight of 4.2 kg, a delay in fine motor skills and speech, and myopia requiring glasses at 6 years. He was diagnosed with Asperger syndrome at 10 years of age, and bilateral, moderate, sensorineural hearing loss at 13 years, prompting the use of hearing aids. There was a family history of hearing loss in multiple distant relatives, but no familial microscopic hematuria or kidney disease requiring dialysis or kidney transplantation. Initial investigations focused on an acute glomerular disease. His GN screen (C3, C4, ANA, ANCA, anti–glomerular basement membrane [GBM] antibodies) was normal, and his kidney ultrasound showed normal kidney lengths. At this stage, the differential diagnosis included Alport syndrome, in view of the hearing loss, and IgA vasculitis, in view of the hypertension and nephrotic range proteinuria.

Making a Diagnosis of Alport Syndrome

Genetic testing for kidney disease is increasingly available and, in families where there is a known Alport variant, extended or cascade testing of other family members is both feasible and important. Where there are atypical clinical features, accelerated decline in kidney function, or where there are discordant presentations among family members with the same pathogenic variant, a kidney biopsy will add additional valuable information. In this patient, light microscopy revealed mesangial expansion but no evidence of FSGS or crescents. There was periglomerular fibrosis and moderate tubular atrophy and interstitial fibrosis (30%–40%). Arteries showed duplication of internal elastic lamina and medial hypertrophy. Immunofluorescence (IgA, IgG, IgM, C3, C1q) was negative. Electron microscopy revealed extensive foot process effacement. The GBM was irregular, with thickness varying between 127 and 807 nm (normal range 297±6.0 nm at 11 years). Almost all GBMs showed lamination and occasional small electron dense particles. No electron dense immune deposits were seen. Immunofluorescence for collagen IV alpha-3,4,5 was not available, although this could also aid diagnosis. The genetic analysis confirmed a homozygous variant in COL4A3. The homozygous variant raised the possibly of consanguinity or consanguinity by descent, where family members are not known to be related. Pathogenicity was assessed following ACMG coding criteria (PVS1_VSTR, PM3_MOD, PM2_MOD, PP4_SUP). This pathogenic frameshift variant in exon 51 c4803del, p(Gly1602Alafs*13) is predicted to lead to a shift in the reading frame, leading to premature termination of protein translation. As such, the mRNA might be targeted for nonsense-mediated decay. The frequency of this variant in the Genome Aggregation Database is low at 0.0008% or two of 246,092 alleles examined. The identification of this variant prompted further genetic testing in the wider family and the identification of heterozygous individuals, who were referred for surveillance of kidney function.

Pathophysiology of Disease Progression

Assembly of the type IV collagen network starts with three alpha chains, which form a triple helix. From the six COL4A genes there are three trimer combinations: alpha-1,1,2; alpha-3,4,5; and alpha-5,5,6. These trimers dimerize at the noncollagenous domain to create heterotrimers, which then oligomerize into the type IV collagen network by crosslinking at the trimer 7S domains. The alpha-3,4,5 network is affected by variants in any one of the three Alport genes. This network is centrally located within the GBM and thought to provide mechanical strength and be less resistant to proteolysis (3). With reduced or absent type IV collagen in the GBM, there is progressive loss of glomerular function with podocyte effacement, persistent proteinuria, and progressive decline in kidney function. There is genotype-phenotype correlation, and individuals with truncating variants reach ESKD sooner than those with missense variants (6). The weakened GBMs in Alport syndrome have an altered matrix composition, which drives aberrant signaling from the GBM to the podocyte. Indeed, deletion or inhibition of integrin receptor signaling prolonged kidney survival in Alport mice (7) and discoid domain receptor 1 activation-mediated podocyte lipotoxicity (8).

Current and Future Treatment Options

Alport GBMs are susceptible to damage from increased intracapillary hydrostatic pressure, and, therefore, minimizing factors that increase mechanical load prolong kidney function. Blockade of the renin-angiotensin-aldosterone system (RAAS) is the standard of care for reducing proteinuria in Alport syndrome and is associated with extended kidney survival of more than 15 years (9). Furthermore, hypertension can accelerate the decline in kidney function and should be controlled. Ambulatory BP monitoring in our patient revealed hypertension on amlodipine monotherapy, and therefore, enalapril and subsequently atenolol were added to maintain BP between the 50th and 90th centiles for age and height. Despite the maximum-tolerated RAAS blockade, there is disease progression in Alport syndrome, highlighting the need for new treatments. A number of new therapies could act as adjunctive agents to reduce proteinuria, consequent tubular protein toxicity, and tubulointerstitial fibrosis. These include anti-microRNA21, which was shown to prolong kidney survival in Alport mice; the combined angiotensin receptor and endothelin receptor inhibitor sparsentan, which has shown promising proteinuria reduction in FSGS; the SGLT2 inhibitor dapagliflozin, which has shown dramatic effects on kidney survival and mortality in patients with nondiabetic CKD; and kidney lipid modifying agents. Bardoxolone has also been investigated in Alport syndrome (unpublished) and shown to increase GFR but also proteinuria. For all potential adjunctive therapies, it will be vital to monitor longer-term safety and efficacy. Because Alport syndrome has a genetic basis, the potential for future curative gene therapy has been investigated in mice with gene replacement or exon skipping approaches (3), and these remain active areas of research.

Subsequent Progress

In this patient, BP control was achieved with amlodipine, enalapril, and atenolol. However, there was persistent proteinuria and decline in kidney function. Enalapril was briefly suspended due to hyperkalemia, and this was successfully managed with dietary potassium restriction and enalapril restarted. At 17 years, with a creatinine of 3.02 mg/dl (Schwartz eGFR 26 ml/min per 1.73 m²), preparation for kidney transplantation began. With regard to living donation, a number of family members are heterozygous for the COL4A3 variant, and this would need to be considered in evaluating kidney health in these potential donors. Because nephrectomy would lead to hyperfiltration in the remaining kidney, an individual’s heterozygous variants in Alport genes are at risk of CKD after donation and, as such, recommendations caution against donation, especially in females with COL4A5 variants (10).

Summary

This patient highlights the need to consider a diagnosis of Alport syndrome to ensure early initiation of RAAS blockade. Where there is no family history of kidney disease, the first clue may be the identification of sensorineural hearing loss, which could prompt urinalysis, and an early diagnosis of the kidney phenotype (Figure 1). Early RAAS blockade in children with Alport syndrome before the onset of proteinuria was shown to be safe (11), and this remains the standard of care to prolong kidney survival in patients with Alport syndrome.

Figure 1.

Who to consider and how to diagnose Alport syndrome. The clinical presentation of Alport syndrome is variable, but the above common presentations should prompt further investigation. Genetic testing is now widely available and the most effective way to diagnose Alport syndrome because it allows the identification of variants that could be examined in other family members. Urine microscopy and kidney biopsy findings will be supportive but not confirmative of an Alport syndrome diagnosis. GBM, glomerular basement membrane.

Disclosures

A. Fornoni is inventor on pending or issued patents (PCT/US11/56272, PCT/US12/62594, PCT/US2019/041730, PCT/US2019/032215, PCT/US13/36484, and PCT 62/674,897) aimed at treating proteinuric kidney diseases, and stands to gain royalties from their future commercialization of these patents; reports having a shareholder agreement with River 3 Renal Corp., which has licensed worldwide rights to develop and commercialize agents for the treatment of kidney disease, and ZyVersa Therapeutics; reports being Chief Scientific Officer and Vice-President of L&F Health LLC.; reports consultancy agreements with Dimerix, Gilead, Janssen, Novartis, ONO, and Zyversa Therapeutics; reports receiving research funding from Boheringer Ingelheim and Roche; and reports serving as a scientific advisor or member of Journal of Clinical Investigation and Kidney International. R. Lennon is a consultant for Travere Therapeutics and reports receiving studentship funding from GlaxoSmithKline; reports serving as a scientific advisor or member of the Kidney Research UK grants panel and the Scientific Advisor Research Network for the Alport Syndrome Foundation; reports serving as a trustee for Alport UK and for Kidneys for Life; and reports receiving funding from The Wellcome Trust and Kidney Research UK.

Funding

R. Lennon is supported by a Wellcome Trust Senior Fellowship award (202860/Z/16/Z). A. Fornoni is supported by the National Institutes of Health grants R01DK117599, R01DK104753, R01CA227493, U54DK083912, UM1DK100846, U01DK116101, and UL1TR000460 (Miami Clinical Translational Science Institute, National Center for Advancing Translational Sciences, and the National Institute on Minority Health and Health Disparities).