Overall manifestations of the patients
Kidney phenotypes
The detailed clinical kidney presentations and outcomes are summarized in Table 1, and the phenotypes and genotypes of the individual patients are listed in Supplemental Table S1. The median age of onset and age at the last visit were 4.4 and 20.8 years, respectively. The most common initial presentation was AUA, followed by macroscopic hematuria, in both males and females. While no female patients presented with NS, approximately 20% of male patients presented with the condition, along with hematuria, at the median onset age of 7.0 (IQR 3.5–10.3) years.
In male patients, UPCR at the initial hospital visit at a median age of 8.9 (IQR 4.1–14.3) years was 1.6 mg/mg (IQR 0.5–4.0 mg/mg), and during follow-up, almost all patients except for one developed overt proteinuria (UPCR > 0.2 mg/mg). More than 90% of female patients developed overt proteinuria during follow-up. All patients exhibited hematuria. Treatment with renin-angiotensin system inhibitors (RASi) was initiated in all male patients at a median age of 9.5 years, except for two patients already in CKD G5 at the initial presentation. Nearly 90% of the female patients started taking RASi at the same median age as the male patients.
Approximately 60% of male patients had reached CKD G5 at the median age of 25.0 ([95% confidence interval (CI)] 23.5–26.5) years, which was estimated by the Kaplan–Meier method (Fig. 1), and nearly 90% had reached CKD G5 by 39 years. In female patients, approximately 20% progressed to kidney failure at the median age of 50.2 (95% CI 39.0–61.5) years, as estimated by Kaplan–Meier analysis, and approximately 30% and 60% reached CKD G5 by 37 and 55 years, respectively (Fig. 1).
Kidney biopsy was performed in 52.3% (68 of 130 patients) of male patients at a median age of 10.4 (IQR 7.1–14.3) and 41.9% (36 of 86 patients) of female patients at a median age of 10.6 (IQR 7.0–22.9) years. The pathological diagnosis was AS in 59 (86.8%) male and 20 (55.6%) female patients, while the remaining patients were diagnosed with immunoglobulin A nephropathy, mesangial proliferative glomerulonephritis, or thin basement membrane disease. On the second biopsy, AS was diagnosed in four male and three female patients.
The expression of the type IV collagen α5 chain was examined by immunofluorescence microscopy in 23 male and 17 female patients. In male patients, the expression was completely absent in 17 (73.9%), weak in 3 (13.0%), and normal in 3 (13.0%) patients. In female patients, 8 (47.1%) showed a mosaic pattern, 7 (41.2%) showed a normal pattern, and 2 (11.8%) showed a total loss.
Extrarenal phenotypes
In male patients, 65.1% developed SNHL at the median age (hearing survival age) of 19.9 (95% CI 13.8–26.0) years, while 14.1% of female patients developed SNHL at the median age of 53.4 (95% CI 47.5–59.2) years, which was estimated by Kaplan–Meier analysis (Table 1). A total of 47.9% (34/71; no data available for 25 patients) of male patients and 40% (4/10; no data available for five patients) of female patients with SNHL were recommended for or wore hearing aids at the median ages of 11.3 (IQR 9.9–16.7) and 45.9 (IQR 33.8–50.8) years, respectively. Ocular examination records were available for 63 males and 21 females. Twenty (31.7%) male patients and two (9.5%) female patients showed ocular abnormalities such as anterior lenticonus or fleck retinopathy.
Genotypes
Among the 124 probands, 56 (45.2%) had missense variants, 23 (18.5%) had abnormal splicing, 19 (15.3%) had nonsense variants, 21 (16.9%) had short frame-shifting insertion or deletion variants, and 5 (4.0%) had other types of variants (Table 2 and Supplemental Table S1). Among the 121 different variants, 93 (76.9%) were novel. Two variants (p.Gly400Val and p.Arg1677Ter) were common in three families, and two other variants (c.466-2A>G and c.991-7T>A) were detected in both families. One male proband (P49 in Supplemental Table S1) had two COL4A5 variants, p.Gly1057Glu and p.Gly1442Cys.
Genotype–phenotype correlations
Male patients
Kidney phenotypes
At the time of onset, NS was significantly more prevalent (P = 0.023, odds ratio [OR] 3.3), and AUA was significantly less prevalent (P = 0.043, OR 0.4) in Group 3 patients when compared to Group 1 or 2 patients (Table 3).
When stratified according to variant types, approximately 50% of Group 1 patients and 70% of Group 3 patients developed CKD G5 at the median age (kidney survival periods) of 30 and 20 years, respectively. The kidney survival period showed a significant difference between Group 1 and Group 3 (P < 0.001, HR 2.8) and between Group 2 and Group 3 (P = 0.002, HR 3.1), which was estimated by Kaplan–Meier analysis. However, there was no difference between Groups 1 and 2. The kidney outcomes according to variant type are described in Table 3 and Fig. 2.
Kidney survival in 120 male patients with X-linked Alport syndrome according to the variant types of COL4A5.
Of the 40 different Gly substitution variants, nine were located between exons 1–20, 29 between exons 21–47, and two in the NC domain. The location of Gly substitution variants (exons 1–20 versus 21–47) did not affect the severity of the kidney phenotypes. Furthermore, the difference in the amino acid substitution for Gly did not correlate with the kidney phenotypes (data not shown).
In 15 families, there were at least two (between two and four) male patients who progressed to CKD G5. Intrafamilial homogeneity in CKD progression (less than 15 [0–10.3] years of age at CKD G5) was observed in 13 families. Conversely, intrafamilial heterogeneity (15 or more [between 16 and 22.9] years of age at CKD G5) was observed in the remaining two families.
Extrarenal phenotypes
SNHL was detected more commonly in Group 3 patients when compared to Group 1 patients (P < 0.001, OR 7.6 [95% CI 2.3–24.8]) (Table 3). Kaplan–Meier analyses estimated that the hearing survival period was significantly shorter in Group 3 than in Group 1 (P < 0.001, HR 5.1) and in Group 2 than in Group 1 (P = 0.032, HR 2.2). However, there was no difference between Groups 2 and 3. (Fig. 3) Ocular abnormalities were also more prevalent in Group 3 than in Group 1 (P < 0.001) and in Group 2 than in Group 1 (P < 0.001) (Table 3).
Hearing survival in 89 male patients with X-linked Alport syndrome according to the variant types of COL4A5.
Female patients
Kidney phenotype
The age of onset was lower in Group 3 than in Group 1 (P = 0.029) and Group 2 (P = 0.005), and Group 3 patients presented AUA less frequently than other groups (P = 0.037) (Table 4). Fourteen (20.6%) of the 68 female patients developed CKD G5. Group 3 patients developed CKD G5 significantly earlier than Group 1 (P = 0.006, HR 5.7), as well as Groups 1 and 2 (P = 0.013, HR 2.7) (Table 4 and Fig. 4).
Kidney survival in 68 female patients with X-linked Alport syndrome according to the variant types of COL4A5.
Extrarenal phenotype
The prevalence of SNHL did not differ among the three groups. Ocular phenotypes were not analyzed owing to data limitations.