When two rare diseases meet: atypical hemolytic uremic syndrome and alkaptonuria in two siblings

Abstract

Atypical hemolytic uremic syndrome (aHUS) is a rare complement-mediated thrombotic microangiopathy (TMA), whereas alkaptonuria (AKU) is an uncommon inherited metabolic disorder. Their coexistence has not previously been described. We report two siblings from a consanguineous family with CD46-associated aHUS and AKU to highlight an unusual diagnostic challenge and the importance of recognizing coexisting inherited diseases. Two brothers experienced recurrent episodes of TMA characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury, while maintaining preserved long-term renal function despite persistent proteinuria. Genetic analysis identified a homozygous CD46 variant consistent with familial aHUS. During evaluation, recurrent black discoloration of urine, chronic back and knee pain, scleral pigmentation, and a history of black-stained diapers since infancy suggested an additional inherited disorder. Subsequent testing confirmed a homozygous pathogenic HGD variant, establishing the diagnosis of AKU. Both patients achieved hematologic remission following complement inhibition and remained free of dialysis throughout follow-up. To our knowledge, this is the first report of the coexistence of aHUS and AKU. These cases demonstrate that persistent or recurrent black discoloration of urine in patients with TMA should not be assumed to result solely from hemolysis. Careful assessment of atypical clinical features, supported by genetic testing when appropriate, may facilitate recognition of coexisting inherited disorders, particularly in consanguineous families, and improve diagnostic accuracy and long-term patient management.

Introduction

Atypical hemolytic uremic syndrome (aHUS) is a rare complement-mediated thrombotic microangiopathy (TMA) associated with dysregulation of the alternative complement pathway resulting in endothelial injury and organ damage. Despite major advances in genetic characterization, unusual clinical findings may complicate the diagnostic evaluation and suggest the presence of additional inherited disorders [1–4]. Alkaptonuria (AKU) is a rare inborn error of tyrosine metabolism that typically presents with progressive ochronosis and characteristic urine discoloration caused by homogentisic acid oxidation [5]. To our knowledge, the coexistence of aHUS and AKU has not previously been reported.

Here we present two brothers from a consanguineous family who were diagnosed with both aHUS and alkaptonuria, creating an unusual diagnostic challenge (Fig. 1a).

Fig. 1.

Pedigree and representative photographs of black discoloration of urine. a Pedigree demonstrating consanguineous parentage and the occurrence of complement-mediated aHUS and alkaptonuria in two brothers. The arrow indicates the proband. b Black discoloration of urine in both patients. The left photograph (Case 1) shows urine that had accumulated in the collection bag while the patient was monitored with an indwelling Foley catheter during hospitalization for acute TMA; the exact duration of urine standing before photography was not documented. The right photograph (Case 2) shows the complete 24-h urine collection

Case presentation

Case 1

The proband was a 24-year-old man of Kurdish ethnicity living in Turkey whose first documented TMA episode occurred at 20 years of age, presenting with nausea, vomiting, abdominal pain, non-immune hemolytic anemia, thrombocytopenia, and acute kidney injury (AKI). He was treated with corticosteroids and plasma exchange (PLEX) without requiring hemodialysis (HD), achieving complete hematologic remission. Renal function normalized (serum creatinine [sCr] 0.73 mg/dL; estimated glomerular filtration rate [eGFR] 133 mL/min/1.73 m²), although low-grade proteinuria (urine protein-to-creatinine ratio [UPCR] 0.8 g/g) persisted.

Three years later, he experienced a second TMA episode that was preceded by an upper respiratory tract infection beginning 3–4 days before symptom onset. He presented with nausea, vomiting, abdominal pain, diarrhea, and longstanding knee and lower back pain. Laboratory evaluation demonstrated recurrent TMA with thrombocytopenia (10 × 10³/mm³), elevated lactate dehydrogenase (2956 U/L), AKI (sCr 2.17 mg/dL), hematuria, and UPCR of 5.2 g/g. ADAMTS13 activity was normal, leading to the discontinuation of PLEX. A striking finding was black discoloration of urine that had accumulated in the Foley catheter collection bag during the acute TMA episode (Fig. 1b, left), prompting further evaluation for an underlying metabolic disorder. Following meningococcal vaccination, eculizumab therapy was initiated (900 mg weekly for four weeks, followed by 1200 mg every two weeks).

Because of persistent proteinuria, ramipril (10 mg/day) and dapagliflozin (10 mg/day) were initiated at month 3 as antiproteinuric and kidney-protective therapy. Longitudinal kidney and hematologic parameters are summarized in Table 1. During follow-up, kidney function remained preserved and proteinuria gradually decreased, although it did not completely resolve. At month 12, sCr was 0.85 mg/dL, eGFR was 127 mL/min/1.73 m², and proteinuria was 2.0 g/g. Blood pressure was 120/80 mmHg, and hematuria was absent. Hematologic remission was sustained, with a hemoglobin level of 17.9 g/dL, platelet count of 240 × 10³/mm³, and LDH of 124 U/L. Chronic musculoskeletal pain persisted.

Table 1.

Longitudinal kidney function, proteinuria, and hematologic follow-up of the two patients

Parameter	Patient	Acute TMA episode	Month 3	Month 6	Month 9	Month 12
Serum creatinine, mg/dL	Patient 1	2.1	0.90	1.02	0.95	0.85
	Patient 2	2.1	0.70	0.73	0.76	0.79
eGFR, mL/min/1.73 m2	Patient 1	43 (nadir 26)	123	108	115	127
	Patient 2	45 (nadir 27)	143	138	133	131
Proteinuria (UPCR), g/g	Patient 1	5.2	3.2	2.7	2.2	2.0
	Patient 2	2.7	1.5	1.2	1.0	0.8
Hemoglobin, g/dL	Patient 1	14.3 (nadir 8.0)	14.9	17.0	17.0	17.9
	Patient 2	15.4 (nadir 8.0)	16.1	17.2	17.0	16.3
Platelet count, ×103/mm3	Patient 1	10 (nadir 10)	250	229	195	240
	Patient 2	78 (nadir 18)	221	237	245	246
LDH, U/L	Patient 1	2956 (peak 3193)	154	144	133	124
	Patient 2	786 (peak 1222)	147	155	156	131
Hematuria	Patient 1	Present	Absent	Absent	Absent	Absent
	Patient 2	Present	Absent	Absent	Absent	Absent
Blood pressure, mmHg	Patient 1	150/90	–	–	–	120/80
	Patient 2	130/80	–	–	–	110/75

Values in parentheses indicate the nadir value for eGFR, hemoglobin, and platelet count and the peak value for LDH during the acute TMA episode. Ramipril (10 mg/day) and dapagliflozin (10 mg/day) were initiated at month 3 in both patients because of persistent proteinuria. Dashes indicate unavailable data

eGFR estimated glomerular filtration rate, LDH lactate dehydrogenase, TMA thrombotic microangiopathy, UPCR urine protein-to-creatinine ratio

Case 2

The proband's 19-year-old brother had experienced three previous TMA episodes between 16 and 17 years of age, all characterized by non-immune hemolytic anemia, thrombocytopenia, AKI, and complete hematologic recovery following supportive treatment without HD. Despite normalization of renal function, persistent proteinuria remained after each episode.

At 19 years of age, he developed a fourth episode, likewise preceded by an upper respiratory tract infection of 3–4 days' duration. Clinical presentation and laboratory findings were similar to those of his brother, including thrombocytopenia (78 × 10³/mm³), elevated LDH (786 U/L), AKI (sCr 2.14 mg/dL), hematuria, proteinuria (UPCR 2.7 g/g), and black discoloration of a 24-h urine collection (Fig. 1b, right). After normal ADAMTS13 activity excluded thrombotic thrombocytopenic purpura, PLEX was discontinued and eculizumab therapy was initiated following meningococcal vaccination.

Because of persistent proteinuria, ramipril (10 mg/day) and dapagliflozin (10 mg/day) were initiated at month 3 as antiproteinuric and kidney-protective therapy. Proteinuria subsequently decreased while kidney function remained preserved (Table 1). At month 12, sCr was 0.79 mg/dL, eGFR was 131 mL/min/1.73 m², and UPCR was 0.8 g/g. Blood pressure was 110/75 mmHg, and hematuria was absent. Hematologic remission was sustained, with a hemoglobin level of 16.3 g/dL, platelet count of 246 × 10³/mm³, and LDH of 131 U/L. Chronic back and knee pain persisted.

Genetic and metabolic evaluation

Genetic testing was performed on peripheral blood samples using next-generation sequencing (NGS)-based clinical exome sequencing on the DNBSEQ-G400 platform, with target enrichment using the Twist Bioscience Clinical Exome panel. Variants potentially relevant to the patients’ clinical phenotypes were identified and interpreted by the performing molecular genetics laboratory.

The remarkably similar clinical course in both brothers—including recurrent infection-triggered TMA episodes, preserved long-term kidney function despite persistent proteinuria, and chronic musculoskeletal symptoms—suggested an underlying inherited disorder. In addition, both had scleral pigmentation, recurrent black discoloration of urine, and a history of black-stained diapers during infancy. Clinical exome sequencing identified the homozygous CD46 c.718 T>C (p.Ser240Pro) variant in both siblings. This variant has previously been reported in patients with aHUS and is supported by functional evidence demonstrating impaired complement regulatory activity [6, 7]. In conjunction with the familial occurrence of recurrent complement-mediated TMA, these findings supported the diagnosis of CD46-associated familial aHUS.

Both brothers also harbored a homozygous HGD c.674G>A (p.Arg225His) variant, which has previously been reported in patients with AKU [8]. In conjunction with the characteristic clinical phenotype, including black discoloration of urine after standing, scleral pigmentation, chronic musculoskeletal symptoms, and a history of black-stained diapers since infancy, this finding established the clinical-molecular diagnosis of AKU. Quantitative urinary homogentisic acid measurement was not available and therefore could not be used for biochemical confirmation or baseline metabolic characterization. During the reported follow-up period, formal metabolic specialist evaluation had not been performed, and disease-specific therapy with nitisinone had not been initiated. Dedicated spinal or joint imaging for ochronotic arthropathy and systematic screening for other ochronosis-related organ involvement had not been performed during the reported follow-up period.

Discussion

To the best of our knowledge, this case report describes the first documented coexistence of familial aHUS and AKU in two brothers, a finding likely facilitated by marked parental consanguinity. The diagnostic challenge arose during TMA episodes, when the characteristic black discoloration of urine in AKU could have been misattributed to hemoglobinuria or hematuria associated with aHUS. Importantly, the coexistence of AKU and aHUS in these siblings should not be interpreted as evidence of a causal or mechanistic relationship between the two disorders. AKU is an autosomal recessive metabolic disorder caused by biallelic pathogenic variants in HGD, whereas CD46-associated aHUS results from impaired complement regulation and represents a genetically and pathophysiologically distinct disorder [1–4, 6]. The homozygous HGD c.674G>A (p.Arg225His) variant identified in our patients has previously been reported in patients with AKU [8]. Thus, the available genetic and clinical evidence supports the interpretation that AKU and aHUS represent two distinct inherited disorders in this family rather than different manifestations of a shared pathogenic mechanism. Although parental consanguinity may have facilitated homozygosity for the HGD variant, the coexistence of HGD and CD46 variants in these siblings does not establish a biological interaction between the two genes.

aHUS results from dysregulated activation of the alternative complement pathway, leading to endothelial injury and systemic TMA [1–4]. CD46 encodes membrane cofactor protein (MCP), a membrane-bound complement regulator that protects host cells by serving as a cofactor for factor I-mediated cleavage of C3b and C4b deposited on endothelial surfaces. Unlike circulating regulators such as complement factor H, MCP functions locally on endothelial cells, making the renal microvasculature particularly susceptible to complement-mediated injury when its activity is impaired [2–4]. The CD46 p.Ser240Pro variant identified in our patients, historically designated p.Ser206Pro, is located within the extracellular complement control protein 4 domain and has previously been reported in patients with aHUS [6, 7]. Functional studies have demonstrated that this variant maintains normal cell-surface expression but markedly impairs C3b binding and factor I-mediated C3b cofactor activity, resulting in defective regulation of complement activation at the cell surface [6]. These functional findings provide a mechanistic basis for its association with complement-mediated TMA. This phenotype was reflected in our patients, who experienced recurrent TMA episodes without requiring dialysis and subsequently achieved sustained hematologic remission with recovery of kidney function (Table 1). Despite preserved eGFR, residual proteinuria persisted in both patients, which may reflect kidney injury following recurrent episodes of complement-mediated endothelial injury [2–4]. In view of persistent proteinuria, ramipril and dapagliflozin were initiated at month 3 as antiproteinuric and kidney-protective therapy, followed by a gradual reduction in proteinuria during subsequent follow-up (Table 1) [9].

The diagnosis of AKU explained several longstanding clinical findings that had preceded the onset of aHUS by many years, including chronic back and knee pain, scleral pigmentation, and recurrent black discoloration of urine. AKU results from deficiency of homogentisate 1,2-dioxygenase, leading to lifelong accumulation of homogentisic acid [5]. Oxidation of homogentisic acid after urine collection produces the characteristic dark pigmentation that represents the earliest clinical manifestation of the disease, whereas ochronosis and degenerative arthropathy usually become evident later in adulthood [5]. In retrospect, these classical manifestations had remained unrecognized despite being present long before the first episode of TMA.

The coexistence of these disorders created an important diagnostic pitfall. Dark urine is frequently encountered during active TMA because of intravascular hemolysis and is therefore commonly attributed to hemoglobinuria. In contrast, urine discoloration in AKU results from oxidation of homogentisic acid and is unrelated to hemolysis [5]. Recognition of this distinction prompted further metabolic evaluation in our patients and ultimately established the diagnosis of a second inherited disorder. Accordingly, persistent or recurrent black discoloration of urine, particularly when accompanied by musculoskeletal symptoms or scleral pigmentation, should prompt consideration of AKU even in patients with an established diagnosis of aHUS.

Recognition of AKU also has important implications for long-term disease-specific management, particularly given the early musculoskeletal symptoms in both patients. Nitisinone is a disease-modifying therapeutic option for AKU that markedly reduces HGA production and may slow clinical disease progression [10]. However, nitisinone treatment increases plasma tyrosine concentrations, and treatment-related hypertyrosinemia may result in ocular complications, particularly corneal keratopathy [10, 11]. Therefore, patients receiving nitisinone require monitoring of plasma tyrosine levels, appropriate dietary management to control tyrosine concentrations, and attention to ocular symptoms and ophthalmologic assessment when indicated [11, 12]. In the present patients, formal metabolic specialist assessment and nitisinone therapy had not been undertaken during the reported follow-up period. The absence of quantitative HGA measurement and comprehensive metabolic assessment represents a limitation of the present report and precluded baseline biochemical characterization and evaluation of disease-specific therapy.

AKU is a progressive multisystem disorder in which ochronotic involvement may affect the spine and major joints as well as cardiovascular and genitourinary systems [5]. Given the early musculoskeletal symptoms in both patients, longitudinal surveillance for ochronotic complications would be clinically relevant. However, dedicated musculoskeletal imaging and systematic multisystem screening had not been performed during the reported follow-up period; therefore, the presence or extent of structural ochronotic involvement could not be assessed.

In conclusion, these cases demonstrate that unusual clinical findings accompanying TMA should not automatically be attributed to aHUS alone. In populations with a high prevalence of consanguinity, the coexistence of multiple inherited disorders, although uncommon, should remain within the differential diagnosis. Recognition of AKU in these siblings clarified their longstanding extrarenal manifestations and highlighted the need for disease-specific metabolic assessment and appropriate long-term multisystem surveillance.

Funding

The authors received no specific grants from any funding agency in the public, commercial, or not-for-profit sectors.

Declarations

Conflict of interest

CK has received honoraria from Alexion and support for meeting registration and travel from Amgen. MTD has received honoraria from Alexion and support for meeting registration and travel from Amgen. SM has received honoraria from Alexion and support for meeting registration and travel from Amgen. BK, RT, and ME declare that they have no competing interests.

Ethical approval

All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its later amendments.

Informed consent

Written informed consent was obtained from both patients for publication of this case report and the accompanying images.