RARE DISEASE
Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome
Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome
Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome
Synonyms: HHH syndrome, ORNT1 deficiency, Ornithine carrier deficiency, Ornithine translocase deficiency, Triple H syndrome
Synonyms: HHH syndrome, ORNT1 deficiency, Ornithine carrier deficiency, Ornithine translocase deficiency, Triple H syndrome
Synonyms: HHH syndrome, ORNT1 deficiency, Ornithine carrier deficiency, Ornithine translocase deficiency, Triple H syndrome
Drug discovery
3
drugs
With orphan designations
Overview
Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive urea cycle disorder caused by pathogenic variants in SLC25A15, disrupting mitochondrial ornithine transport. This results in hyperammonemia, persistently elevated plasma ornithine, and homocitrullinuria. Clinical presentation spans neonatal hyperammonemic crises (lethargy, vomiting, seizures) to later-onset neurological deficits (spastic paraplegia, ataxia, intellectual disability) and chronic liver dysfunction [1][3][6]. Diagnosis relies on biochemical triad confirmation and genetic testing. Management prioritizes protein restriction, citrulline/arginine supplementation, and ammonia scavengers (e.g., sodium benzoate), with acute crises requiring hemodialysis [1][7][12].
Burden
Lifetime risk of metabolic decompensation from infections, fasting, or stressors [1][12].
Irreversible neurological sequelae (developmental delay, spasticity) despite treatment, particularly with delayed diagnosis [6][7][12].
Chronic liver dysfunction and coagulopathy in ~50% of patients, necessitating ongoing monitoring [1][6].
Therapies
Protein-restricted diet with essential amino acid supplementation [1][3][7].
Chronic citrulline/arginine to maintain urea cycle function and sodium benzoate/phenylbutyrate for ammonia control [3][7][12].
Acute hyperammonemia: Hemodialysis, intravenous nitrogen scavengers, and glucose infusions [3][7].
Categories: rare genetic diseases, rare inborn errors of metabolism, rare neurological diseases
Research Papers
39 drug discovery papers about Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, with 1 first-in-class and 1 next-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
39 drug discovery papers about Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, with 1 first-in-class and 1 next-in-class emerging drug candidates forecasted to outperform the average preclinical success rate. Recent publications:
categories:
Small molecules
gene therapies
2025-04-26 | Urea cycle defects in adulthood: clinical presentation, diagnosis and treatment in genetically encoded hepatic metabolic disorders with a potential for encephalopathy.
Hyperammonaemia is an important cause for encephalopathy. Ammonia is the waste product of amino acid degradation and cannot be excreted via urine. Ammonia is metabolized to water-soluble urea via the urea cycle. Hyperammonaemia not only occurs during acute liver failure, but also in rare genetically determined defects of enzymes or transporters involved in the urea cycle resulting in elevated ammonia concentrations. Enzyme defects include deficiency of carbamylphosphate synthase, N-acetylglutamate synthase, ornithine transcarbamylase, argininosuccinate lyase and arginase, transporter defects are citrin deficiency and HHH-syndrome. These urea cycle defects (UCD) mostly manifest for the first time during the neonatal period, infancy or childhood, however first clinical manifestations including encephalopathy may be observed in adulthood in milder forms. Therefore, physicians treating adults should be aware of clinical symptoms in UCD to make a timely diagnosis and initiate treatment. In adulthood, clinical symptoms are often uncharacteristic including headache, avoidance of high-protein food, psychiatric symptoms triggered by heavy exercise or delivery of a child, autism, attention deficit, lethargy, developmental delay and epilepsy. Elevated ammonia concentrations in blood are the biochemical hallmark. Some UCDs can be diagnosed at metabolite level, others only at genetic level. Treatment consists of eucaloric, low-protein diet supplemented with essential amino acids and vitamins/trace elements, and intake of arginine or citrulline. Pharmacological scavengers of nitrogen are benzoate and butyrate. If conservative therapy fails, hemodialysis should be considered. Prompt treatment during acute crises is essential for optimal outcome. Liver transplantation is considered in metabolically unstable patients. For arginase deficiency, enzyme replacement therapy is available.
2009-06-16 | The Human and Mouse SLC25A29 Mitochondrial Transporters Rescue the Deficient Ornithine Metabolism in Fibroblasts of Patients With the Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) Syndrome
The hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a disorder of the urea cycle (UCD) and ornithine degradation pathway caused by mutations in the mitochondrial ornithine transporter (ORNT1). Unlike other UCDs, HHH syndrome is characterized by a less severe and variable phenotype that we believe may, in part, be due to genes with redundant function to ORNT1, such as the previously characterized ORNT2 gene. We reasoned that SLC25A29, a member of the same subfamily of mitochondrial carrier proteins as ORNT1 and ORNT2, might also have overlapping function with ORNT1. Here, we report that both the human and mouse SLC25A29, previously identified as mitochondrial carnitine/acyl-carnitine transporter-like, when overexpressed transiently also rescues the impaired ornithine transport in cultured HHH fibroblasts. Moreover, we observed that, in the mouse, the Slc25a29 message is more significantly expressed in the CNS and cultured astrocytes when compared with the liver and kidney. These results suggest a potential physiologic role for the SLC25A29 transporter in the oxidation of fatty acids, ornithine degradation pathway, and possibly the urea cycle. Our results show that SLC25A29 is the third human mitochondrial ornithine transporter, designated as ORNT3, which may contribute to the milder and variable phenotype seen in patients with HHH syndrome.
1999-06-29 | Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrome is caused by mutations in a gene encoding a mitochondrial ornithine transporter.
Neurospora crassa ARG13 and Saccharomyces cerevisiae ARG11 encode mitochondrial carrier family (MCF) proteins that transport ornithine across the mitochondrial inner membrane. We used their sequences to identify EST candidates that partially encode orthologous mammalian transporters. We thereby identified such a gene (ORNT1) that maps to 13q14 and whose expression, similar to that of other urea cycle (UC) components, was high in liver and varied with changes in dietary protein. ORNT1 expression restores ornithine metabolism in fibroblasts from patients with hyperammonaemia-hyperornithinaemia-homocitrullinuria (HHH) syndrome. In a survey of 11 HHH probands, we identified 3 ORNT1 mutant alleles that account for 21 of 22 possible mutant ORNT1 genes in our patients: F188delta, which is common in French-Canadian HHH patients and encodes an unstable protein; E180K, which encodes a stable, properly targeted protein that is inactive; and a 13q14 microdeletion. Our results show that ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome.
small molecules
2025-11-27 | Impact of high-dose vitamins on a patient with hyperornithinemia-hyperammonemia-homocitrullinuria syndrome
Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive metabolic disorder caused by a mutation of SLC25A15 encoding for the mitochondrial ornithine carrier. Recent study suggested that impaired mitochondrial function may play a major role in central nervous system impairment in HHH syndrome. We hypothesized that if the mitochondrial dysfunction was associated with neurological impairment in HHH syndrome, antioxidants might be effective to avoid aggravation of the brain injury. We presented the clinical outcome in a patient with HHH syndrome undergoing a high-dose vitamin treatment and discussed the effect of this treatment. The male patient was definitively diagnosed by compound heterogeneous pathogenic variants in the SLC25A15 gene. He was started vitamin B1, C and E therapy at age of 3 years. The blood lactate (Lac) levels gradually decreased and serum β-hydroxybutyrate/acetoacetate (β-OHB/AcAc) ratio also decreased at age of 7 years. A verbal development score in Kyoto Scale of Psychological Development was 65 at age of 3 years. However, quotient of Verbal Comprehension Index in Wechsler Intelligence Scale for Children - Fourth Edition increased to 109 at age of 7 years during this vitamin therapy. The outcome in our patient suggested that high-dose vitamin therapy might contribute to preventing cognitive disability. Moreover, the decreased blood Lac level and serum β-OHB/AcAc ratio also suggested that the effect of this therapy might be due to improving the mitochondrial function such as citric acid cycle . This case report suggest that the high-dose vitamin therapy may enhance mitochondrial function in HHH syndrome.
2025-09-05 | What are the effective treatments for managing symptoms of hyperornithinemia-hyperammonemia-homocitrullinuria syndrome?
Effective management of HHH syndrome involves dietary protein restriction, essential amino acid supplementation, use of nitrogen-scavenging drugs, and potentially liver transplantation.
2024-06-21 | Postoperative hyperammonemic encephalopathy due to unexpected constipation in a patient with hyperornithinemia-hyperammonemia-homocitrullinuria syndrome: a case report.
Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive urea cycle disorder associated with a high risk of exacerbation of hyperammonemia during the perioperative period. Here, we describe an adult patient with HHH syndrome who developed hyperammonemic encephalopathy secondary to postoperative constipation. A 52-year-old patient with HHH syndrome underwent intrathecal baclofen pump insertion for lower limb spasticity under general anesthesia. The surgery was uneventful, without any increase in serum ammonia levels. However, after surgery, he was constipated, and on postoperative day (POD) 3, he fell into a coma with an exacerbation of hyperammonemia (894 µg/dL). After administering a glycerin enema, he defecated, leading to a rapid decrease in serum ammonia levels to 165 µg/dL. He regained consciousness, and serum ammonia levels remained stable as long as he defecated. We suggest strict management of defecation during the perioperative period to prevent hyperammonemia in patients with HHH syndrome.
2024-03-07 | Amyloidogenic Propensity of Metabolites in the Uric Acid Pathway and Urea Cycle Critically Impacts the Etiology of Metabolic Disorders.
Novel insights into the etiology of metabolic disorders have recently been uncovered through the study of metabolite amyloids. In particular, inborn errors of metabolism (IEMs), including gout, Lesch-Nyhan syndrome (LNS), xanthinuria, citrullinemia, and hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, are attributed to the dysfunction of the urea cycle and uric acid pathway. In this study, we endeavored to understand and mechanistically characterize the aggregative property exhibited by the principal metabolites of the urea cycle and uric acid pathway, specifically hypoxanthine, xanthine, citrulline, and ornithine. Employing scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM), we studied the aggregation profiles of the metabolites. Insights obtained through molecular dynamics (MD) simulation underscore the vital roles of π-π stacking and hydrogen bonding interactions in the self-assembly process, and thioflavin T (ThT) assays further corroborate the amyloid nature of these metabolites. The in vitro MTT assay revealed the cytotoxic trait of these assemblies, a finding that was substantiated by in vivo assays employing the Caenorhabditis elegans (C. elegans) model, which revealed that the toxic effects were more pronounced and dose-specific in the case of metabolites that had aged via longer preincubation. We hence report a compelling phenomenon wherein these metabolites not only aggregate but transform into a soft, ordered assembly over time, eventually crystallizing upon extended incubation, leading to pathological implications. Our study suggests that the amyloidogenic nature of the involved metabolites could be a common etiological link in IEMs, potentially providing a unified perspective to study their pathophysiology, thus offering exciting insights into the development of targeted interventions for these metabolic disorders.
2022-02-18 | Reversible Leukoencephalopathy in a Man with Childhood-onset Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome.
A 49-year-old Japanese man had shown developmental delay, learning difficulties, epilepsy, and slowly progressive gait disturbance in elementary school. At 46 years old, he experienced repeated drowsiness with or without generalized convulsions, and hyperammonemia was detected. Brain magnetic resonance imaging detected multiple cerebral white matter lesions. An electroencephalogram showed diffuse slow basic activities with 2- to 3-Hz δ waves. Genetic tests confirmed a diagnosis of hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome. Leukoencephalopathy was resolved following the administration of L-arginine and lactulose with a decrease in plasma ammonia levels and glutamine-glutamate peak on magnetic resonance spectroscopy. Leukoencephalopathy in HHH syndrome may be reversible with the resolution of hyperammonemia-induced glutamine toxicity.
cell therapies
2025-10-23 | Liver transplantation can prevent the progression of neurological damage in hyperornithinemia-hyperammonemia-homocitrullinuria syndrome and maintain long-term metabolic stability - The largest single-center experience.
Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) syndrome is a rare urea cycle disorder caused by mutations in the SLC25A15 gene, leading to metabolic and neurological impairments. Liver transplantation (LT) may restore urea cycle function and prevent disease progression. This retrospective study analyzed six patients with HHH syndrome who underwent LT between 2016 and 2022. Pre- and post-transplant evaluations included biochemical tests, genetic analysis, neurological assessments, and quality-of-life measures. LT successfully normalized metabolic parameters (ammonia and amino acid levels) and allowed patients to resume normal diets. Early transplantation resulted in neurological improvement in 5 of 6 patients (83.3%), including reduced lower limb spasticity and improved walking ability. Two patients (33.3%) achieved nearly normal gait, and one patient (16.7%) recovered to normal motor function within three months after LT. Quality-of-life scores improved in 2 patients (33.3%). The overall survival rate was 83.3%, with one patient dying from unrelated causes 5 years post-transplant. No significant long-term complications were observed in the surviving patients. Liver transplantation is an effective treatment for HHH syndrome, halting neurological decline and improving quality of life. Early LT before irreversible damage provides the best outcomes, making it a viable option for patients with progressive symptoms unresponsive to conventional therapies. Not applicable.
gene therapies
2025-04-26 | Urea cycle defects in adulthood: clinical presentation, diagnosis and treatment in genetically encoded hepatic metabolic disorders with a potential for encephalopathy.
Hyperammonaemia is an important cause for encephalopathy. Ammonia is the waste product of amino acid degradation and cannot be excreted via urine. Ammonia is metabolized to water-soluble urea via the urea cycle. Hyperammonaemia not only occurs during acute liver failure, but also in rare genetically determined defects of enzymes or transporters involved in the urea cycle resulting in elevated ammonia concentrations. Enzyme defects include deficiency of carbamylphosphate synthase, N-acetylglutamate synthase, ornithine transcarbamylase, argininosuccinate lyase and arginase, transporter defects are citrin deficiency and HHH-syndrome. These urea cycle defects (UCD) mostly manifest for the first time during the neonatal period, infancy or childhood, however first clinical manifestations including encephalopathy may be observed in adulthood in milder forms. Therefore, physicians treating adults should be aware of clinical symptoms in UCD to make a timely diagnosis and initiate treatment. In adulthood, clinical symptoms are often uncharacteristic including headache, avoidance of high-protein food, psychiatric symptoms triggered by heavy exercise or delivery of a child, autism, attention deficit, lethargy, developmental delay and epilepsy. Elevated ammonia concentrations in blood are the biochemical hallmark. Some UCDs can be diagnosed at metabolite level, others only at genetic level. Treatment consists of eucaloric, low-protein diet supplemented with essential amino acids and vitamins/trace elements, and intake of arginine or citrulline. Pharmacological scavengers of nitrogen are benzoate and butyrate. If conservative therapy fails, hemodialysis should be considered. Prompt treatment during acute crises is essential for optimal outcome. Liver transplantation is considered in metabolically unstable patients. For arginase deficiency, enzyme replacement therapy is available.
2009-06-16 | The Human and Mouse SLC25A29 Mitochondrial Transporters Rescue the Deficient Ornithine Metabolism in Fibroblasts of Patients With the Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) Syndrome
The hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a disorder of the urea cycle (UCD) and ornithine degradation pathway caused by mutations in the mitochondrial ornithine transporter (ORNT1). Unlike other UCDs, HHH syndrome is characterized by a less severe and variable phenotype that we believe may, in part, be due to genes with redundant function to ORNT1, such as the previously characterized ORNT2 gene. We reasoned that SLC25A29, a member of the same subfamily of mitochondrial carrier proteins as ORNT1 and ORNT2, might also have overlapping function with ORNT1. Here, we report that both the human and mouse SLC25A29, previously identified as mitochondrial carnitine/acyl-carnitine transporter-like, when overexpressed transiently also rescues the impaired ornithine transport in cultured HHH fibroblasts. Moreover, we observed that, in the mouse, the Slc25a29 message is more significantly expressed in the CNS and cultured astrocytes when compared with the liver and kidney. These results suggest a potential physiologic role for the SLC25A29 transporter in the oxidation of fatty acids, ornithine degradation pathway, and possibly the urea cycle. Our results show that SLC25A29 is the third human mitochondrial ornithine transporter, designated as ORNT3, which may contribute to the milder and variable phenotype seen in patients with HHH syndrome.
1999-06-29 | Hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrome is caused by mutations in a gene encoding a mitochondrial ornithine transporter.
Neurospora crassa ARG13 and Saccharomyces cerevisiae ARG11 encode mitochondrial carrier family (MCF) proteins that transport ornithine across the mitochondrial inner membrane. We used their sequences to identify EST candidates that partially encode orthologous mammalian transporters. We thereby identified such a gene (ORNT1) that maps to 13q14 and whose expression, similar to that of other urea cycle (UC) components, was high in liver and varied with changes in dietary protein. ORNT1 expression restores ornithine metabolism in fibroblasts from patients with hyperammonaemia-hyperornithinaemia-homocitrullinuria (HHH) syndrome. In a survey of 11 HHH probands, we identified 3 ORNT1 mutant alleles that account for 21 of 22 possible mutant ORNT1 genes in our patients: F188delta, which is common in French-Canadian HHH patients and encodes an unstable protein; E180K, which encodes a stable, properly targeted protein that is inactive; and a 13q14 microdeletion. Our results show that ORNT1 encodes the mitochondrial ornithine transporter involved in UC function and is defective in HHH syndrome.
small molecules
2025-11-27 | Impact of high-dose vitamins on a patient with hyperornithinemia-hyperammonemia-homocitrullinuria syndrome
Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive metabolic disorder caused by a mutation of SLC25A15 encoding for the mitochondrial ornithine carrier. Recent study suggested that impaired mitochondrial function may play a major role in central nervous system impairment in HHH syndrome. We hypothesized that if the mitochondrial dysfunction was associated with neurological impairment in HHH syndrome, antioxidants might be effective to avoid aggravation of the brain injury. We presented the clinical outcome in a patient with HHH syndrome undergoing a high-dose vitamin treatment and discussed the effect of this treatment. The male patient was definitively diagnosed by compound heterogeneous pathogenic variants in the SLC25A15 gene. He was started vitamin B1, C and E therapy at age of 3 years. The blood lactate (Lac) levels gradually decreased and serum β-hydroxybutyrate/acetoacetate (β-OHB/AcAc) ratio also decreased at age of 7 years. A verbal development score in Kyoto Scale of Psychological Development was 65 at age of 3 years. However, quotient of Verbal Comprehension Index in Wechsler Intelligence Scale for Children - Fourth Edition increased to 109 at age of 7 years during this vitamin therapy. The outcome in our patient suggested that high-dose vitamin therapy might contribute to preventing cognitive disability. Moreover, the decreased blood Lac level and serum β-OHB/AcAc ratio also suggested that the effect of this therapy might be due to improving the mitochondrial function such as citric acid cycle . This case report suggest that the high-dose vitamin therapy may enhance mitochondrial function in HHH syndrome.
2025-09-05 | What are the effective treatments for managing symptoms of hyperornithinemia-hyperammonemia-homocitrullinuria syndrome?
Effective management of HHH syndrome involves dietary protein restriction, essential amino acid supplementation, use of nitrogen-scavenging drugs, and potentially liver transplantation.
2024-06-21 | Postoperative hyperammonemic encephalopathy due to unexpected constipation in a patient with hyperornithinemia-hyperammonemia-homocitrullinuria syndrome: a case report.
Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a rare autosomal recessive urea cycle disorder associated with a high risk of exacerbation of hyperammonemia during the perioperative period. Here, we describe an adult patient with HHH syndrome who developed hyperammonemic encephalopathy secondary to postoperative constipation. A 52-year-old patient with HHH syndrome underwent intrathecal baclofen pump insertion for lower limb spasticity under general anesthesia. The surgery was uneventful, without any increase in serum ammonia levels. However, after surgery, he was constipated, and on postoperative day (POD) 3, he fell into a coma with an exacerbation of hyperammonemia (894 µg/dL). After administering a glycerin enema, he defecated, leading to a rapid decrease in serum ammonia levels to 165 µg/dL. He regained consciousness, and serum ammonia levels remained stable as long as he defecated. We suggest strict management of defecation during the perioperative period to prevent hyperammonemia in patients with HHH syndrome.
2024-03-07 | Amyloidogenic Propensity of Metabolites in the Uric Acid Pathway and Urea Cycle Critically Impacts the Etiology of Metabolic Disorders.
Novel insights into the etiology of metabolic disorders have recently been uncovered through the study of metabolite amyloids. In particular, inborn errors of metabolism (IEMs), including gout, Lesch-Nyhan syndrome (LNS), xanthinuria, citrullinemia, and hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, are attributed to the dysfunction of the urea cycle and uric acid pathway. In this study, we endeavored to understand and mechanistically characterize the aggregative property exhibited by the principal metabolites of the urea cycle and uric acid pathway, specifically hypoxanthine, xanthine, citrulline, and ornithine. Employing scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM), we studied the aggregation profiles of the metabolites. Insights obtained through molecular dynamics (MD) simulation underscore the vital roles of π-π stacking and hydrogen bonding interactions in the self-assembly process, and thioflavin T (ThT) assays further corroborate the amyloid nature of these metabolites. The in vitro MTT assay revealed the cytotoxic trait of these assemblies, a finding that was substantiated by in vivo assays employing the Caenorhabditis elegans (C. elegans) model, which revealed that the toxic effects were more pronounced and dose-specific in the case of metabolites that had aged via longer preincubation. We hence report a compelling phenomenon wherein these metabolites not only aggregate but transform into a soft, ordered assembly over time, eventually crystallizing upon extended incubation, leading to pathological implications. Our study suggests that the amyloidogenic nature of the involved metabolites could be a common etiological link in IEMs, potentially providing a unified perspective to study their pathophysiology, thus offering exciting insights into the development of targeted interventions for these metabolic disorders.
2022-02-18 | Reversible Leukoencephalopathy in a Man with Childhood-onset Hyperornithinemia-Hyperammonemia-Homocitrullinuria Syndrome.
A 49-year-old Japanese man had shown developmental delay, learning difficulties, epilepsy, and slowly progressive gait disturbance in elementary school. At 46 years old, he experienced repeated drowsiness with or without generalized convulsions, and hyperammonemia was detected. Brain magnetic resonance imaging detected multiple cerebral white matter lesions. An electroencephalogram showed diffuse slow basic activities with 2- to 3-Hz δ waves. Genetic tests confirmed a diagnosis of hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome. Leukoencephalopathy was resolved following the administration of L-arginine and lactulose with a decrease in plasma ammonia levels and glutamine-glutamate peak on magnetic resonance spectroscopy. Leukoencephalopathy in HHH syndrome may be reversible with the resolution of hyperammonemia-induced glutamine toxicity.
cell therapies
2025-10-23 | Liver transplantation can prevent the progression of neurological damage in hyperornithinemia-hyperammonemia-homocitrullinuria syndrome and maintain long-term metabolic stability - The largest single-center experience.
Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) syndrome is a rare urea cycle disorder caused by mutations in the SLC25A15 gene, leading to metabolic and neurological impairments. Liver transplantation (LT) may restore urea cycle function and prevent disease progression. This retrospective study analyzed six patients with HHH syndrome who underwent LT between 2016 and 2022. Pre- and post-transplant evaluations included biochemical tests, genetic analysis, neurological assessments, and quality-of-life measures. LT successfully normalized metabolic parameters (ammonia and amino acid levels) and allowed patients to resume normal diets. Early transplantation resulted in neurological improvement in 5 of 6 patients (83.3%), including reduced lower limb spasticity and improved walking ability. Two patients (33.3%) achieved nearly normal gait, and one patient (16.7%) recovered to normal motor function within three months after LT. Quality-of-life scores improved in 2 patients (33.3%). The overall survival rate was 83.3%, with one patient dying from unrelated causes 5 years post-transplant. No significant long-term complications were observed in the surviving patients. Liver transplantation is an effective treatment for HHH syndrome, halting neurological decline and improving quality of life. Early LT before irreversible damage provides the best outcomes, making it a viable option for patients with progressive symptoms unresponsive to conventional therapies. Not applicable.
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Drug Discovery Landscape
3 orphan drug designations for Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, including 1 approved therapy.
3 orphan drug designations for Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, including 1 approved therapy.
Drug | Therapy type | Regulator | Orphan designation | Approval | Sponsor |
|---|---|---|---|---|---|
Sodium benzoate | small molecules | EMA | 2016-08-29 | — | Lucane Pharma SA |
Heterologous human adult liver-derived progenitor cells | cell therapies | EMA | 2013-07-17 | — | Cellaion |
Glyceryl tri-(4-phenylbutyrate) [Ravicti] | small molecules | EMA | 2010-06-10 | 2015-12-01 | Immedica Pharma AB |
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