Many inborn errors of metabolism affect pathways involved in the synthesis of a metabolite that has an important biochemical or physiological function, and adverse effects of the disorder can be attributed to the lack of this metabolite. Thus, there is the opportunity for treatment by 'product replacement'. One of the disorders in the pathways for the synthesis of bile acids from cholesterol, 3β-hydroxy-Δ5-C27-steroid dehydrogenase deficiency, causes cholestatic liver disease in infancy that can be treated very effectively with chenodeoxycholic acid (CDCA) and/or cholic acid (CA). There are several other enzyme deficiencies that can cause liver disease in infancy that improve with CDCA or CA or both (alongside a reduction of abnormal bile acids or alcohols); however, individuals with the same gene variant(s) may remain asymptomatic or have transient liver dysfunction that resolves spontaneously. In some disorders, the more usual presentation is with neurological disease later in childhood or in adolescence or adult life, for example, cerebrotendinous xanthomatosis (CTX), α-methylacyl-CoA racemase deficiency, and oxysterol 7α-hydroxylase deficiency. Treatment with CDCA has been dramatically effective in the neurological disease of CTX. In the disorders of peroxisome biogenesis, liver disease is a part of the clinical picture although neurological symptoms tend to be predominant. Treatment with CDCA and CA (or CA alone) leads to a reduction in the levels of C27 bile acids. Some trials suggest this treatment leads to significant improvement in clinical status and liver function tests; others do not. Defects in individual peroxisomal enzymes and transporters vary in their clinical presentations. Treatment of acyl-CoA oxidase 2 deficiency with ursodeoxycholic acid is discussed.

Retinitis pigmentosa (RP) is a progressive inherited retinal dystrophy, characterized by the degeneration of photoreceptors, presenting as a rod-cone dystrophy. Approximately 20-30% of patients with RP also exhibit extra-ocular manifestations in the context of a syndrome. This manuscript discusses the broad spectrum of syndromes associated with RP, pathogenic mechanisms, clinical manifestations, differential diagnoses, clinical management approaches, and future perspectives. Given the diverse clinical and genetic landscape of syndromic RP, the diagnosis may be challenging. However, an accurate and timely diagnosis is essential for optimal clinical management, prognostication, and potential treatment. Broadly, the syndromes associated with RP can be categorized into ciliopathies, inherited metabolic disorders, mitochondrial disorders, and miscellaneous syndromes. Among the ciliopathies associated with RP, Usher syndrome and Bardet-Biedl syndrome are the most well-known. Less common ciliopathies include Cohen syndrome, Joubert syndrome, cranioectodermal dysplasia, asphyxiating thoracic dystrophy, Mainzer-Saldino syndrome, and RHYNS syndrome. Several inherited metabolic disorders can present with RP, including Zellweger spectrum disorders, adult Refsum disease, α-methylacyl-CoA racemase deficiency, certain mucopolysaccharidoses, ataxia with vitamin E deficiency, abetalipoproteinemia, several neuronal ceroid lipofuscinoses, mevalonic aciduria, PKAN/HARP syndrome, PHARC syndrome, and methylmalonic acidaemia with homocystinuria type cobalamin (cbl) C disease. Due to the mitochondria's essential role in supplying continuous energy to the retina, disruption of mitochondrial function can lead to RP, as seen in Kearns-Sayre syndrome, NARP syndrome, primary coenzyme Q10 deficiency, SSBP1-associated disease, and long chain 3-hydroxyacyl-CoA dehydrogenase deficiency. Lastly, Cockayne syndrome and PERCHING syndrome can present with RP, but they do not fit the abovementioned hierarchy and are thus categorized as miscellaneous. Several first-in-human clinical trials are underway or in preparation for some of these syndromic forms of RP.

The aim of this study was to present a case of a young female patient with asymptomatic retinal dysfunction associated with alpha-methylacyl-CoA (alpha-methylacyl-CoA racemase) racemase deficiency. This was a retrospective analysis of the medical notes of a single patient. Detailed slit-lamp examination was completed by Optos color fundus photography and enhanced depth imaging optical coherence tomography. Genetic testing was conducted to establish the diagnosis, and the patient was also referred to the Department of Neurology for further assessment. Dilated fundoscopy and ophthalmic imaging revealed bilateral retinal pigment epithelium abnormalities that could be associated with a genetic retinal disorder. Indeed, genetic testing showed that this lady was homozygous for alpha-methylacyl-CoA racemase (OMIM 604489; Gene ID 23600) variant NM 014324.6: c.154T>C; p.(Ser52Pro). She had no detectable neurological deficit. Alpha-methylacyl-CoA racemase deficiency is a rare genetic condition that can potentially contribute to retinal dystrophy through various mechanisms. In addition, it may lead to a wide spectrum of systemic signs and symptoms. Interestingly, in contrast to other reported studies, the patient was completely asymptomatic, with no evidence of systemic disorders.

Bile acid synthesis defects (BASDs) can be severely disabling involving the liver and nervous system, potentially due to elevated levels of toxic C27-bile acid intermediates. Cholic acid (CA) supplementation is hypothesized to decrease bile acid production, stimulate bile secretion and -flow, and slowing down disease progression. This systematic review assesses the clinical and biochemical effectiveness, and safety of CA in BASDs patients. A systematic review of MEDLINE, Embase and clinical trial registries (ClinicalTrials.gov, ICTRP registry) using controlled MeSH- and Emtree terms. From 526 articles 70 publications were deemed eligible for inclusion based on title and abstract. 14 publications were included after full-text assessment comprising case reports and -series with 1-35 patients (162 patients in total) receiving 1 week to 16,5 years of CA treatment. All presented data on effectiveness, 8 studies also presented data on safety. The included population concerned patients with Zellweger spectrum disorders (n = 73), 3β-Hydroxy-Δ5-C27-steroid oxidoreductase deficiency (n = 62), cerebrotendinous xanthomatosis (n = 22), Δ4-3-oxosteroid 5β-reductase deficiency (n = 13), and α-methylacyl-CoA racemase deficiency (n = 3). Main outcomes concerned liver disease (12 studies), general physical examinations, biochemical outcomes, and safety (9 studies), and fat-soluble vitamin absorption (7 studies). The overall risk of bias score was considered to be critical (1 study), serious (4 studies), and moderate (9 studies). Major issues were missing data (10 studies), generalized data (8 studies), and no wash-out between treatments (4 studies). More controlled studies are required as the available data is insufficient to draw definite conclusions on the effectiveness and safety of CA treatment in BASD patients. Establishing an independent international disease registry could better utilize existing real-world data.

Alpha-methylacyl-CoA-racemase (AMACR) deficiency (MIM#604489) is a peroxisomal disorder resulting in the accumulation of pristanic acid, dihydroxycholestanoic acid (DHCA), and trihydroxycholestanoic acid (THCA), with variable clinical features and age of onset from infancy to late adulthood. The purpose of this report is to define clinical variations and follow-up data in AMACR deficiency emphasizing treatment with a review of cases reported in the literature. Here, four patients, from two families, diagnosed with AMACR deficiency and showing phenotypic heterogeneity are presented. A 10-month-old-female presented with coagulopathy, hepatic dysfunction, and elevated pristanic acid, DHCA, and THCA levels. Genetic testing confirmed a homozygous variant c.596G>A in the AMACR gene. Her brother who had macrovesicular hepatosteatosis and elevated pristanic acid levels was diagnosed with family screening. The third patient presented with rhabdomyolysis following a strenuous exercise without any other complaint. Homozygous novel c.1006G>A variant was found on the AMACR gene. His asymptomatic sister carrying the same variant also had elevated pristanic acid levels. They had normal neuropsychologic evaluation. Dietary treatment with low phytanic and pristanic acid content was recommended to the patients but all showed poor compliance. The sibling pairs were followed for periods of 11 and 7 years, respectively. AMACR deficiency is usually described as an adult-onset disorder with neuropsychological problems. The characterization of natural history and new clinical phenotypes may support earlier diagnosis and treatment.