Endogenous galactose formation in galactose-1-phosphate uridyltransferase deficiency

Restoring galactose metabolism without restoring GALT rescues both compromised survival in larvae and an adult climbing deficit in a GALT-null D. Melanogaster model of classic galactosemia

Classic galactosemia (CG) is an autosomal recessive disorder that results from profound deficiency of galactose-1-phosphate uridylyltransferase (GALT), the middle enzyme in the highly conserved Leloir pathway of galactose metabolism. That galactose metabolism is disrupted in patients with CG, and in GALT-null microbial, cell culture, and animal models of CG, has been known for many years. However, whether the long-term developmental complications of CG result from disrupted galactose metabolism alone, or from loss of some independent moonlighting function of GALT, in addition to disrupted galactose metabolism, has been posed but never resolved. Here, we addressed this question using a GALT-null Drosophila melanogaster model of CG engineered to express uridine diphosphate (UDP)-glucose/galactose pyrophosphorylase (UGGP), a plant enzyme that effectively bypasses GALT in the Leloir pathway by converting substrates uridine triphosphate (UTP) plus galactose-1-phosphate (gal-1P) into products UDP-galactose plus pyrophosphate (PPi). While GALT and UGGP share one substrate (gal-1P) and one product (UDP-galactose), they are structurally and evolutionarily unrelated enzymes. It is therefore extremely unlikely that they would also share a moonlighting function. We found that GALT-null flies expressing UGGP showed not only partial rescue of metabolic abnormalities and acute larval sensitivity to dietary galactose, as expected, but also full rescue of an adult motor deficit otherwise seen in this model. By extension, these results may offer insights to the underlying bases of at least some acute and long-term complications experienced by patients with CG.

Lactose and Galactose Content in Spanish Cheeses: Usefulness in the Dietary Treatment of Patients with Galactosaemia

In galactosaemia, a strict galactose-free diet is necessary to prevent or resolve acute symptoms in infants. However, because the body produces up to 10 times more galactose than is found in a galactose-restricted diet, excessively restrictive diets should be avoided in children and adults to prevent nutritional deficiencies. Since cheese is a nutritional source of the calcium necessary for bone health, the latest international guidelines on the management of classical galactosaemia (2017) allow the consumption of cured cheeses with less than 25 mg of galactose/100 g and recommend that each country verifies the adequacy of the cheeses, since most mature cheeses do not always have a lower galactose content. In total, 32 cheese samples were purchased (19 Spanish and 13 international cheeses), and their lactose and galactose contents were analysed using ion chromatography with pulsed amperometric detection (IC-PAD). Five Spanish cheeses contained less than 25 mg of galactose/100 g: García Baquero semi-cured cheese; Hacendado, Gran Reserva and Mahón cured cheeses; and García Baquero Reserva 12-month cured cheese. In addition, eight international cheeses were confirmed as suitable: Comté, Gouda, Gruyere, Maasdam, Parmigiano, Edam, Emmental, and some samples of Cheddar. In addition to the well-known low-galactose Swiss and Dutch cheeses, according to the current results, five Spanish cheeses can be safely consumed. The greater availability of types of cheese favours better bone health in patients with galactosaemia.

Pathophysiology of long-term complications in classic galactosemia: What we do and do not know

Despite many decades of research involving both human subjects and model systems, the underlying pathophysiology of long-term complications in classic galactosemia (CG) remains poorly understood. In this review, intended for those already familiar with galactosemia, we focus on the big questions relating to outcomes, mechanism, and markers, drawing on relevant literature where available, attempting to navigate inconsistencies where they appear, and acknowledging gaps in knowledge where they persist.

Messenger RNA as a personalized therapy: The moment of truth for rare metabolic diseases

Inborn errors of metabolism (IEM) encompass a group of monogenic diseases affecting both pediatric and adult populations and currently lack effective treatments. Some IEM such as familial hypercholesterolemia or X-linked protoporphyria are caused by gain of function mutations, while others are characterized by an impaired protein function, causing a metabolic pathway blockage. Pathophysiology classification includes intoxication, storage and energy-related metabolic disorders. Factors specific to each disease trigger acute metabolic decompensations. IEM require prompt and effective care, since therapeutic delay has been associated with the development of fatal events including severe metabolic acidosis, hyperammonemia, cerebral edema, and death. Rapid expression of therapeutic proteins can be achieved hours after the administration of messenger RNAs (mRNA), representing an etiological solution for acute decompensations. mRNA-based therapy relies on modified RNAs with enhanced stability and translatability into therapeutic proteins. The proteins produced in the ribosomes can be targeted to specific intracellular compartments, the cell membrane, or be secreted. Non-immunogenic lipid nanoparticle formulations have been optimized to prevent RNA degradation and to allow safe repetitive administrations depending on the disease physiopathology and clinical status of the patients, thus, mRNA could be also an effective chronic treatment for IEM. Given that the liver plays a key role in most of metabolic pathways or can be used as bioreactor for excretable proteins, this review focuses on the preclinical and clinical evidence that supports the implementation of mRNA technology as a promising personalized strategy for liver metabolic disorders such as acute intermittent porphyria, ornithine transcarbamylase deficiency or glycogen storage disease.

Neonatal GALT gene replacement offers metabolic and phenotypic correction through early adulthood in a rat model of classic galactosemia

Classic galactosemia (CG) results from profound deficiency of galactose-1-P uridylyltransferase (GALT). Despite early detection by newborn screening and lifelong dietary restriction of galactose, most patients grow to experience a range of long-term complications. Recently, we developed and characterized a GALT-null rat model of CG and demonstrated that AAV9-hGALT, administered by tail vein injection to neonatal pups, dramatically improved plasma, liver, and brain galactose metabolites at 2 weeks posttreatment. Here we report a time-course study of GALT restoration in rats treated as neonates with scAAV9-hGALT and harvested at 8, 14, 30, and 60 days. Cohorts of rats in the two older groups were weaned to diets containing either 1% or 3% of calories from galactose. As expected, GALT activity in all treated animals peaked early and then diminished over time, most notably in liver, ostensibly due to dilution of the nonreplicating episomal vector as transduced cells divided. All treated rats showed dramatic metabolic rescue through 1 month, and those weaned to the lower galactose diet showed continued strong metabolic rescue into adulthood (2 months). Prepubertal growth delay and cataracts were both partially rescued by treatment. Finally, we found that UDP glucose pyrophosphorylase (UGP), which offers a metabolic bypass around missing GALT, was 3-fold more active in brain samples from adult rats than from young pups, offering a possible explanation for the improved ability of older GALT-null rats to metabolize galactose. Combined, these results document promising metabolic and phenotypic efficacy of neonatal GALT gene replacement in a rat model of classic galactosemia.

Two consecutive pregnancies in a patient with premature ovarian insufficiency in the course of classic galactosemia and a review of the literature

AIM

To present a case report of a patient with classic galactosemia and the Q188R/K285N GALT mutation, who conceived spontaneously twice despite severe ovarian failure. A review of the literature is included.

MATERIALS AND METHODS

A 20-year-old patient with classic galactosemia and premature ovarian insufficiency (POI) came to our attention. We performed a routine hormonal and ultrasound examination confirming low ovarian reserve. Due to low rates of pregnancies in individuals with POI (5%-10%), we were almost certain of the infeasibility of pregnancy.

RESULTS

Surprisingly, several months after hospitalization, the patient conceived without any medical intervention and less than a year after the first birth she became pregnant again. While reviewing the literature, 90 pregnancies among galactosemic patients were identified.

CONCLUSIONS

Ovarian failure is a long-term diet-independent complication of classic galactosemia, pertaining to about 90% of affected individuals. This case confirms its unpredicted course, as even the presence of unfavorable factors (absence of spontaneous puberty, early diagnosis of POI, undetectable AMH) may not preclude the chance for conception.

Galactosemia: Towards Pharmacological Chaperones

Galactosemia is a rare inherited metabolic disease resulting from mutations in the four genes which encode enzymes involved in the metabolism of galactose. The current therapy, the removal of galactose from the diet, is inadequate. Consequently, many patients suffer lifelong physical and cognitive disability. The phenotype varies from almost asymptomatic to life-threatening disability. The fundamental biochemical cause of the disease is a decrease in enzymatic activity due to failure of the affected protein to fold and/or function correctly. Many novel therapies have been proposed for the treatment of galactosemia. Often, these are designed to treat the symptoms and not the fundamental cause. Pharmacological chaperones (PC) (small molecules which correct the folding of misfolded proteins) represent an exciting potential therapy for galactosemia. In theory, they would restore enzyme function, thus preventing downstream pathological consequences. In practice, no PCs have been identified for potential application in galactosemia. Here, we review the biochemical basis of the disease, identify opportunities for the application of PCs and describe how these might be discovered. We will conclude by considering some of the clinical issues which will affect the future use of PCs in the treatment of galactosemia.

A pilot study of neonatal GALT gene replacement using AAV9 dramatically lowers galactose metabolites in blood, liver, and brain and minimizes cataracts in GALT-null rat pups

Classic galactosemia (CG) is a rare metabolic disorder that results from profound deficiency of galactose-1-P uridylyltransferase (GALT). Despite early detection by newborn screening and rapid and lifelong dietary restriction of galactose, which is the current standard of care, most patients grow to experience a broad constellation of long-term complications. The mechanisms underlying these complications remain unclear and likely differ by tissue. Here we conducted a pilot study testing the safety and efficacy of GALT gene replacement using our recently-described GALT-null rat model for CG. Specifically, we administered AAV9.CMV.HA-hGALT to seven GALT-null rat pups via tail vein injection on day 3 of life; eight GALT-null pups injected with PBS served as the negative control, and four GALT+ heterozygous pups injected with PBS served as the positive control. All pups were returned to their nursing mothers, weighed daily, and euthanized for tissue collection 2 weeks later. Among the AAV9-injected pups in this study, we achieved GALT levels in liver ranging from 64% to 595% wild-type, and in brain ranging from 3% to 42% wild-type. In liver, brain, and blood samples from these animals we also saw a striking drop in galactose, galactitol, and gal-1P. Finally, all treated GALT-null pups showed dramatic improvement in cataracts relative to their mock-treated counterparts. Combined, these results demonstrate that GALT restoration in both liver and brain of GALT-null rats by neonatal tail vein administration using AAV9 is not only attainable but effective.

A galactose-1-phosphate uridylyltransferase-null rat model of classic galactosemia mimics relevant patient outcomes and reveals tissue-specific and longitudinal differences in galactose metabolism

Classic galactosemia (CG) is a potentially lethal inborn error of metabolism, if untreated, that results from profound deficiency of galactose-1-phosphate uridylyltransferase (GALT), the middle enzyme of the Leloir pathway of galactose metabolism. While newborn screening and rapid dietary restriction of galactose prevent or resolve the potentially lethal acute symptoms of CG, by mid-childhood, most treated patients experience significant complications. The mechanisms underlying these long-term deficits remain unclear. Here we introduce a new GALT-null rat model of CG and demonstrate that these rats display cataracts, cognitive, motor, and growth phenotypes reminiscent of patients outcomes. We further apply the GALT-null rats to test how well blood biomarkers, typically followed in patients, reflect metabolic perturbations in other, more relevant tissues. Our results document that the relative levels of galactose metabolites seen in GALT deficiency differ widely by tissue and age, and that red blood cell Gal-1P, the marker most commonly followed in patients, shows no significant association with Gal-1P in other tissues. The work reported here establishes our outbred GALT-null rats as an effective model for at least four complications characteristic of CG, and sets the stage for future studies addressing mechanism and testing the efficacy of novel candidate interventions.

Early neurological complications in children with classical galactosemia and p.gln188arg mutation

BACKGROUND

Despite implementation of a controlled diet, children with classical galactosemia (CG) may develop a variety of developmental and cognitive problems. In this study, we examined the early developmental status of, as well as the neurological and neuroradiological findings for, children with CG.

METHODS

We retrospectively evaluated 46 galactosemia patients who were followed between 2003 and 2017. We included those who exhibited CG and p.gln188arg homozygous mutation without concomitant disease and who had undergone detailed neurological examination, brain magnetic resonance imaging (MRI), and Denver II developmental testing.

RESULTS

The mean ages at the time of the most recent neurological examination and Denver II testing were 48.5 ± 28.5 months and 34.4 ± 18.2 months, respectively. Developmental delay was defined as developmental age ≥ 20% lower than chronological age. The results were normal in 25 patients and delayed ≥ 20% in least in one domain, primarily in language development, in 21 patients. Brain MRI was abnormal in 22 patients.

CONCLUSIONS

This analysis of the youngest children with the same genetic mutation reported thus far showed that, despite treatment, developmental delays and abnormalities on brain MRI may begin at an early age.

Hereditary galactosemia

Hereditary galactosemia is an inborn error of carbohydrate metabolism. Galactose is metabolized by Leloir pathway enzymes; galactokinase (GALK), galactose-1-phosphate uridylyltransferase (GALT) and UDP-galactose 4-epimerase (GALE). The defects in these enzymes cause galactosemia in an autosomal recessive manner. The severe GALT deficiency, or classic galactosemia, is life-threatening in the newborn period. The treatment for classic galactosemia is dietary restriction of lactose. Although implementation of lactose restricted diet is efficient in resolving the acute complications, it is not sufficient to prevent long-term complications affecting the brain and female gonads, the two main target organs of damage. Implementation of molecular genetics diagnostic tools and GALT enzyme assays are instrumental in distinguishing classic galactosemia from clinical and biochemical variant forms of GALT deficiency. Better understanding of mechanisms responsible for the phenotypic variation even within the same genotype is essential to provide appropriate counseling for families. Utilization of a lactose restricted diet is also recommended for GALK deficiency and some rare forms of GALE deficiency. Novel modes of therapies are being explored; they may be beneficial if access issues to the affected tissues are circumvented and optimum use of therapeutic window is achieved.

Rigor of non-dairy galactose restriction in early childhood, measured by retrospective survey, does not associate with severity of five long-term outcomes quantified in 231 children and adults with classic galactosemia

One of many vexing decisions faced by parents of an infant with classic galactosemia (CG) is how carefully to restrict non-dairy galactose from their growing child's diet. Until recently, many experts recommended vigorous lifelong dietary restriction of milk and all high-galactose dairy products as well as some non-dairy sources of galactose such as legumes and specific fruits and vegetables. Recently, experts have begun to relax their recommendations. The new recommendations, that restrict only high galactose dairy products, were made in the face of uncertainty, however, because no sufficiently powered study had been reported testing for possible association between rigor of non-dairy galactose restriction and severity of long-term outcomes in CG. Here we describe the largest study of diet and outcomes in CG reported to date, conducted using information gathered from 231 patients with CG and 71 unaffected sibling controls. We compared rigor of dietary galactose restriction, measured using a 4-point scale by a retrospective parent-response survey, with outcomes including growth, adaptive behaviors, receipt of speech therapy, receipt of special educational services, and for girls and women, a plasma marker of ovarian function (AMH). Our results confirmed the expected differences between patients and controls, but among patients showed no significant association between rigor of non-dairy galactose restriction in early childhood and any of the outcomes quantified. Indeed, some weak associations were seen suggesting that rigorous restriction of non-dairy galactose may be deleterious rather than beneficial. Despite limitations, these findings support the ongoing trend toward diet liberalization with regard to non-dairy sources of galactose for children and adults with classic galactosemia.

Sweet and sour: an update on classic galactosemia

Classic galactosemia is a rare inherited disorder of galactose metabolism caused by deficient activity of galactose-1-phosphate uridylyltransferase (GALT), the second enzyme of the Leloir pathway. It presents in the newborn period as a life-threatening disease, whose clinical picture can be resolved by a galactose-restricted diet. The dietary treatment proves, however, insufficient in preventing severe long-term complications, such as cognitive, social and reproductive impairments. Classic galactosemia represents a heavy burden on patients' and their families' lives. After its first description in 1908 and despite intense research in the past century, the exact pathogenic mechanisms underlying galactosemia are still not fully understood. Recently, new important insights on molecular and cellular aspects of galactosemia have been gained, and should open new avenues for the development of novel therapeutic strategies. Moreover, an international galactosemia network has been established, which shall act as a platform for expertise and research in galactosemia. Herein are reviewed some of the latest developments in clinical practice and research findings on classic galactosemia, an enigmatic disorder with many unanswered questions warranting dedicated research.

Classical Galactosaemia and CDG, the N-Glycosylation Interface. A Review

Classical galactosaemia is a rare disorder of carbohydrate metabolism caused by galactose-1-phosphate uridyltransferase (GALT) deficiency (EC 2.7.7.12). The disease is life threatening if left untreated in neonates and the only available treatment option is a long-term galactose restricted diet. While this is lifesaving in the neonate, complications persist in treated individuals, and the cause of these, despite early initiation of treatment, and shared GALT genotypes remain poorly understood. Systemic abnormal glycosylation has been proposed to contribute substantially to the ongoing pathophysiology. The gross N-glycosylation assembly defects observed in the untreated neonate correct over time with treatment. However, N-glycosylation processing defects persist in treated children and adults.Congenital disorders of glycosylation (CDG) are a large group of over 100 inherited disorders affecting largely N- and O-glycosylation.In this review, we compare the clinical features observed in galactosaemia with a number of predominant CDG conditions.We also summarize the N-glycosylation abnormalities, which we have described in galactosaemia adult and paediatric patients, using an automated high-throughput HILIC-UPLC analysis of galactose incorporation into serum IgG with analysis of the corresponding N-glycan gene expression patterns and the affected pathways.

Inherited Metabolic Disorders: Aspects of Chronic Nutrition Management

The introduction of newborn screening and the development of new therapies have led to an expanding population of patients with inherited metabolic disorders, and these patients are now entering adulthood. Dietary therapy is the mainstay of treatment for many of these disorders, and thus, trained metabolic dietitians are critical members of the multidisciplinary team required for management of such patients. The main goals of dietary therapy in inborn errors of metabolism are the maintenance of normal growth and development while limiting offending metabolites and providing deficient products. Typically, the offending metabolite is either significantly reduced or removed completely from the diet and then reintroduced in small quantities until blood levels are within the normal range. Such treatment is required in infancy, childhood, and adulthood and requires careful monitoring of micronutrient and macronutrient intake throughout the life span. The goal of this review is to highlight the basic principles of chronic nutrition management of the inborn errors of protein, carbohydrate, and fat metabolism.