Molecular and structural analyses of maple syrup urine disease and identification of a founder mutation in a Portuguese Gypsy community

Treatment of maple syrup urine disease: Benefits, risks, and challenges of liver transplantation

Maple syrup urine disease (MSUD) is caused by a deficiency in the activity of the branched-chain α-ketoacid dehydrogenase (BCKD) complex, promoting the accumulation of the branched-chain amino acids (BCAA) leucine, isoleucine, and valine, as well as their respective α-keto acids. MSUD is an autosomal recessive hereditary metabolic disorder characterized by ketoacidosis, ataxia, coma, and mental and psychomotor retardation. The mechanisms involved in the brain damage caused by MSUD are not fully understood. Early diagnosis and treatment, as well as proper control of metabolic decompensation crises, are crucial for patients' survival and for a better prognosis. The recommended treatment consists of a high-calorie diet with restricted protein intake and specific formulas containing essential amino acids, except those accumulated in MSUD. This treatment will be maintained throughout life, being adjusted according to the patients' nutritional needs and BCAA concentration. Because dietary treatment may not be sufficient to prevent neurological damage in MSUD patients, other therapeutic strategies have been studied, including liver transplantation. With transplantation, it is possible to obtain an increase of about 10% of the normal BCKD in the body, an amount sufficient to maintain amino acid homeostasis and reduce metabolic decompensation crises. However, the experience related to this practice is very limited when considering the shortage of liver for transplantation and the risks related to the surgical procedure and immunosuppression. Thus, the purpose of this review is to survey the benefits, risks, and challenges of liver transplantation in the treatment of MSUD.

Administration of branched-chain amino acids alters epigenetic regulatory enzymes in an animal model of Maple Syrup Urine Disease

Maple Syrup Urine Disease (MSUD) is an autosomal recessive inherited disorder that affects the activity of the branched-chainα-keto acid dehydrogenase complex (BCDK). This deficiency on BCDK complex results in the accumulation of branched-chain amino acids (BCAA) leucine, isoleucine, valine, and their corresponding α-keto acids. Epigenetic changes can negatively affect the metabolism of BCAA. These changes are catalyzed by the epigenetic regulatory enzymes, e.g., DNA methyltransferase (DNMT), histone deacetylases (HDAC), and histone acetyltransferases (HAT). However, the impacts of BCAA administration on the activity of epigenetic regulatory enzymes in the brain of MSUD patients are still unknown. In this study, we aimed to demonstrate the impact of BCAA administration on the activity of DNMT, HDAC, and HAT in the brain structures of infant rats, an animal model of MSUD. For that, we administered a BCAA pool to infant rats for 21 days. We demonstrated that BCAA administration significantly increased the DNMT and HDAC activities in the hippocampus and striatum, but not in the cerebral cortex of MSUD infant rats. A positive correlation was observed between HDAC and DNMT activities in the hippocampus and striatum of animals exposed to BCAA injections. Our results showed that the BCAA administration could modulate epigenetic regulatory enzymes, mainly DNMT and HDAC, in the brains of infant rats. Therefore, we suggest that the increase in the activity of DNMT and HDAC in the hippocampus and striatum could partially explain the neurological impairments presented in animal models of MSUD.

Maple syrup urine disease in Brazilian patients: variants and clinical phenotype heterogeneity

Background

Maple syrup urine disease (MSUD) is an autosomal recessive inherited metabolic disease caused by deficient activity of the branched-chain α-keto acid dehydrogenase (BCKD) enzymatic complex. BCKD is a mitochondrial complex encoded by BCKDHA, BCKDHB, DBT, and DLD genes. MSUD is predominantly caused by Variants in BCKDHA, BCKDHB, and DBT genes encoding the E1α, E1β, and E2 subunits of BCKD complex, respectively. The aim of this study was to characterize the genetic basis of MSUD by identifying the point variants in BCKDHA, BCKDHB, and DBT genes in a cohort of Brazilian MSUD patients and to describe their phenotypic heterogeneity. It is a descriptive cross-sectional study with 21 MSUD patients involving molecular genotyping by Sanger sequencing.

Results

Eight new variants predicted as pathogenic were found between 30 variants (damaging and non-damaging) identified in the 21 patients analyzed: one in the BCKDHA gene (p.Tyr120Ter); five in the BCKDHB gene (p.Gly131Val, p.Glu146Glnfs * 13, p.Phe149Cysfs * 9, p.Cys207Phe, and p.Lys211Asn); and two in the DBT gene (p.Glu148Ter and p.Glu417Val). Seventeen pathogenic variants were previously described and five variants showed no pathogenicity according to in silico analysis.

Conclusion

Given that most of the patients received late diagnoses, the study results do not allow us to state that the molecular features of MSUD variant phenotypes are predictive of clinical severity.

Identification of novel mutations in BCKDHB and DBT genes in Vietnamese patients with maple sirup urine disease

Background

Maple sirup urine disease (MSUD) is an autosomal recessive inherited metabolic disorder. The disease‐causing mutations can affect the BCKDHA, BCKDHB, and DBT genes encoding for the E1α, E1β, and E2 subunits of the multienzyme branched‐chain α‐keto acid dehydrogenase (BCKDH) complex. In the present study, novel pathogenic variants in BCKDHB and DBT genes were identified in three Vietnamese families with MSUD.

Methods

Three newborn patients from three unrelated Vietnamese families were diagnosed with MSUD at the Metabolic Clinic, National Hospital of Pediatrics. Blood samples of 11 relatives from two generations of the three families diagnosed with MSUD were analyzed using exome and Sanger sequencing analyses.

Results

Novel pathogenic variants in BCKDHB (c.1103C>T, c.989A>G, and c.704G>A), and DBT (c.263_265delAAG) genes were identified in three pediatric patients with MSUD.

Conclusions

We have identified novel pathogenic variants in the MSUD‐related genes in the pedigree of the three patient's families. Our findings expand the mutational spectrum of MSUD and provide the scientific basis for genetic counseling for the patient's families.

Diagnostic and clinical utility of whole genome sequencing in a cohort of undiagnosed Chinese families with rare diseases

Rare diseases are usually chronically debilitating or even life-threatening with diagnostic and therapeutic challenges in current clinical practice. It has been estimated that 80% of rare diseases are genetic in origin, and thus genome sequencing-based diagnosis offers a promising alternative for rare-disease management. In this study, 79 individuals from 16 independent families were performed for whole-genome sequencing (WGS) in an effort to identify the causative mutations for 16 distinct rare diseases that are largely clinically intractable. Comprehensive analysis of variations, including simple nucleotide variants (SNVs), copy-number variations (CNVs), and structural variations (SVs), was implemented using the WGS data. A flexible analysis pipeline that allowed a certain degree of misclassification of disease status was developed to facilitate the identification of causative variants. As a result, disease-causing variants were identified in 10 of the 16 investigated diseases, yielding a diagnostic rate of 62.5%. Additionally, new potentially pathogenic variants were discovered for two disorders, including IGF2/INS-IGF2 in mitochondrial disease and FBN3 in Klippel–Trenaunay–Weber syndrome. Our WGS analysis not only detected a CNV associated with 3p deletion syndrome but also captured a simple sequence repeat (SSR) variation associated with Machado–Joseph disease. To our knowledge, this is the first time the clinical WGS analysis of short-read sequences has been used successfully to identify a causative SSR variation that perfectly segregates with a repeat expansion disorder. After the WGS analysis, we confirmed the initial diagnosis for three of 10 established disorders and modified or corrected the initial diagnosis for the remaining seven disorders. In summary, clinical WGS is a powerful tool for the diagnosis of rare diseases, and its diagnostic clarity at molecular levels offers important benefits for the participating families.

Four novel mutations of the BCKDHA, BCKDHB and DBT genes in Iranian patients with maple syrup urine disease

BACKGROUND

Maple syrup urine disease (MSUD) is a rare metabolic autosomal recessive disorder caused by dysfunction of the branched-chain α-ketoacid dehydrogenase (BCKDH) complex. Mutations in the BCKDHA, BCKDHB and DBT genes are responsible for MSUD. The current study analyzed seven Iranian MSUD patients genetically and explored probable correlations between their genotype and phenotype.

METHODS

The panel of genes, including BCKDHA, BCKDHB and DBT, was evaluated, using routine the polymerase chain reaction (PCR)-sequencing method. In addition, protein modeling (homology and threading modeling) of the deduced novel mutations was performed. The resulting structures were then analyzed, using state-of-the-art bioinformatics tools to better understand the structural and functional effects caused by mutations.

RESULTS

Seven mutations were detected in seven patients, including four novel pathogenic mutations in BCKDHA (c.1198delA, c.629C>T), BCKDHB (c.652C>T) and DBT (c.1150A>G) genes. Molecular modeling of the novel mutations revealed clear changes in the molecular energy levels and stereochemical traits of the modeled proteins, which may be indicative of strong correlations with the functional modifications of the genes. Structural deficiencies were compatible with the observed phenotypes.

CONCLUSIONS

Any type of MSUD can show heterogeneous clinical manifestations in different ethnic groups. Comprehensive molecular investigations would be necessary for differential diagnosis.

Different gene preferences of maple syrup urine disease in the aboriginal tribes of Taiwan

BACKGROUND

Maple syrup urine disease (MSUD) is a rare inborn error of metabolism caused by a deficiency of the branched-chain α-ketoacid dehydrogenase (BCKD) complex. Mutations in any one of the three different genes encoding for the BCKD components, namely, BCKDHA, BCKDHB, and DBT, may be responsible for this disease. In Taiwan, few MSUD cases were diagnosed clinically, and most of these patients are from Aboriginal tribes.

MATERIALS AND METHODS

To identify and detect the carrier frequency of MSUD in Taiwanese Aboriginal tribes, we performed biochemical and molecular studies from peripheral blood in MSUD patients and dried blood on filter paper in the enrolled screened populations.

RESULTS

Homozygous A208T and I281T of BCKDHA were found in two patients from Hans (non-Aboriginal Taiwanese), respectively; compound heterozygous mutations of the DBT gene [4.7 kb deletion/c.650-651insT (L217F or L217fsX223) and c.650-651insT/c.88-89delAT] were found in two patients from Amis, respectively, after direct DNA sequencing and polymerase chain reaction-restriction fragment length polymorphism studies. There were no cases of deleted 4.7-kb heterozygote out of 302 normal people (Hans, n = 125; Atayal, n = 156; and Saisiyat, n = 21); by contrast, the DBT mutations c.650-651insT and deleted 4.7-kb heterozygote were noted in 2/121 and 1/121, respectively, from the general population of the Amis, a southeastern Taiwanese tribe.

CONCLUSION

Although the Taiwanese Austronesian Aboriginal tribes are considered to share a common origin, different gene preferences of MSUD were noted. The novel DBT mutation c.650-651insT was more prevalent than the deleted 4.7-kb heterozygote in the Amis population. The reported 4.7-kb deletion indicating a possible founder mutation may be preserved in the southern and eastern, but not in northern Aboriginal tribes of Taiwan.

Molecular characterization of maple syrup urine disease patients from Tunisia

Maple syrup urine disease (MSUD) is a rare disorder of branched-chain amino acids (BCAA) metabolism caused by the defective function of branched-chain α-ketoacid dehydrogenase complex (BCKD). The disease causal mutations can occur either in BCKDHA, BCKDHB or DBT genes encoding respectively the E1α, E1β and E2 subunits of the complex. In this study we report the molecular characterization of 3 Tunisian patients with the classic form of MSUD. Two novel putative mutations have been identified: the alteration c.716A>G (p.Glu239Gly) in BCKDHB and a small deletion (c.1333_1336delAATG; p.Asn445X) detected in DBT gene.

A novel regulatory defect in the branched-chain α-keto acid dehydrogenase complex due to a mutation in the PPM1K gene causes a mild variant phenotype of maple syrup urine disease

This article describes a hitherto unreported involvement of the phosphatase PP2Cm, a recently described member of the branched-chain α-keto acid dehydrogenase (BCKDH) complex, in maple syrup urine disease (MSUD). The disease-causing mutation was identified in a patient with a mild variant phenotype, involving a gene not previously associated with MSUD. SNP array-based genotyping showed a copy-neutral homozygous pattern for chromosome 4 compatible with uniparental isodisomy. Mutation analysis of the candidate gene, PPM1K, revealed a homozygous c.417_418delTA change predicted to result in a truncated, unstable protein. No PP2Cm mutant protein was detected in immunocytochemical or Western blot expression analyses. The transient expression of wild-type PPM1K in PP2Cm-deficient fibroblasts recovered 35% of normal BCKDH activity. As PP2Cm has been described essential for cell survival, apoptosis and metabolism, the impact of its deficiency on specific metabolic stress variables was evaluated in PP2Cm-deficient fibroblasts. Increases were seen in ROS levels along with the activation of specific stress-signaling MAP kinases. Similar to that described for the pyruvate dehydrogenase complex, a defect in the regulation of BCKDH caused the aberrant metabolism of its substrate, contributing to the patient's MSUD phenotype--and perhaps others.

Analysis of gene mutations in Chinese patients with maple syrup urine disease

OBJECTIVE

Maple syrup urine disease (MSUD) is predominantly caused by mutations in the BCKDHA, BCKDHB and DBT genes, which encode for the E1α, E1β and E2 subunits of the branched-chain α-keto acid dehydrogenase complex, respectively. The aim of this study was to screen DNA samples from 16 Chinese MSUD patients and assess a potential correlation between genotype and phenotype.

METHODS

BCKDHA, BCKDHB and DBT genes were analyzed by polymerase chain reaction (PCR) and direct sequencing. Segments bearing novel mutations were identified by PCR-restriction fragment length polymorphism (PCR-RFLP) analysis.

RESULTS

Within the variant alleles, 28 mutations (28/32, 87.5%), were detected in 15 patients, while one patient displayed no mutations. Mutations were comprised of 20 different: 6 BCKDHA gene mutations in 4 cases, 10 BCKDHB gene mutations in 8 cases and 4 DBT gene mutations in 3 cases. From these, 14 were novel, which included 3 mutations in the BCKDHA gene, 7 in the BCKDHB gene and 4 in the DBT gene. Only two patients with mutations in the BCKDHB and DBT genes were thiamine-responsive and presented a better clinical outcome.

CONCLUSION

We identified 20 different mutations within the BCKDHA, BCKDHB and DBT genes among 16 Chinese MSUD patients, including 14 novel mutations. The majority were non-responsive to thiamine, associating with a worse clinical outcome. Our data provide the basis for further genotype-phenotype correlation studies in these patients, which will be beneficial for early diagnosis and in directing the approach to clinical intervention.

Identification of two novel BCKDHA mutations in a Chinese patient with maple syrup urine disease

Maple syrup urine disease (MSUD) is an autosomal recessive metabolic disorder affecting branched-chain amino acids. Mutations in the BCKDHA, BCKDHB, and DBT gene impair the branched-chain alpha-ketoacid dehydrogenase (BCKD) complex, resulting in the accumulation of branched-chain amino acids and branched-chain alpha-ketoacid in tissues and plasma. This leads to mental and physical retardation, feeding problems, and a maple syrup odor in the urine. In this study, we describe the clinical and biochemical manifestations of a sporadic mutation in a neonate with classic MSUD. Analysis of the BCKDHA gene revealed a compound heterozygous mutation consisting of two novel missense mutations (p.L103P and p.R265P). Viewing the protein with PyMOL indicated that the p.L103P and p.R265P mutations were, respectively, located in the helical region and core domains of the BCKD's Ela component. The p.L103P mutation affected the hydrophobic cores and is predicted to shorten the helix; the p.R265P mutation can predictably affect the cofactor binding site by ligating the associated manganese ion. In conclusion, we identified two novel missense mutations in the BCKDHA gene in a Chinese patient with MSUD.

Functional characterization of the novel intronic nucleotide change c.288+9C>T within the BCKDHA gene: understanding a variant presentation of maple syrup urine disease

Mutations in any of the three different genes--BCKDHA, BCKDHB, and DBT--encoding for the E1α, E1β, and E2 catalytic components of the branched-chain α-ketoacid dehydrogenase complex can cause maple syrup urine disease (MSUD). Disease severity ranges from the classic to the mildest variant types and precise genotypes, mostly based on missense mutations, have been associated to the less severe presentations of the disease. Herein, we examine the consequences at the messenger RNA (mRNA) level of the novel intronic alteration c.288+9C>T found in heterozygous fashion in a BCKDHA variant MSUD patient who also carries the nucleotide change c.745G>A (p.Gly249Ser), previously described as a severe change. Direct analysis of the processed transcripts from the patient showed--in addition to a low but measurable level of normal mRNA product--an aberrantly spliced mRNA containing a 7-bp fragment of intron 2, which could be rescued when the patient's cells were treated with emetine. This aberrant transcript with a premature stop codon would be unstable, supporting the possible activation of nonsense-mediated mRNA decay pathway. Consistent with this finding, minigene splicing assays demonstrated that the point mutation c.288+9C>T is sufficient to create a cryptic splice site and cause the observed 7-bp insertion. Furthermore, our results strongly suggest that the c.288+9C>T allele in the patient generates both normal and aberrant transcripts that could sustain the variant presentation of the disease, highlighting the importance of correct genotyping to establish genotype-phenotype correlations and as basis for the development of therapeutic interventions.

Four novel mutations identified in Norwegian patients result in intermittent maple syrup urine disease when combined with the R301C mutation

Maple syrup urine disease (MSUD) is caused by a defect in branched chain alpha-ketoacid dehydrogenase complex (BCKD), an essential metabolon for the catabolism of the branched chain amino acids. Here, we report four novel mutations in the DBT gene, encoding the transacylase subunit (E2) of BCKD, resulting in intermittent MSUD in seven Norwegian patients. The patients had episodes with neurological symptoms including lethargy and/or ataxia during childhood infections. All seven patients were heterozygous for the annotated R301C mutation. The second allelic mutations were identified in five patients; one nonsense mutation (G62X), two missense mutations (W84C and R376C) and a mutation in the 3' untranslated region (UTR; c. *358A>C) in two patients. These four novel mutations result in near depletion of E2 protein, and the common R301C protein contributes predominantly to the residual (14%) cellular BCKD activity. Structural analyses of the mutations implied that the W84C and R376C mutations affect stability of intramolecular domains in E2, while the R301C mutation likely disturbs E2 trimer assembly as previously reported. The UTR mutated allele coincided with a strong reduction in mRNA levels, as did the non-R301C specific allele in two patients where the second mutation could not be identified. In summary, the pathogenic effect of the novel mutations is depletion of cellular protein, and the intermittent form of MSUD appears to be attributed to the residual R301C mutant protein in these patients.

Incidence of maple syrup urine disease in Portugal

Maple syrup urine disease is an autosomal recessive disorder of branched-chain amino acids metabolism with a worldwide frequency of 1/185,000 live newborns. In Portugal, the incidence of the disease has not been assessed. Based on the review of the cases diagnosed by tandem mass spectrometry an incidence of 1/86,800 live newborns was estimated in Portugal, indicating that the disease is more frequent in this country than reported in most populations.

Maple syrup urine disease due to a new large deletion at BCKDHA caused by non-homologous recombination

Maple syrup urine disease (MSUD) is a rare disorder of branched-chain amino acid (BCAA) metabolism caused by the defective function of branched-chain α-ketoacid dehydrogenase complex (BCKD). Many MSUD-causing mutations have already been described in genes that encode the complex (BCKDHA, BCKDHB and DBT), but up to now only four large deletions are known, all located in the DBT gene. In a previous study we identified a Portuguese MSUD patient with a homozygous deletion of exons 2, 3 and 4 at the BCKDHA gene; however, the corresponding breakpoints and, consequently, the exact deletion extension were not identified. Here, using long-range PCR and sequencing methodologies we were able to refine the characterization of this gross rearrangement. A genomic DNA loss of about 13.8 kb was detected, starting at intron 1 and ending at intron 4, thus encompassing exons 2, 3 and 4. Molecular characterization showed that the deletion junction contained a short sequence whose motif was CGGG. Since this motif is present in introns 1 and 4 of normal genomic DNA, we have hypothesized that non-homologous recombination was the mechanism underlying the identified large deletion, within which the CGGG could be derived either from intron 1 or from intron 4.