A novel SLC5A6 homozygous variant in a family with multivitamin-dependent neurometabolic disorder: Phenotype expansion and long-term follow-up

Expanded Clinical Phenotype and the Role of Untargeted Metabolomics Analysis in Confirming the Diagnosis of Sodium-Dependent Multivitamin Transporter Deficiency

The sodium-dependent multivitamin transporter (SMVT) is a ubiquitously expressed sodium-solute symporter that transports pantothenic acid, biotin, and α-lipoic acid across the intestinal epithelia and blood-brain barrier. Severe biallelic loss-of-function variants in SLC5A6 (MIM #604024) lead to SMVT deficiency (SMVTD, MIM #618973), which classically presents with developmental delay, brain atrophy, epilepsy, sensorineural hearing loss, peripheral neuropathy, and gastrointestinal, cutaneous, and immunologic abnormalities. We describe a 25-year-old female with autism spectrum disorder (ASD), intellectual disability, agenesis of the corpus callosum (ACC), and epilepsy who presented at 15 years of age with a severe metabolic crisis characterized by hyperammonemia, lactic acidosis, and rhabdomyolysis. Trio exome sequencing (ES) identified compound heterozygous variants in SLC5A6 . Plasma untargeted metabolomics analysis demonstrated reduced pantothenate and coenzyme A with elevated long-chain fatty acids, indicating impaired fatty acid oxidation, functionally validating ES results, and confirming a diagnosis of SMVTD. Targeted replacement with biotin, lipoic acid, and pantothenic acid improved her neurocognitive function and metabolic control. Our patient, the oldest reported at diagnosis, expands the phenotype of SMVTD to include rhabdomyolysis, ACC, and ASD. Our study suggests that integrating ES and untargeted metabolomics in undiagnosed patients with suspected inborn errors of metabolism may help identify this ultra-rare disorder.

Prenatal Brain Abnormalities in Sodium-Dependent Multivitamin Transporter Deficiency

In treatable neurometabolic disorders, early diagnosis and prompt initiation of treatment are key to improved survival and outcomes. Biallelic variants in SLC5A6 cause sodium-dependent multivitamin transporter deficiency (OMIM # 618973), which is treatable with high-dose pantothenic acid, biotin, and alpha lipoic acid. So far, the onset has been postnatal in all reported patients. A review of the prenatal imaging showed ventriculomegaly at 32 weeks of gestation. After birth, the infant exhibited developmental regression. At 6 months of age, he developed acute metabolic shock in the setting of a urinary tract infection. Rapid exome analysis identified compound heterozygous pathogenic variants in SLC5A6 and confirmed the diagnosis of sodium-dependent multivitamin transporter deficiency. After the infant was initiated on treatment with high-dose biotin and pantothenic acid, we noted dramatic improvements in the hematological, cutaneous, cardiac, and gastrointestinal symptoms. Although the infant showed no further regression, he continued to have significant psychomotor disability. A review of the findings during pregnancy showed that the infant already had enlarged ventricles in late pregnancy, and neuroimaging on day 12 of birth showed signs of energy failure in late pregnancy. Our review of previously reported patients suggested no clear genotype-phenotype correlations, but there was little intrafamilial variability in disease onset. The present observation, for the first time, provides clinical evidence of a fetal onset in sodium-dependent multivitamin transporter deficiency. The high risk of recurrence and the low intrafamilial variability in disease onset suggest that identification and prenatal treatment in the high-risk group may be of significance.

The evolving spectrum of complex inherited neuropathies

PURPOSE OF REVIEW

Inherited peripheral neuropathies can be divided into those diseases in which peripheral neuropathy is the sole or main feature of the disease (Charcot-Marie-Tooth disease) and those in which peripheral neuropathy is just one feature of a more complex syndrome. In recent years there has been a substantial expansion in the number of genes associated with complex neuropathy syndromes.

RECENT FINDINGS

This review will focus on emerging themes in this group of diseases, namely the increasing number of diseases due to repeat expansions; the emergence of both recessive and dominant negative alleles in the same gene producing a common phenotype and diseases in which there is selective loss of the allele from haematopoietic stem cells making genetic diagnosis on blood derived DNA problematic.

SUMMARY

In this review we provide a practical approach to investigating and diagnosing patients with peripheral neuropathy as part of a complex syndrome and provide an updated table of the genes associated with this group of diseases.

Genome sequencing enables diagnosis and treatment of SLC5A6 neuropathy

The sodium-dependent multivitamin transporter encoded by SLC5A6 is responsible for uptake of biotin, pantothenic acid, and α-lipoic acid. Thirteen individuals from eight families are reported with pathogenic biallelic SLC5A6 variants. Phenotype ranges from multisystem metabolic disorder to childhood-onset peripheral motor neuropathy. We report three additional affected individuals with biallelic SLC5A6 variants. In Family A, a male proband (AII:1) presenting in early childhood with gross motor regression, motor axonal neuropathy, recurrent cytopenia and infections, and failure to thrive was diagnosed at 12 years of age via genome sequencing (GS) with a paternal NM_021095.4:c.393+2T>C variant and a maternal c.1285A>G p.(Ser429Gly) variant. An uncle with recurrent cytopenia and peripheral neuropathy was subsequently found to have the same genotype. We also report an unrelated female with peripheral neuropathy homozygous for the c.1285A>G p.(Ser429Gly) recurrent variant identified in seven reported cases, including this study. RT-PCR studies on blood mRNA from AII:1 showed c.393+2T>C caused mis-splicing with all canonically spliced transcripts in AII:1 containing the c.1285A>G variant. SLC5A6 mRNA expression in AII:1 fibroblasts was ~50% of control levels, indicative of nonsense-mediated decay of mis-spliced transcripts. Biotin uptake studies on AII:1 fibroblasts, expressing the p.(Ser429Gly) variant, showed an ~90% reduction in uptake compared to controls. Targeted treatment of AII:1 with biotin, pantothenic acid, and lipoic acid resulted in clinical improvement. Health Economic analyses showed implementation of GS as an early investigation could have saved $ AUD 105,988 and shortened diagnostic odyssey and initiation of treatment by up to 7 years.

Biotin-functionalized nanoparticles: an overview of recent trends in cancer detection

Electrochemical bio-sensing is a potent and efficient method for converting various biological recognition events into voltage, current, and impedance electrical signals. Biochemical sensors are now a common part of medical applications, such as detecting blood glucose levels, detecting food pathogens, and detecting specific cancers. As an exciting feature, bio-affinity couples, such as proteins with aptamers, ligands, paired nucleotides, and antibodies with antigens, are commonly used as bio-sensitive elements in electrochemical biosensors. Biotin-avidin interactions have been utilized for various purposes in recent years, such as targeting drugs, diagnosing clinically, labeling immunologically, biotechnology, biomedical engineering, and separating or purifying biomolecular compounds. The interaction between biotin and avidin is widely regarded as one of the most robust and reliable noncovalent interactions due to its high bi-affinity and ability to remain selective and accurate under various reaction conditions and bio-molecular attachments. More recently, there have been numerous attempts to develop electrochemical sensors to sense circulating cancer cells and the measurement of intracellular levels of protein thiols, formaldehyde, vitamin-targeted polymers, huwentoxin-I, anti-human antibodies, and a variety of tumor markers (including alpha-fetoprotein, epidermal growth factor receptor, prostate-specific Ag, carcinoembryonic Ag, cancer antigen 125, cancer antigen 15-3, etc.). Still, the non-specific binding of biotin to endogenous biotin-binding proteins present in biological samples can result in false-positive signals and hinder the accurate detection of cancer biomarkers. This review summarizes various categories of biotin-functional nanoparticles designed to detect such biomarkers and highlights some challenges in using them as diagnostic tools.

A new case of sodium-dependent multivitamin transporter defect occurring as a life-threatening condition responsive to early vitamin supplementation and literature review

BACKGROUND

Biallelic pathogenic variants in SLC5A6 resulting in sodium-dependent multivitamin transporter (SMVT) defect have recently been described as a vitamin-responsive inborn error of metabolism mimicking biotinidase deficiency. To our knowledge, only 16 patients have been reported so far with various clinical phenotypes such as neuropathy and other neurologic impairments, gastro-intestinal dysfunction and failure to thrive, osteopenia, immunodeficiency, metabolic acidosis, hypoglycemia, and recently severe cardiac symptoms.

METHODS

We describe a case report of a 5-month-old girl presenting two recurrent episodes of metabolic decompensation and massive cardiac failure in the course of an infectious disease. We compare clinical, biological, and genetic findings of this patient to previous literature collected from Pubmed database (keywords: Sodium-dependent multivitamin transporter (SMVT), SMVT defect/disorder/deficiency, SLC5A6 gene/mutation).

RESULTS

We highlight the life-threatening presentation of this disease, the stagnation of psychomotor development, the severe and persistent hypogammaglobulinemia, and additionally, the successful clinical response on early vitamin supplementation (biotin 15 mg a day and pantothenic acid 100 mg a day). Metabolic assessment showed a persistent increase of urinary 3-hydroxyisovaleric acid (3-HIA) as previously reported in this disease in literature.

CONCLUSION

SMVT deficiency is a vitamin-responsive inborn error of metabolism that can lead to a wide range of symptoms. Increased and isolated excretion of urinary 3-hydroxyisovaleric acid may suggest, in the absence of markedly reduced biotinidase activity, a SMVT deficiency. Prompt supplementation with high doses of biotin and pantothenic acid should be initiated while awaiting results of SLC5A6 sequencing as this condition may be life-threatening.

Recurrent "outsider" intronic variation in the SLC5A6 gene causes severe mixed axonal and demyelinating neuropathy, cyclic vomiting and optic atrophy in 3 families from Maghreb

Sodium dependent multivitamin transporter (SMVT) deficiency is a very rare autosomal recessive disorder characterized by multisystemic clinical manifestations due to combined biotin, panthotenic acid and lipoic acid deficiency. About 10 families have been described so far. Accurate diagnosis is crucial because of the possibility of a supplementation treatment with proven efficacy. Here we describe 4 new patients (3 additional families) originating from the same world region (Algeria, Maghreb). All patients, born form consanguineous parents, were homozygous carriers of the same intronic variation, outside of canonical sites, in the SLC5A6 gene encoding SMVT. RNA study in one family allowed confirming the pathogenic effect of the variation and re-classifying this variant of uncertain significance as pathogenic, opening the possibility of genetic counseling and treatment. The identification of the same variation in three distinct and apparently unrelated families is suggestive of a founder effect. The phenotype of all patients was very similar, with systematic optic atrophy (initially considered as a very rare sign), severe cyclic vomiting, and rapidly progressive mixed axonal and demyelinating sensory motor neuropathy.