Maple syrup urine disease (MSUD) is an inborn error of metabolism caused by defects in the branched-chain α-ketoacid dehydrogenase complex, which results in elevations of the branched-chain amino acids (BCAAs) in plasma, α-ketoacids in urine, and production of the pathognomonic disease marker, alloisoleucine. The disorder varies in severity and the clinical spectrum is quite broad with five recognized clinical variants that have no known association with genotype. The classic presentation occurs in the neonatal period with developmental delay, failure to thrive, feeding difficulties, and maple syrup odor in the cerumen and urine, and can lead to irreversible neurological complications, including stereotypical movements, metabolic decompensation, and death if left untreated. Treatment consists of dietary restriction of BCAAs and close metabolic monitoring. Clinical outcomes are generally good in patients where treatment is initiated early. Newborn screening for MSUD is now commonplace in the United States and is included on the Recommended Uniform Screening Panel (RUSP). We review this disorder including its presentation, screening and clinical diagnosis, treatment, and other relevant aspects pertaining to the care of patients.

Sphingolipid imbalance is the culprit in a variety of neurological diseases, some affecting the myelin sheath. We have used whole exome sequencing in patients with undetermined leukoencephalopathies to uncover the endoplasmic reticulum lipid desaturase DEGS1 as the causative gene in nineteen patients from thirteen unrelated families. Shared features among the cases include severe motor arrest, early nystagmus, dystonia, spasticity and profound failure to thrive. MRI showed hypomyelination, thinning of corpus callosum and progressive thalami and cerebellar atrophy, suggesting a critical role of DEGS1 in myelin development and maintenance. This enzyme converts dihydroceramide (DhCer) into ceramide (Cer) in the final step of the de novo biosynthesis pathway. We detected a marked increase of the substrate DhCer and DhCer/Cer ratios in patient’s fibroblasts and muscle. Further, we used a knockdown approach for disease modelling in Danio rerio, followed by a preclinical test with the first-line treatment for multiple sclerosis, fingolimod (FTY720, Gilenya). The enzymatic inhibition of ceramide synthase, one step prior to DEGS1 in the pathway, by fingolimod, reduced the critical DhCer/Cer imbalance and the severe locomotor disability, increasing the number of myelinating oligodendrocytes in the zebrafish model. These proof-of-concept results pave the way to clinical translation.

Congenital hyperinsulinaemic hypoglycaemia (HH) can occur in isolation or it may present as part of a wider syndrome. For approximately 40%-50% of individuals with this condition, sequence analysis of the known HH genes identifies a causative mutation. Identifying the underlying genetic aetiology in the remaining cases is important as a genetic diagnosis will inform on recurrence risk, may guide medical management and will provide valuable insights into β-cell physiology. We sequenced the exome of a child with persistent diazoxide-responsive HH, mild aortic insufficiency, severe hypotonia, and developmental delay as well as the unaffected parents. This analysis identified a de novo mutation, p.G403D, in the proband's CACNA1D gene. CACNA1D encodes the main L-type voltage-gated calcium channel in the pancreatic β-cell, a key component of the insulin secretion pathway. The p.G403D mutation had been reported previously as an activating mutation in an individual with primary hyper-aldosteronism, neuromuscular abnormalities, and transient hypoglycaemia. Sequence analysis of the CACNA1D gene in 60 further cases with HH did not identify a pathogenic mutation. Identification of an activating CACNA1D mutation in a second patient with congenital HH confirms the aetiological role of CACNA1D mutations in this disorder. A genetic diagnosis is important as treatment with a calcium channel blocker may be an option for the medical management of this patient.

Mucopolysaccharidosis type I (MPS I) is caused by the deficiency of α-l-iduronidase, leading to the storage of dermatan and heparan sulfate. There is a broad phenotypical spectrum with the presence or absence of neurological impairment. The classical form is known as Hurler syndrome, the intermediate form as Hurler–Scheie, and the most attenuated form is known as Scheie syndrome. Phenotype seems to be largely influenced by genotype. Patients usually develop several somatic symptoms such as abdominal hernias, extensive dermal melanocytosis, thoracolumbar kyphosis odontoid dysplasia, arthropathy, coxa valga and genu valgum, coarse facial features, respiratory and cardiac impairment. The diagnosis is based on the quantification of α-l-iduronidase coupled with glycosaminoglycan analysis and gene sequencing. Guidelines for treatment recommend hematopoietic stem cell transplantation for young Hurler patients (usually at less than 30 months of age). Intravenous enzyme replacement is approved and is the standard of care for attenuated—Hurler–Scheie and Scheie—forms (without cognitive impairment) and for the late-diagnosed severe—Hurler—cases. Intrathecal enzyme replacement therapy is under evaluation, but it seems to be safe and effective. Other therapeutic approaches such as gene therapy, gene editing, stop codon read through, and therapy with small molecules are under development. Newborn screening is now allowing the early identification of MPS I patients, who can then be treated within their first days of life, potentially leading to a dramatic change in the disease’s progression. Supportive care is very important to improve quality of life and might include several surgeries throughout the life course.