Inborn errors of metabolism with 3-methylglutaconic aciduria as discriminative feature: proper classification and nomenclature.

Increased urinary 3-methylglutaconic acid excretion is a relatively common finding in metabolic disorders, especially in mitochondrial disorders. In most cases 3-methylglutaconic acid is only slightly elevated and accompanied by other (disease specific) metabolites. There is, however, a group of disorders with significantly and consistently increased 3-methylglutaconic acid excretion, where the 3-methylglutaconic aciduria is a hallmark of the phenotype and the key to diagnosis. Until now these disorders were labelled by roman numbers (I-V) in the order of discovery regardless of pathomechanism. Especially, the so called "unspecified" 3-methylglutaconic aciduria type IV has been ever growing, leading to biochemical and clinical diagnostic confusion. Therefore, we propose the following pathomechanism based classification and a simplified diagnostic flow chart for these "inborn errors of metabolism with 3-methylglutaconic aciduria as discriminative feature". One should distinguish between "primary 3-methylglutaconic aciduria" formerly known as type I (3-methylglutaconyl-CoA hydratase deficiency, AUH defect) due to defective leucine catabolism and the--currently known--three groups of "secondary 3-methylglutaconic aciduria". The latter should be further classified and named by their defective protein or the historical name as follows: i) defective phospholipid remodelling (TAZ defect or Barth syndrome, SERAC1 defect or MEGDEL syndrome) and ii) mitochondrial membrane associated disorders (OPA3 defect or Costeff syndrome, DNAJC19 defect or DCMA syndrome, TMEM70 defect). The remaining patients with significant and consistent 3-methylglutaconic aciduria in whom the above mentioned syndromes have been excluded, should be referred to as "not otherwise specified (NOS) 3-MGA-uria" until elucidation of the underlying pathomechanism enables proper (possibly extended) classification.

Specific combination of compound heterozygous mutations in 17ß-hydroxysteroid dehydrogenase type 4 (HSD17B4) defines a new subtype of D-bifunctional protein deficiency.

BACKGROUND: D-bifunctional protein (DBP) deficiency is typically apparent within the first month of life with most infants demonstrating hypotonia, psychomotor delay and seizures. Few children survive beyond two years of age. Among patients with prolonged survival all demonstrate severe gross motor delay, absent language development, and severe hearing and visual impairment. DBP contains three catalytically active domains; an N-terminal dehydrogenase, a central hydratase and a C-terminal sterol carrier protein-2-like domain. Three subtypes of the disease are identified based upon the domain affected; DBP type I results from a combined deficiency of dehydrogenase and hydratase activity; DBP type II from isolated hydratase deficiency and DBP type III from isolated dehydrogenase deficiency. Here we report two brothers (16½ and 14 years old) with DBP deficiency characterized by normal early childhood followed by sensorineural hearing loss, progressive cerebellar and sensory ataxia and subclinical retinitis pigmentosa. METHODS AND RESULTS: Biochemical analysis revealed normal levels of plasma VLCFA, phytanic acid and pristanic acid, and normal bile acids in urine; based on these results no diagnosis was made. Exome analysis was performed using the Agilent SureSelect 50Mb All Exon Kit and the Illumina HiSeq 2000 next-generation-sequencing (NGS) platform. Compound heterozygous mutations were identified by exome sequencing and confirmed by Sanger sequencing within the dehydrogenase domain (c.101C>T; p.Ala34Val) and hydratase domain (c.1547T>C; p.Ile516Thr) of the 17ß-hydroxysteroid dehydrogenase type 4 gene (HSD17B4). These mutations have been previously reported in patients with severe-forms of DBP deficiency, however each mutation was reported in combination with another mutation affecting the same domain. Subsequent studies in fibroblasts revealed normal VLCFA levels, normal C26:0 but reduced pristanic acid beta-oxidation activity. Both DBP hydratase and dehydrogenase activity were markedly decreased but detectable. CONCLUSIONS: We propose that the DBP phenotype seen in this family represents a distinct and novel subtype of DBP deficiency, which we have termed type IV based on the presence of a missense mutation in each of the domains of DBP resulting in markedly reduced but detectable hydratase and dehydrogenase activity of DBP. Given that the biochemical testing in plasma was normal in these patients, this is likely an underdiagnosed form of DBP deficiency.

L-2-hydroxyglutaric aciduria diagnosed in a young adult with progressive cerebellar ataxia and facial dyskinesia.

INTRODUCTION: L-2-hydroxyglutaric aciduria is a rare metabolic disorder with quite typical radiological abnormalities and various clinical symptoms. OBSERVATION: A 19-year-old girl presented with ataxia, facial dyskinesia, and mild cognitive impairment. Cerebral magnetic resonance imaging demonstrated subcortical white matter T2 abnormalities and a suggestive rim hyperintensity around the caudate nuclei and the putamen. Diagnosis was confirmed by increased 2-hydroxyglutaric acid in urine and a genetic study (Gly260Ala mutation in the L-2-hydroxyglutarate dehydrogenase (L2HGDH) gene). DISCUSSION: This case highlights the movement disorder onset and radiological aspects that should indicate the L-2-hydroxyglutaric aciduria diagnosis.

Normomorphic sialidosis in two female adults with severe neurologic disease and without sialyl oligosacchariduria.

Two female patients of German origin, aged 38 and 21 years, with myoclonus epilepsy and cerebellar ataxia, but without dysmorphic signs and dementia, were found to excrete normal amounts of sialyl oligosaccharides in their urine. The younger patient showed cherry red spots in her ocular fundi. The older patient had a brother with an autopsy-proven neuronal storage disease compatible with sialidosis, and in her rectal biopsy lamellar inclusion bodies were detected. Enzyme assays in cultured fibroblasts of both patients revealed a profound but incomplete deficiency of oligosaccharide sialidase activity and normal beta-galactosidase activity. Adult sialidosis was diagnosed in both patients. In their fibroblasts, moderate elevations of bound sialic acid could also be measured. The small residual sialidase activity, which in the older patient had a normal KM value, is considered responsible for the late onset and slow clinical course of the disease. It is concluded that in adult sialidosis the extraneural storage process can be difficult to demonstrate in terms of metabolite accumulation or excretion during the course of intraneuronal storage.

Cerebellar ataxia and hypergonadotropic hypogonadism in two kindreds. Chance concurrence, pleiotropism or linkage?

Two kindreds with Marinesco-Sjögren's syndrome in three sibships are described. In five of the six affected, but in none of the unaffected sibs, a hypergonadotropic hypogonadism was observed. In one of the kindreds a high degree of inbreeding was revealed, and inbreeding likely also existed in the other kindred. The two families were not related. Marinesco-Sjögren's syndrome is known to be a distinct clinical entity, governed by autosomal recessive inheritance, and this also applies to hypergonadotropic hypogonadism. Several heredo-degenerative nervous disorders are accompanied by a hypogonadotropic hypogonadism, which is believed to be secondary to the neurological disorder, as in traumatic paraplegia. A hypergonadotropic hypoganadism cannot readily be explained in this way. We consider genetic linkage between two independent disorders as the most likely explanation for the observed concurrence.