AGC1 Deficiency: Pathology and Molecular and Cellular Mechanisms of the Disease.

AGC1/Aralar/Slc25a12 is the mitochondrial carrier of aspartate-glutamate, the regulatory component of the NADH malate-aspartate shuttle (MAS) that transfers cytosolic redox power to neuronal mitochondria. The deficiency in AGC1/Aralar leads to the human rare disease named "early infantile epileptic encephalopathy 39" (EIEE 39, OMIM # 612949) characterized by epilepsy, hypotonia, arrested psychomotor neurodevelopment, hypo myelination and a drastic drop in brain aspartate (Asp) and N-acetylaspartate (NAA). Current evidence suggest that neurons are the main brain cell type expressing Aralar. However, paradoxically, glial functions such as myelin and Glutamine (Gln) synthesis are markedly impaired in AGC1 deficiency. Herein, we discuss the role of the AGC1/Aralar-MAS pathway in neuronal functions such as Asp and NAA synthesis, lactate use, respiration on glucose, glutamate (Glu) oxidation and other neurometabolic aspects. The possible mechanism triggering the pathophysiological findings in AGC1 deficiency, such as epilepsy and postnatal hypomyelination observed in humans and mice, are also included. Many of these mechanisms arise from findings in the aralar-KO mice model that extensively recapitulate the human disease including the astroglial failure to synthesize Gln and the dopamine (DA) mishandling in the nigrostriatal system. Epilepsy and DA mishandling are a direct consequence of the metabolic defect in neurons due to AGC1/Aralar deficiency. However, the deficits in myelin and Gln synthesis may be a consequence of neuronal affectation or a direct effect of AGC1/Aralar deficiency in glial cells. Further research is needed to clarify this question and delineate the transcellular metabolic fluxes that control brain functions. Finally, we discuss therapeutic approaches successfully used in AGC1-deficient patients and mice.

[Anti-NMDA receptor antibody-positive meningoencephalitis with SIADH and CNS demyelination: A case report].

After a 34-year-old female developed a headache and high fever, she was diagnosed with aseptic meningitis. On admission, neurological examinations revealed cerebellar limb ataxia, horizontal gaze paretic nystagmus, and pyramidal tract signs. Laboratory tests showed hyponatremia (129 mEq/l). Five days after admission, convulsions in the upper limbs due to the severe hyponatremia (108 mEq/l) were noted. In addition, serum antidiuretic hormone levels were markedly increased to 18.5 pg/ml. Brain MRI showed multiple small inflammatory lesions in the subcortical cerebral white matter, thalamus, and around the third ventricular diencephalic regions. Pulse corticosteroid treatment promptly improved her symptoms. Although tests for serum anti-aquaporin 4, anti-myelin oligodendrocyte glycoprotein, and anti-voltage-gated potassium channel antibodies were negative, cerebrospinal fluid samples tested positive for anti-N-methyl-D-aspartate (NMDA) receptor antibodies. Oral prednisolone administration was continued, but she developed paresthesia in her upper and lower extremities and gaze-evoked nystagmus three months after the first attack. MRI showed that the previously observed high-intensity regions were decreased, but a new area of high intensity was observed in ventral regions through the lower midbrain to the pons. Because pulse corticosteroid treatment was again effective, we continued the oral prednisolone treatment. This case presented none of the characteristic symptoms of anti-NMDA receptor antibody encephalitis during the clinical course other than repeated demyelinating encephalitis and severe syndrome of inappropriate antidiuretic hormone secretion (SIADH). Additional clinical observations are needed to better understand the underlying pathology of the NMDA receptor antibodies in the cerebrospinal fluid in this case.

Mutations in SZT2 result in early-onset epileptic encephalopathy and leukoencephalopathy.

Early-onset epileptic encephalopathies (EOEEs) are a genetically heterogeneous collection of severe epilepsies often associated with psychomotor regression. Mutations in SZT2, a known seizure threshold regulator gene, are a newly identified cause of EOEE. We present an individual with EOEE, macrocephaly, and developmental regression with compound heterozygous mutations in SZT2 as identified by whole exome sequencing. Serial imaging characterized the novel finding of progressive loss of central myelination. This case expands our clinical understanding of the SZT2-phenotype and emphasizes the role of this gene in the diagnostic investigation for EOEE and leukoencephalopathies.

Expanded spectrum of Pelizaeus-Merzbacher-like disease: literature revision and description of a novel GJC2 mutation in an unusually severe form.

Homozygous or compound heterozygous mutations in the GJC2 gene, encoding the gap junction protein connexin47 (Cx47), cause the autosomal recessive hypomyelinating Pelizaeus-Merzbacher-like disease (PMLD1, MIM# 608804). Although clinical and neuroradiological findings resemble those of the classic Pelizaeus-Merzbacher disease, PMLD patients usually show a greater level of cognitive and motor functions. Unpredictably a homozygous missense GJC2 mutation (p.Glu260Lys) was found in a patient presenting with a very severe clinical picture characterised by congenital nystagmus and severe neurological impairment. Also magnetic resonance imaging was unusually severe, showing an abnormal supra- and infratentorial white matter involvement extending to the spinal cord. The novel p.Glu260Lys (c.778G>A) mutation, occurring in a highly conserved motif (SRPTEK) of the Cx47 extracellular loop-2 domain, was predicted, by modelling analysis, to break a 'salt bridge network', crucial for a proper connexin-connexin interaction to form a connexon, thus hampering the correct formation of the connexon pore. The same structural analysis, extended to the previously reported missense mutations, predicted that most changes were expected to have less severe impact on protein functions, correlating with the mild PMLD1 form of the patients. Our study expands the spectrum of PMLD1 and provides evidence that the extremely severe clinical and neuroradiological PMLD1 form of our patient likely correlates with the predicted impairment of gap junction channel assembly resulting from the detrimental effect of the new p.Glu260Lys mutant allele on Cx47 protein.

[Leucodystrophy induced by late onset 3-hydroxy-3-methylglutaric aciduria].

3-Hydroxy-3-methylglutaric aciduria is a rare disorder of organic acid metabolism caused by 3-hydroxy-3-methylglutaryl-coenzyme A lyase deficiency. The disorder was common in neonatal or infant period. Here a case of late onset 3-hydroxy-3-methylglutaric aciduria complicated by leucodystrophy was reported. The patient was a 7-year-old boy. He presented with progressive headache, drowsiness and vomiting. Hepatic lesions, ketosis and leucopenia were found. Symmetrical diffused leucodystrophy was shown by MRI. Blood levels of isovalerylcarnitine and acetylcarnitine increased significantly. Urinary levels of 3-hydroxy-3-methylglutaric, 3-methylglutaconic, 3-hydroxyglutaric acids and 3-methyl-crotonylglycine increased significantly. Symptoms were released by intravenous infusion of L-carnitine and glucose. After treatment for 6 months, urinary levels of 3-hydroxy-3-methylglutaric aciduria decreased in the boy and his health improved.

Magnetic resonance findings in leucodystrophies and MS.

White matter diseases are a frequent diagnosis problem in adult patients. They are divided into leucodystrophy, defined by abnormal white matter from the beginning, and leucoencephalopathy, with an initial normal white matter. In addition, two different natures have to be considered: vascular and non-vascular. Vascular diseases are mainly acquired and related to atherosclerosis. Genetic vascular disorders are mostly secondary to Notch3 mutations, defined as cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL). Occurrence of leucoaraiosis and lacunae on T2 sequences, and microbleeds on Gradient Echo sequences, strongly suggest this diagnosis. Some magnetic resonance (MR) patterns can help to identify genetic leucodystrophies, such as childhood ataxia with central nervous system hypomyelination/leucoencephalopathy with vanishing white matter disease (progressive rarefaction and cystic degeneration of the affected white matter, replaced by water); Alexander disease (hypointense signals on T2 sequences involving grey matter, brainstem and cervical cord, with marked atrophy); megalencephalic leucoencephalopathy with subcortical cysts (diffuse, symmetrical white matter lesions, with constant frontoparietal and anterotemporal subcortical cysts); leucoencephalopathy with brainstem and spinal cord involvement and high lactates syndrome (extensive demyelination, involvement of the brainstem, i.e. cerebellar peduncles, intraparenchymal and mesencephalic trigeminal nerves and spinal cord, mainly in the lateral corticospinal tracts and dorsal columns). Half of the genetic adult leucodystrophies remain without any precise diagnosis. This review describes MR in the adult leucoencephalopathies and in multiple sclerosis (MS). The first part will focus on MR patterns of vascular and nonvascular adult leucoencephalopathies, the second part on MR findings in MS and MS-related diseases. Specific MR patterns in both diseases will be summarized and compared.

Leukoencephalopathies associated with inborn errors of metabolism in adults.

The discovery of a leukoencephalopathy is a frequent situation in neurological practice and the diagnostic approach is often difficult given the numerous possible aetiologies, which include multiple acquired causes and genetic diseases including inborn errors of metabolism (IEMs). It is now clear that IEMs can have their clinical onset from early infancy until late adulthood. These diseases are particularly important to recognize because specific treatments often exist. In this review, illustrated by personal observations, we give an overview of late-onset leukoencephalopathies caused by IEMs.

Deficiency of hyccin, a newly identified membrane protein, causes hypomyelination and congenital cataract.

We describe a new autosomal recessive white matter disorder ('hypomyelination and congenital cataract') characterized by hypomyelination of the central and peripheral nervous system, progressive neurological impairment and congenital cataract. We identified mutations in five affected families, resulting in a deficiency of hyccin, a newly identified 521-amino acid membrane protein. Our study highlights the essential role of hyccin in central and peripheral myelination.

TREMs in the immune system and beyond.

Triggering receptors expressed by myeloid cells (TREMs) belong to a rapidly expanding family of receptors that include activating and inhibitory isoforms encoded by a gene cluster linked to the MHC. TREM1 and TREM2 activate myeloid cells by signalling through the adaptor protein DAP12. TREM1 triggers phagocyte secretion of pro-inflammatory chemokines and cytokines, amplifying the inflammation that is induced by bacteria and fungi. TREM2 activates monocyte-derived dendritic cells and regulates osteoclast development. Remarkably, TREM2 deficiency leads to a severe disease that is characterized by bone cysts and demyelination of the central nervous system, which results in dementia, implying that the function of TREM2 extends beyond the immune system.