Akiko Ichinohe

A novel KCNQ4 one-base deletion in a large pedigree with hearing loss: implication for the genotype–phenotype correlation

AbstractAutosomal-dominant, nonsyndromic hearing impairment is clinically and genetically heterogeneous. We encountered a large Japanese pedigree in which nonsyndromic hearing loss was inherited in an autosomal-dominant fashion. A genome-wide linkage study indicated linkage to the DFNA2 locus on chromosome 1p34. Mutational analysis of KCNQ4 encoding a potassium channel revealed a novel one-base deletion in exon 1, c.211delC, which generated a profoundly truncated protein without transmembrane domains (p.Q71fsX138). Previously, six missense mutations and one 13-base deletion, c.211_223del, had been reported in KCNQ4. Patients with the KCNQ4 missense mutations had younger-onset and more profound hearing loss than patients with the 211_223del mutation. In our current study, 12 individuals with the c.211delC mutation manifested late-onset and pure high-frequency hearing loss. Our results support the genotype-phenotype correlation that the KCNQ4 deletions are associated with later-onset and milder hearing impairment than the missense mutations. The phenotypic difference may be caused by the difference in pathogenic mechanisms: haploinsufficiency in deletions and dominant-negative effect in missense mutations.

Glycine cleavage system in neurogenic regions.

The glycine cleavage system (GCS) is the essential enzyme complex for degrading glycine and supplying 5,10‐methylenetetrahydrofolate for DNA synthesis. Inherited deficiency of this system causes nonketotic hyperglycinemia, characterized by severe neurological symptoms and frequent association of brain malformations. Although high levels of glycine have been considered to cause the above‐mentioned problems, the detailed pathogenesis of this disease is still unknown. Here we show that GCS is abundantly expressed in rat embryonic neural stem/progenitor cells in the neuroepithelium, and this expression is transmitted to the radial glia–astrocyte lineage, with prominence in postnatal neurogenic regions. These data indicate that GCS plays important roles in neurogenesis, and suggest that disturbance of neurogenesis induced by deficiency of GCS may be the main pathogenesis of nonketotic hyperglycinemia.

Heterozygous GLDC and GCSH gene mutations in transient neonatal hyperglycinemia

Transient neonatal hyperglycinemia is clinically or biochemically indistinguishable from nonketotic hyperglycinemia at onset. In the case of transient neonatal hyperglycinemia, the elevated plasma and cerebrospinal fluid glycine levels are normalized within 2 to 8 weeks. To elucidate the pathogenesis of transient neonatal hyperglycinemia, we studied three patients by screening mutations in the genes that encode three components of the glycine cleavage system. Heterozygous mutations were identified in all of the three patients, suggesting that transient neonatal hyperglycinemia develops in some heterozygous carriers for nonketotic hyperglycinemia.

Novel mutations in the P-protein (glycine decarboxylase) gene in patients with glycine encephalopathy (non-ketotic hyperglycinemia) ☆

Eight novel mutations were found in the P-protein (glycine decarboxylase) gene (GLDC) of the glycine cleavage system (EC 2.1.1.10) by screening five exons of the gene in patients with glycine encephalopathy (NKH). The mutations identified were of eight single base changes: a one-base deletion 1054del A, a splice site mutation IVS18-2A-->G and six amino acid substitutions A283P, A313P, P329T, R410K, P700A, and G762R.

Model Mice for Mild-Form Glycine Encephalopathy: Behavioral and Biochemical Characterizations and Efficacy of Antagonists for the Glycine Binding Site of N-Methyl D-Aspartate Receptor

Glycine encephalopathy (GE) is caused by an inherited deficiency of the glycine cleavage system (GCS) and characterized by accumulation of glycine in body fluids and various neurologic symptoms. Coma and convulsions develop in neonates in typical GE while psychomotor retardation and behavioral abnormalities in infancy and childhood are observed in mild GE. Recently, we have established a transgenic mouse line (low-GCS) with reduced GCS activity (29% of wild-type (WT) C57BL/6) and accumulation of glycine in the brain (Stroke, 2007; 38:2157). The purpose of the present study is to characterize behavioral features of the low-GCS mouse as a model of mild GE. Two other transgenic mouse lines were also analyzed: high-GCS mice with elevated GCS activity and low-GCS-2 mice with reduced GCS activity. As compared with controls, low-GCS mice manifested increased seizure susceptibility, aggressiveness and anxiety-like activity, which resembled abnormal behaviors reported in mild GE, whereas high-GCS mice were less sensitive to seizures, hypoactive and less anxious. Antagonists for the glycine-binding site of the N-methyl-D-aspartate receptor significantly ameliorated elevated locomotor activity and seizure susceptibility in the low-GCS mice. Our results suggest the usefulness of low-GCS mice as a mouse model for mild GE and a novel therapeutic strategy.

Development and Aging Expression of Cystathionine-Beta Synthase in the Temporal Lobe and Cerebellum of Down Syndrome Patients

Down’s syndrome (DS) is characterized by intellectual disabilities and Alzheimer-type dementia. Cystathionine-β synthase (CBS) plays a role in the production of hydrogen sulfide (H2S) in the brain and is encoded by a gene which is localized in the DS critical region of chromosome 21. The expression of CBS was compared during development and aging. Therefore, cerebri (temporal lobe) and cerebelli from fetuses and adults were studied in 22 normal controls and 21 DS patients using immunohistochemical methods. During normal development, CBS-positive astrocytes appeared in the cortical layer at 14 weeks of gestation (GW), remained elevated until 34 GW and decreased slightly during the infantile period (1–11 months of age). A few CBS-positive neurons were transiently found in the deep cortical layer only at 35 GW. In the cerebellum, CBS-positive Bergmann glial cells were found from 26 GW to adulthood in both controls and DS patients. In DS, there were greater numbers of CBS-positive astrocytes, situated predominantly in granular cell layers II and IV, compared to the controls. In addition, CBS-positive astrocytes were also increased around senile plaques in adults with DS. The high level of CBS in the granular cell layers in DS may be related to an abnormal development of inhibitory neurons and may be responsible for the pathogenesis of intellectual disability, while the increased CBS observed in adult DS may be related to the pathogenesis of Alzheimer-type dementia.

Early and Late Development of Notch3 in Human Brains

Notch1, 2, and 3, cell surface proteins, exhibit distinct spatial and temporal expression in a variety of regions in the developing brain, and the mutation of Notch3 is the cause of a hereditary disease, CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy). Notch system has been thought to have important functions during early neural development, but its function and significance in the developing central nervous system remain elusive. We demonstrated early and temporal Notch3 expression in progenitors of neurons and glial cells at an early fetal stage, and late and increasing Notch3 expression until adulthood in the vessels of human brains. This persistent Notch3 expression might influence the pathogenesis of CADASIL. Notch3 expression at a temporally early stage of neurons and glial cells, and in a period of permanent vessel development suggests the different and important roles of this protein during human brain development. Additionally, abnormal expression of Notch3 was detected in cortical neurons of type 2 lissencephaly and tuberous sclerosis brains. These data suggest that Notch3 might be a marker of delayed or abnormal maturation of neurons associated with migration and maturation disorders.

Pathogenesis and Prevention of Pontosubicular Necrosis

Perinatal pontosubicular neuron necrosis (PSN) has similar characteristics to apoptosis, such as karyorrhexis, cellular atrophy and little glial reaction. In PSN pathogenesis, the developmental discrepancy between neurons exhibiting late maturation and vessels showing an early increase in the basis pontis and subiculum may predispose fetuses and neonates to PSN. Ischemia, hyperoxygenemia, hypocarbia and hypoglycemia have been reported to be causal factors. Apoptotic neurons are associated with activation of caspase-3, and remaining neurons are accompanied by growth factors in glial cells. Nestin, which is abundant in multipotential stem cells, is reactivated in cells surrounding the subacute and chronic lesions. Thus, local plasticity may be activated both in necrotic and apoptotic cell death, and the period of regeneration is important for the treatment of PSN.