[Neurodevelopmental impact of a mutation in the RHOBTB2 gene]. / Impact neurodéveloppemental d'une mutation sur le gène RHOBTB2.

RHOBTB2 was first described as epileptogenic when it presents a missense variant in 2016 and studied more specifically in 2018. It is a gene that causes rare, but potentially severe childhood epileptic encephalopathy. In 2021, research confirmed that heterozygous mutations of RHOBTB2 included other clinical signs besides these encephalopathies. Thus, these infantile epilepsies are mainly associated with highly variable phenotypes, with developmental delay, post-traumatic encephalitis, paroxysmal movement disorders and iconographic brain damage. In this work, after presenting a clinical case, we will recall the role of RhoGTPases on neuronal development. We will then discuss a study which highlighted the neurodevelopmental impact of mutations on the RHOBTB2 gene by carrying out work on Drosophila melanogaster flies. Finally, we will compare the presented clinical case with a literature review.

Le gène RHOBTB2 est décrit pour la première fois comme épileptogène alors qu'il présente un variant faux-sens en 2016, puis est étudié plus précisément en 2018. Il s'agit d'un gène qui est à l'origine d'encéphalopathies épileptiques infantiles rares, mais pouvant être sévères. En 2021, des recherches ont confirmé que les mutations hétérozygotes de RHOBTB2 englobaient d'autres signes cliniques que ces encéphalopathies. Ainsi, ces épilepsies infantiles sont associées, principalement, avec des phénotypes fortement variables, à un retard développemental, à des encéphalites post-traumatiques, à des troubles paroxystiques des mouvements et à des atteintes iconographiques de l'encéphale. Dans ce travail, après avoir présenté un cas clinique, nous rappellerons le rôle des RhoGTPases sur le développement neuronal. Nous discuterons ensuite d'une étude qui a mis en évidence l'impact neurodéveloppemental de mutations sur le gène RHOBTB2 en réalisant des travaux sur des mouches Drosophila melanogaster. Pour terminer, nous mettrons le cas clinique présenté en parallèle avec une revue de la littérature réalisée par rapport à ce gène.

Mutations in APOPT1, encoding a mitochondrial protein, cause cavitating leukoencephalopathy with cytochrome c oxidase deficiency.

Cytochrome c oxidase (COX) deficiency is a frequent biochemical abnormality in mitochondrial disorders, but a large fraction of cases remains genetically undetermined. Whole-exome sequencing led to the identification of APOPT1 mutations in two Italian sisters and in a third Turkish individual presenting severe COX deficiency. All three subjects presented a distinctive brain MRI pattern characterized by cavitating leukodystrophy, predominantly in the posterior region of the cerebral hemispheres. We then found APOPT1 mutations in three additional unrelated children, selected on the basis of these particular MRI features. All identified mutations predicted the synthesis of severely damaged protein variants. The clinical features of the six subjects varied widely from acute neurometabolic decompensation in late infancy to subtle neurological signs, which appeared in adolescence; all presented a chronic, long-surviving clinical course. We showed that APOPT1 is targeted to and localized within mitochondria by an N-terminal mitochondrial targeting sequence that is eventually cleaved off from the mature protein. We then showed that APOPT1 is virtually absent in fibroblasts cultured in standard conditions, but its levels increase by inhibiting the proteasome or after oxidative challenge. Mutant fibroblasts showed reduced amount of COX holocomplex and higher levels of reactive oxygen species, which both shifted toward control values by expressing a recombinant, wild-type APOPT1 cDNA. The shRNA-mediated knockdown of APOPT1 in myoblasts and fibroblasts caused dramatic decrease in cell viability. APOPT1 mutations are responsible for infantile or childhood-onset mitochondrial disease, hallmarked by the combination of profound COX deficiency with a distinctive neuroimaging presentation.

Mutation of the iron-sulfur cluster assembly gene IBA57 causes fatal infantile leukodystrophy.

Leukodystrophies are a heterogeneous group of severe genetic neurodegenerative disorders. A multiple mitochondrial dysfunctions syndrome was found in an infant presenting with a progressive leukoencephalopathy. Homozygosity mapping, whole exome sequencing, and functional studies were used to define the underlying molecular defect. Respiratory chain studies in skeletal muscle isolated from the proband revealed a combined deficiency of complexes I and II. In addition, western blotting indicated lack of protein lipoylation. The combination of these findings was suggestive for a defect in the iron-sulfur (Fe/S) protein assembly pathway. SNP array identified loss of heterozygosity in large chromosomal regions, covering the NFU1 and BOLA3, and the IBA57 and ABCB10 candidate genes, in 2p15-p11.2 and 1q31.1-q42.13, respectively. A homozygous c.436C > T (p.Arg146Trp) variant was detected in IBA57 using whole exome sequencing. Complementation studies in a HeLa cell line depleted for IBA57 showed that the mutant protein with the semi-conservative amino acid exchange was unable to restore the biochemical phenotype indicating a loss-of-function mutation of IBA57. In conclusion, defects in the Fe/S protein assembly gene IBA57 can cause autosomal recessive neurodegeneration associated with progressive leukodystrophy and fatal outcome at young age. In the affected patient, the biochemical phenotype was characterized by a defect in the respiratory chain complexes I and II and a decrease in mitochondrial protein lipoylation, both resulting from impaired assembly of Fe/S clusters.

Polymerase gamma deficiency (POLG): clinical course in a child with a two stage evolution from infantile myocerebrohepatopathy spectrum to an Alpers syndrome and neuropathological findings of Leigh's encephalopathy.

AIMS: Description of the clinical course in a child compound heterozygous for POLG1 mutations, neuropathology findings and results of dietary treatment based on fasting avoidance and long chain triglycerides (LCT) restriction. RESULTS: At 3(1/2) months of age the patient presented with severe hypoglycemia, hyperlactatemia, moderate ketosis and hepatic failure. Fasting hypoglycemia occurred 8 h after meals. The hypoglycemia did not respond to glucagon. She was supplemented with IV glucose and/or frequent feedings, but developed liver insufficiency which was reversed by long-chain triglyceride (LCT) restriction. Alpha-foeto-protein (AFP) levels were elevated and returned to low values after dietary treatment. Liver biopsy displayed cirrhosis, bile ductular proliferation, steatosis, isolated complex IV defect in part of the liver mitochondria, and mitochondrial DNA depletion (27% of control values). Two heterozygous mutations (p. [Ala467Thr] + p. [Gly848Ser]) were found in the POLG1 gene. At 3 years of age she progressively developed refractory mixed type seizures including a focal component and psychomotor regression which fulfilled the criteria of Alpers syndrome (AS) although the initial presentation was compatible with infantile myocerebrohepatopathy spectrum (MCHS). She died at 5 years of age of respiratory insufficiency. Neuropathologic investigation revealed lesions in the right striatal area and the inferior colliculi typical for Leigh's encephalopathy. CONCLUSION: The present patient showed an evolution from infantile MCHS to AS, and dietary treatment seemed to slow the progression of liver failure. In spite of the late clinical features of AS, it extends the neuropathological spectrum of AS and polymerase gamma deficiency (POLG) to Leigh syndrome lesions.

Blepharophimosis-mental retardation (BMR) syndromes: A proposed clinical classification of the so-called Ohdo syndrome, and delineation of two new BMR syndromes, one X-linked and one autosomal recessive.

We report on 11 patients from 8 families with a blepharophimosis and mental retardation syndrome (BMRS) phenotype. Using current nosology, five sporadic patients have Ohdo syndrome, associated with congenital hypothyroidism in two of them (thus also compatible with a diagnosis of Young-Simpson syndrome). In two affected sibs with milder phenotype, compensated hypothyroidism was demonstrated. In another family, an affected boy was born to the unaffected sister of a previously reported patient. Finally, in the last sibship, two affected boys in addition had severe microcephaly and neurological anomalies. A definitive clinical and etiologic classification of BMRS is lacking, but closer phenotypic analysis should lead to a more useful appraisal of the BMRS phenotype. We suggest discontinuing the systematic use of the term "Ohdo syndrome" when referring to patients with BMRS. We propose a classification of BMRS into five groups: (1) del(3p) syndrome, (possibly overlooked in older reports); (2) BMRS, Ohdo type, limited to the original patients of Ohdo; (3) BMRS SBBYS (Say-Barber/Biesecker/Young-Simpson) type, with distinctive dysmorphic features and inconstant anomalies including heart defect, optic atrophy, deafness, hypoplastic teeth, cleft palate, joint limitations, and hypothyroidism. BMRS type SBBYS is probably an etiologically heterogeneous phenotype, as AD and apparently AR forms exist; (4) BMRS, MKB (Maat-Kievit-Brunner) type, with coarse, triangular face, which is probably sex-linked; (5) BMRS V (Verloes) type, a probable new type with severe microcephaly, hypsarrhythmia, adducted thumbs, cleft palate, and abnormal genitalia, which is likely autosomal recessive. Types MKB and V are newly described here.