Extended phenotype of pontocerebellar hypoplasia with infantile spinal muscular atrophy

Pontocerebellar hypoplasia (PCH) is rarely associated with anterior horn cell disease and designated as PCH-1. This phenotype is characterized by severe muscle weakness and hypotonia starting prenatally or at birth with a life span not exceeding a few months in most cases. Milder disease courses with later onset and longer survival are normally not diagnosed as PCH-1. We describe the clinical and neuroradiological findings in nine patients out of six siblingships with evidence of cerebellar defects and early onset spinal muscular atrophy (SMA), representing a broad spectrum of clinical variability. In all patients, the diagnosis of SMA (Werdnig-Hoffmann disease) was made on the basis of electrophysiological data and muscle biopsy; however, genetic testing failed to confirm the diagnosis of infantile SMA with a gene defect on chromosome 5q and resulted in clinical reevaluation. Age at onset was after a normal period in the first months of life in three siblingships and pre- and postnatally in the other three families. Life span was 2-4 years in patients with later onset, and age at death occurred after birth or within months in the more severe group. Two siblingships showed discordant ages at death despite similar treatment. In contrast to the previous definition of PCH-1, our observations suggest the existence of milder phenotypes with pontocerebellar hypoplasia or olivopontocerebellar atrophy in combination with anterior horn cell loss. A pontine involvement is not necessarily seen by neuroimaging methods. The genetic basis of PCH-1 remains to be determined. The gene locus for infantile SMA on chromosome 5q could be excluded by linkage studies. Parental consanguinity and affected siblings make autosomal recessive inheritance most likely.     

Homozygous EXOSC3 Mutation c.92G→C,p.G31A is a Founder Mutation Causing Severe Pontocerebellar Hypoplasia Type 1 Among the Czech Roma

Abstract

Pontocerebellar hypoplasia type 1 (PCH1) is characterized by cerebellar and anterior horn motor neuron degeneration and loss, signs of spinal muscular atrophy plus. Patients manifest severe perinatal weakness, hypotonia, and respiratory insufficiency, causing death frequently before the age of 1 year. Recently, causative mutations in EXOSC3 were reported in a majority of PCH1 patients, but the detailed clinical phenotype caused by EXOSC3 mutations, genotype-phenotype correlations, and prevalent mutations in specific ethnic groups is not yet known.

Three unrelated Czech Roma patients with PCH1 were investigated clinically, electrophysiologically, neuroradiologically, and neuropathologically (patients 1 and 2). The entire coding region of the EXOSC3 gene, including the adjacent intron sequences, was sequenced in all three patients. The same mutation c.92G→C, p.G31A in EXOSC3 was found in all three affected patients in homozygous state and in heterozygous state in the parents from two of the families. Haplotype analysis with four flanking microsatellite markers showed identical haplotype in 9 out of 11 haplotypes carrying the c.92G→C, p.G31A mutation. Furthermore, four heterozygotes for this mutation were found in anonymous DNA samples from 90 unrelated Roma individuals. All four of these samples shared the same haplotype. No heterozygous sample was found among 120 anonymous DNA samples from Czech non-Roma individuals with no familial relation. It may therefore be concluded that EXOSC3 c.92G→C, p.G31A mutation is a founder mutation with high prevalence among the Czech Roma causing a similar and particularly severe phenotype of PCH1. These observations from the Czech Roma may have consequences also for other Roma from other countries.

PCH1 caused by EXOSC3 founder mutation c.92G→C, p.G31A extends the list of autosomal recessive disorders rare among the general population but more frequent among Roma at least in the Czech Republic.     

Pontocerebellar hypoplasia with rhombencephalosynapsis and microlissencephaly expands the spectrum of PCH type 1B

Rhombencephalosynapsis is a rare cerebellar malformation developing during embryogenesis defined by vermian agenesis or hypogenesis with fusion of the cerebellar hemispheres. It occurs either alone or in association with other cerebral and/or extracerebral anomalies. Its association with microlissencephaly is exceedingly rare and to date, only a heterozygous de novo missense variant in ADGRL2, a gene encoding Adhesion G-Protein-Coupled Receptor L2, has been identified. We report on two siblings of Roma origin presenting with severe growth retardation, fetal akinesia, microlissencephaly and small cerebellum with vermian agenesis. Neuropathological studies revealed extreme paucity in pontine transverse fibres, rudimentary olivary nuclei and rhombencephalosynapsis with vanishing spinal motoneurons in both fetuses. Comparative fetus-parent exome sequencing revealed in both fetuses a homozygous variant in exon 1 of the EXOSC3 gene encoding a core component of the RNA exosome, c.92G > C; p.(Gly31Ala). EXOSC3 accounts for 40%–75% of patients affected by ponto-cerebellar hypoplasia with spinal muscular atrophy (PCH1B). The c.92G > C variant is a founder mutation in the Roma population and has been reported in severe PCH1B. PCH1B is characterized by a broad phenotypic spectrum, ranging from mild phenotypes with spasticity, mild to moderate intellectual disability, pronounced distal muscular and cerebellar atrophy/hypoplasia, to severe phenotypes with profound global developmental delay, progressive microcephaly and atrophy of the cerebellar hemispheres. In PCH1B, the usual cerebellar lesions affect mainly the hemispheres with relative sparing of vermis that radically differs from rhombencephalosynapsis. This unusual foetal presentation expands the spectrum of PCH1B and highlights the diversity of rhombencephalosynapsis etiologies.     

Expanded PCH1D phenotype linked to EXOSC9 mutation

Pontocerebellar Hypoplasia type 1 is a rare heterogeneous neurodegenerative disorder with multiple subtypes linked to dysfunction of the exosome complex. Patients with mutations in exosome subunits exhibit a generally lethal phenotype characterized by cerebellar and pontine hypoplasia in association with spinal motor neuropathy and multiple systemic and neurologic features. Recently, two variants in the novel PCH1 associated protein EXOSC9 p.(Leu14Pro) and p.(Arg161*) have been identified in 4 unrelated patients exhibiting a severe phenotype involving cerebellar hypoplasia, axonal motor neuropathy, hypotonia, feeding difficulties, and respiratory insufficiency (PCH1D). We report clinical and molecular characterization of 2 unrelated patients exhibiting a relatively milder phenotype involving hypotonia, brachycephaly, cerebellar atrophy, psychomotor delay, as well as lactic acidosis and aberrant CNS myelination, resulting from the recurring homozygous missense mutation NM_001034194.1: c.41T>C; p.(Leu14Pro) in the EXOSC9 gene. We review the clinical picture of the EXOSC9-related PCH disorder.     

Widespread Lewy body and tau accumulation in childhood and adult onset dystonia-parkinsonism cases with PLA2G6 mutations

The 2 major types of neurodegeneration with brain iron accumulation (NBIA) are the pantothenate kinase type 2 (PANK2)-associated neurodegeneration (PKAN) and NBIA2 or infantile neuroaxonal dystrophy (INAD) due to mutations in the phospholipase A2, group VI (PLA2G6) gene. We have recently demonstrated clinical heterogeneity in patients with mutations in the PLA2G6 gene by identifying a poorly defined subgroup of patients who present late with dystonia and parkinsonism. We report the clinical and genetic features of 7 cases with PLA2G6 mutations. Brain was available in 5 cases with an age of death ranging from 8 to 36 years and showed widespread alpha-synuclein-positive Lewy pathology, which was particularly severe in the neocortex, indicating that the Lewy pathology spread corresponded to Braak stage 6 and was that of the “diffuse neocortical type”. In 3 cases there was hyperphosphorylated tau accumulation in both cellular processes as threads and neuronal perikarya as pretangles and neurofibrillary tangles. Later onset cases tended to have less tau involvement but still severe alpha-synuclein pathology. The clinical and neuropathological features clearly represent a link between PLA2G6 and parkinsonian disorders.     

Extension of the phenotype of biallelic loss‐of‐function mutations in SLC25A46 to the severe form of pontocerebellar hypoplasia type I

Biallelic mutations in SLC25A46, encoding a modified solute transporter involved in mitochondrial dynamics, have been identified in a wide range of conditions such as hereditary motor and sensory neuropathy with optic atrophy type VIB (OMIM: *610826) and congenital lethal pontocerebellar hypoplasia (PCH). To date, 18 patients from 13 families have been reported, presenting with the key clinical features of optic atrophy, peripheral neuropathy, and cerebellar atrophy. The course of the disease was highly variable ranging from severe muscular hypotonia at birth and early death to first manifestations in late childhood and survival into the fifties. Here we report on 4 patients from 2 families diagnosed with PCH who died within the first month of life from respiratory insufficiency. Patients from 1 family had pathoanatomically proven spinal motor neuron degeneration (PCH1). Using exome sequencing, we identified biallelic disease‐segregating loss‐of‐function mutations in SLC25A46 in both families. Our study adds to the definition of the SLC25A46‐associated phenotypic spectrum that includes neonatal fatalities due to PCH as the severe extreme.     

Clinical and genetic delineation of neurodegeneration with brain iron accumulation

Neurodegeneration with brain iron accumulation (NBIA) describes a group of progressive neurodegenerative disorders characterised by high brain iron and the presence of axonal spheroids, usually limited to the central nervous system. Mutations in the PANK2 gene account for the majority of NBIA cases and cause an autosomal recessive inborn error of coenzyme A metabolism called pantothenate kinase associated neurodegeneration (PKAN). More recently, it was found that mutations in the PLA2G6 gene cause both infantile neuroaxonal dystrophy (INAD) and, more rarely, an atypical neuroaxonal dystrophy that overlaps clinically with other forms of NBIA. High brain iron is also present in a portion of these cases. Clinical assessment, neuroimaging, and molecular genetic testing all play a role in guiding the diagnostic evaluation and treatment of NBIA.     

TSEN54 mutations cause pontocerebellar hypoplasia type 5

Pontocerebellar hypoplasia (PCH) is a group of autosomal recessive neurodegenerative disorders characterized by prenatal onset of stunted brain growth and progressive atrophy predominantly affecting cerebellum, pons and olivary nuclei, and to a lesser extent also the cerebral cortex. Six subtypes (PCH1-6) were described and genes for four types (PCH1, 2, 4 and 6) were identified. Mutations in the tRNA splicing endonuclease subunit (TSEN) genes 54, 2 and 34 are found in PCH2 and PCH4. One family with severe prenatal onset of PCH has been the only representative of PCH5 published so far, and the molecular genetic status of PCH5 has not been ascertained until now. We screened the previously reported PCH5 family for mutations in the TSEN54 gene. The PCH5 patient was found to be the result of compound heterozygosity for the common TSEN54 mutation (p.A307S) plus a novel splice site mutation. The mutations associated with PCH5 are similar to what has been reported in PCH4. Thus, PCH5, PCH4 and PCH2 represent a spectrum of clinical manifestations caused by different mutations in the TSEN genes. We, therefore, propose to classify PCH2, PCH4 and PCH5 as TSEN mutation spectrum disorders.     

Impairment of the tRNA-splicing endonuclease subunit 54 (tsen54) gene causes neurological abnormalities and larval death in zebrafish models of pontocerebellar hypoplasia

Pontocerebellar hypoplasia (PCH) represents a group (PCH1-6) of neurodegenerative autosomal recessive disorders characterized by hypoplasia and/or atrophy of the cerebellum, hypoplasia of the ventral pons, progressive microcephaly and variable neocortical atrophy. The majority of PCH2 and PCH4 cases are caused by mutations in the TSEN54 gene; one of the four subunits comprising the tRNA-splicing endonuclease (TSEN) complex. We hypothesized that TSEN54 mutations act through a loss of function mechanism. At 8 weeks of gestation, human TSEN54 is expressed ubiquitously in the brain, yet strong expression is seen within the telencephalon and metencephalon. Comparable expression patterns for tsen54 are observed in zebrafish embryos. Morpholino (MO) knockdown of tsen54 in zebrafish embryos results in loss of structural definition in the brain. This phenotype was partially rescued by co-injecting the MO with human TSEN54 mRNA. A developmental patterning defect was not associated with tsen54 knockdown; however, an increase in cell death within the brain was observed, thus bearing resemblance to PCH pathophysiology. Additionally, N-methyl-N-nitrosourea mutant zebrafish homozygous for a tsen54 premature stop-codon mutation die within 9 days post-fertilization. To determine whether a common disease pathway exists between TSEN54 and other PCH-related genes, we also monitored the effects of mitochondrial arginyl-tRNA synthetase (rars2; PCH1?and PCH6) knockdown in zebrafish. Comparable brain phenotypes were observed following the inhibition of both genes. These data strongly support the hypothesis that TSEN54 mutations cause PCH through a loss of function mechanism. Also we suggest that a common disease pathway may exist between TSEN54- and RARS2-related PCH, which may involve a tRNA processing-related mechanism.     

Novel splice‐site mutations and a large intragenic deletion in PLA2G6 associated with a severe and rapidly progressive form of infantile neuroaxonal dystrophy

Tonelli A, Romaniello R, Grasso R, Cavallini A, Righini A, Bresolin N, Borgatti R, Bassi MT. Novel splice‐site mutations and a large intragenic deletion in PLA2G6 associated with a severe and rapidly progressive form of infantile neuroaxonal dystrophy. Infantile neuroaxonal dystrophy, INAD, is a severe progressive psychomotor disorder with infantile onset and characterized by the presence of axonal spheroids throughout the central and peripheral nervous systems. A subset of INAD patients shows also brain iron accumulation which represents instead the distinctive feature of the idiopathic neurodegeneration with brain iron accumulation, NBIA. These diseases share the same causative gene, PLA2G6, encoding iPLA2‐VIA, a calcium‐independent phospholipase. Mutations that lead to a complete absence of protein are associated with a severe INAD profile, while compound heterozygous mutations with possibly a residual protein activity are instead associated with the less severe NBIA phenotype. Here we describe two INAD patients both with an unusually rapid disease progression and a peculiar neuroradiological presentation in one of them. Compound heterozygosity for a large intragenic deletion and a nonsense mutation was found in one of them while the other is carrying two novel splice‐site mutations. Breakpoint‐sequence analysis suggests a non‐allelic‐homologous‐recombination (NAHR) event, probably underlying the rearrangement. These findings, while supporting the genotype–phenotype correlation already observed in INAD patients, provide the first sequence characterization of a genomic rearrangement in PLA2G6 gene, thus orienting the search for missing mutant alleles in PLA2G6 related diseases.     

Exclusion of 5 functional candidate genes for distal hereditary motor neuropathy type II (distal HMN II) linked to 12q24.3

Distal hereditary motor neuropathies (distal HMNs) are characterised by degeneration of anterior horn cells of the spinal cord resulting in muscle weakness and atrophy. Distal HMN type II is genetically linked to chromosome 12q24.3 and located within a 13 cM region flanked by markers D12S86 and D12S340. We previously excluded the human phospholipase A2 group 1B gene ( PLA2G1B ) as the disease causing gene. Here, we report the mutation analysis of five other candidate genes localised within the distal HMN II region: the cytoskeletal proteins paxillin ( PXN ) and restin ( RSN ); the acidic ribosomal phosphoprotein, large P0 subunit ( RPLP0 ); a nucleoside diphosphate kinase ( NME2B ); and the β 3 subunit of the voltage-gated calcium channel ( CACNB3 ). DNA sequencing of the coding regions was performed but no disease causing mutations could be identified, hence excluding these five genes for distal HMN type II.     

Biallelic variant in AGTPBP1 causes infantile lower motor neuron degeneration and cerebellar atrophy

Infantile hereditary lower motor neuron disorders beyond 5q–spinal muscular atrophy (5q‐SMA) are usually caused by mutations other than deletions or mutations in SMN1. In addition to motor neuron degeneration, further neurologic or multisystemic pathologies in non‐5q‐SMAs are not seldom. Some of the non‐5q‐SMA phenotypes, such as pontocerebellar hypoplasia (PCH1), have been classified later as a different disease group due to distinctive primary pathologies. Likewise, a novel phenotype, childhood‐onset neurodegeneration with cerebellar atrophy (CONDCA) has been described recently in individuals with lower motor neuron disorder and cerebellar atrophy due to biallelic loss‐of‐function variants in AGTPBP1 that encodes cytosolic carboxypeptidase 1 (CCP1). Here we present two individuals with CONDCA in whom a biallelic missense AGTPBP1 variant (NM_001330701.1:c.2396G>T, p.Arg799Leu) was identified by whole exome sequencing. Affected individuals in this report correspond to the severe infantile spectrum of the disease and underline the severe pathogenic effect of this missense variant. This report is the second in the literature that delineates the pathogenic effects of biallelic AGTPBP1 variants presenting the recently described CONDCA disease.     

Allelic heterogeneity of SMARD1 at the IGHMBP2 locus

Spinal Muscular Atrophy with Respiratory Distress (SMARD) is an autosomal recessive disorder characterized by neurogenic muscular atrophy due to progressive anterior horn cell degeneration and early life-threatening respiratory failure ascribed to diaphragmatic dysfunction. SMARD is clinically and genetically heterogeneous. SMARD type 1 is characterized by onset of respiratory failure within the first weeks of life and has been ascribed to mutations in the immunoglobulin mu-binding protein 2 (IGHMBP2) gene on chromosome 11q13-q21. We report here the identification of nine novel IGHMBP2 mutations in five SMARD1 patients, including seven missense [ c.587A>G (p.Gln196Arg), c.647C>T (p.Pro216Leu), c.752T>C (p.Leu251Pro), c.1693G>A (p.Asp565Asn), c.1730T>C (p.Leu577Pro), c.1807C>T (p.Arg603Cys), c.1909C>T (p.Arg637Cys)] and two nonsense mutations [ c.1488C>A (p.Cys496X), c.2368C>T (p.Arg790X)]. Interestingly, 7 of 9 mutations occurred at highly conserved residues of the putative DNA helicase domain. The identification of novel IGHMBP2 variants will hopefully help diagnosing SMARD1 and contribute to a better functional characterization of IGHMBP2 gene product.     

Severe, fetal-onset form of olivopontocerebellar hypoplasia in three sibs: PCH type 5?

We present three siblings with a precise onset of fetal seizure-like activity who had severe olivopontocerebellar hypoplasia (OPCH) and degeneration. Autopsies at 20, 27, and 37 weeks gestation showed diffuse central nervous system volume loss that was most marked for the cerebellum and brain stem structures. Neuropathological abnormalities included dysplastic, C-shaped inferior olivary nuclei, absent or immature dentate nuclei, and cell paucity more marked for the cerebellar vermis than the hemispheres. Delayed development was seen in layer 2 of the cerebral cortex and in Purkinje cells of the cerebellum. Prenatal monitoring defined a developmental window of 16-18 weeks gestation when ultrasonic assessment of cerebellar width was used for prenatal diagnosis. We discuss our findings in the context of the differential diagnosis for infantile (O)PCH and propose a classification scheme for the pontocerebellar hypoplasias. These patients represent the earliest reported with OPCH and provide unique information regarding the developmental neuropathology of this condition.     

Novel EXOSC3 pathogenic variant results in a mild course of neurologic disease with cerebellum involvement

EXOSC3-related autosomal recessive neurodevelopmental disorders are rare entities with variable clinical course and prognosis. They are characterized by hypoplasia of cerebellar structures and pons, degeneration of the anterior horn cells and motor as well as neurocognitive impairment. Phenotypic expression is variable with an overall poor outcome. Current research suggests clear genotype-phenotype correlations among EXOSC3-pathogenic-variants carriers. Homozygosity for the EXOSC3 variant c.395A > C, p.(Asp132Ala) is proposed to lead to a rather mild phenotype compared to compound-heterozygous EXOSC3-pathogenic-variants carriers with lethal neurological disease in very early childhood. In this study, we report two siblings (21- and 8-year-old) affected by PCH1B with an unusual presentation. We identified compound heterozygosity for the well-established EXOSC3 variant c.395A > C, p.(Asp132Ala) and the novel variant c.572G > A, p.(Gly191Asp), expanding the genetic spectrum. Phenotypic presentation of the siblings was strikingly different from that of literature reports with a surprisingly mild disease manifestation and an unexpected intrafamilial variability. This study demonstrates the extensive clinical heterogeneity and the broad phenotypic spectrum associated with EXOSC3-associated disorders. Enlargement of sample sizes and reports of novel cases will be essential for the delineation of associated phenotypes.     

A de novo missense mutation in the inositol 1,4,5‐triphosphate receptor type 1 gene causing severe pontine and cerebellar hypoplasia: Expanding the phenotype of ITPR1‐related spinocerebellar ataxia's

We report a de novo missense mutation (c.7649T>A) in the inositol, 1,4,5 triphosphate receptor type 1 (ITPR1) gene in a patient with severe pontocerebellar hypoplasia. The mutation results in an amino acid substitution of a highly conserved isoleucine by asparagine (p. I2550N) in the transmembrane domain. Mutations and deletions of the ITPR1 gene are associated with several types of autosomal dominant spinocerebellar ataxia, varying in age of onset and severity. Patients have signs of cerebellar ataxia and at most, a mild cerebellar atrophy on MRI. In contrast, the patient we report here has profound cerebellar and pontine hypoplasia. Our finding therefore further expands the spectrum of ITPR1‐related ataxias. © 2016 Wiley Periodicals, Inc.     

Spectrum of malformations of the hindbrain (cerebellum, pons, and medulla) in a cohort of children with high rate of parental consanguinity

We review 25 patients with a spectrum of hindbrain (cerebellum, pons, and medulla) malformations from a cohort of children with high parental consanguinity rate. Twenty-three of the 25 patients were born to consanguineous parents. The patients were classified in four groups. Eleven patients of 6 families had malformation of the hindbrain and midbrain with molar tooth sign (10 patients of 5 families with typical Joubert syndrome), 5 patients showed severe supratentorial anomalies in addition to the hindbrain malformations, 5 patients had pontocerebellar or cerebellar hypoplasia with anterior horn cell disease in the spinal cord (spinal muscular atrophy), and 4 patients showed malformations affecting predominantly the hindbrain without substantial involvement of other systems. A locus for Joubert syndrome was previously identified on chromosome 9q34.3 in two families, and a second locus on chromosome 11p12-q13.3 in another family. A third Joubert syndrome locus has been mapped at 6q23 and a mutation in the AHI1 gene at this site has been found recently in a further family from this cohort. Delineation of homogeneous subgroups of patients with hindbrain malformations and molecular genetic analysis of these groups may lead to identification of further loci, genes and mutations responsible for the malformations.