Genetic background of ataxia in children younger than 5 years in Finland.

OBJECTIVE: To characterize the genetic background of molecularly undefined childhood-onset ataxias in Finland. METHODS: This study examined a cohort of patients from 50 families with onset of an ataxia syndrome before the age of 5 years collected from a single tertiary center, drawing on the advantages offered by next generation sequencing. A genome-wide genotyping array (Illumina Infinium Global Screening Array MD-24 v.2.0) was used to search for copy number variation undetectable by exome sequencing. RESULTS: Exome sequencing led to a molecular diagnosis for 20 probands (40%). In the 23 patients examined with a genome-wide genotyping array, 2 additional diagnoses were made. A considerable proportion of probands with a molecular diagnosis had de novo pathogenic variants (45%). In addition, the study identified a de novo variant in a gene not previously linked to ataxia: MED23. Patients in the cohort had medically actionable findings. CONCLUSIONS: There is a high heterogeneity of causative mutations in this cohort despite the defined age at onset, phenotypical overlap between patients, the founder effect, and genetic isolation in the Finnish population. The findings reflect the heterogeneous genetic background of ataxia seen worldwide and the substantial contribution of de novo variants underlying childhood ataxia.

Phosphorylation of Parkin at serine 65 is essential for its activation in vivo.

Mutations in PINK1 and Parkin result in autosomal recessive Parkinson's disease (PD). Cell culture and in vitro studies have elaborated the PINK1-dependent regulation of Parkin and defined how this dyad orchestrates the elimination of damaged mitochondria via mitophagy. PINK1 phosphorylates ubiquitin at serine 65 (Ser65) and Parkin at an equivalent Ser65 residue located within its N-terminal ubiquitin-like domain, resulting in activation; however, the physiological significance of Parkin Ser65 phosphorylation in vivo in mammals remains unknown. To address this, we generated a Parkin Ser65Ala (S65A) knock-in mouse model. We observe endogenous Parkin Ser65 phosphorylation and activation in mature primary neurons following mitochondrial depolarization and reveal this is disrupted in ParkinS65A/S65A neurons. Phenotypically, ParkinS65A/S65A mice exhibit selective motor dysfunction in the absence of any overt neurodegeneration or alterations in nigrostriatal mitophagy. The clinical relevance of our findings is substantiated by the discovery of homozygous PARKIN (PARK2) p.S65N mutations in two unrelated patients with PD. Moreover, biochemical and structural analysis demonstrates that the ParkinS65N/S65N mutant is pathogenic and cannot be activated by PINK1. Our findings highlight the central role of Parkin Ser65 phosphorylation in health and disease.

Genetic Basis of Severe Childhood-Onset Cardiomyopathies.

BACKGROUND: Childhood cardiomyopathies are progressive and often lethal disorders, forming the most common cause of heart failure in children. Despite severe outcomes, their genetic background is still poorly characterized. OBJECTIVES: The purpose of this study was to characterize the genetics of severe childhood cardiomyopathies in a countrywide cohort. METHODS: The authors collected a countrywide cohort, KidCMP, of 66 severe childhood cardiomyopathies from the sole center in Finland performing cardiac transplantation. For genetic diagnosis, next-generation sequencing and subsequent validation using genetic, cell biology, and computational approaches were used. RESULTS: The KidCMP cohort presents remarkable early-onset and severe disorders: the median age of diagnosis was 0.33 years, and 17 patients underwent cardiac transplantation. The authors identified the pathogenic variants in 39% of patients: 46% de novo, 34% recessive, and 20% dominantly-inherited. The authors report NRAP underlying childhood dilated cardiomyopathy, as well as novel phenotypes for known heart disease genes. Some genetic diagnoses have immediate implications for treatment: CALM1 with life-threatening arrhythmias, and TAZ with good cardiac prognosis. The disease genes converge on metabolic causes (PRKAG2, MRPL44, AARS2, HADHB, DNAJC19, PPA2, TAZ, BAG3), MAPK pathways (HRAS, PTPN11, RAF1, TAB2), development (NEK8 and TBX20), calcium signaling (JPH2, CALM1, CACNA1C), and the sarcomeric contraction cycle (TNNC1, TNNI3, ACTC1, MYH7, NRAP). CONCLUSIONS: Childhood cardiomyopathies are typically caused by rare, family-specific mutations, most commonly de novo, indicating that next-generation sequencing of trios is the approach of choice in their diagnosis. Genetic diagnoses may suggest intervention strategies and predict prognosis, offering valuable tools for prioritization of patients for transplantation versus conservative treatment.

ATPase-deficient mitochondrial inner membrane protein ATAD3A disturbs mitochondrial dynamics in dominant hereditary spastic paraplegia.

De novo mutations in ATAD3A (ATPase family AAA-domain containing protein 3A) were recently found to cause a neurological syndrome with developmental delay, hypotonia, spasticity, optic atrophy, axonal neuropathy, and hypertrophic cardiomyopathy. Using whole-exome sequencing, we identified a dominantly inherited heterozygous variant c.1064G > A (p.G355D) in ATAD3A in a mother presenting with hereditary spastic paraplegia (HSP) and axonal neuropathy and her son with dyskinetic cerebral palsy, both with disease onset in childhood. HSP is a clinically and genetically heterogeneous disorder of the upper motor neurons. Symptoms beginning in early childhood may resemble spastic cerebral palsy. The function of ATAD3A, a mitochondrial inner membrane AAA ATPase, is yet undefined. AAA ATPases form hexameric rings, which are catalytically dependent on the co-operation of the subunits. The dominant-negative patient mutation affects the Walker A motif, which is responsible for ATP binding in the AAA module of ATAD3A, and we show that the recombinant mutant ATAD3A protein has a markedly reduced ATPase activity. We further show that overexpression of the mutant ATAD3A fragments the mitochondrial network and induces lysosome mass. Similarly, we observed altered dynamics of the mitochondrial network and increased lysosomes in patient fibroblasts and neurons derived through differentiation of patient-specific induced pluripotent stem cells. These alterations were verified in patient fibroblasts to associate with upregulated basal autophagy through mTOR inactivation, resembling starvation. Mutations in ATAD3A can thus be dominantly inherited and underlie variable neurological phenotypes, including HSP, with intrafamiliar variability. This finding extends the group of mitochondrial inner membrane AAA proteins associated with spasticity.

SNCA mutation p.Ala53Glu is derived from a common founder in the Finnish population.

Mutations in SNCA are rare causes of familial Parkinson's disease (PD). We have previously described a novel p.Ala53Glu mutation in 2 Finnish families. To assess this mutation's frequency among Finnish PD patients, we screened 110 PD patients (mean age-of-onset 60 years) from Western Finland by Sanger sequencing of the third coding exon of SNCA. In addition, a sample of 47 PD subjects (mean age-of-onset 53 years) originating from Southern and Eastern Finland were studied using next-generation sequencing covering SNCA. Only one new individual with the p.Ala53Glu mutation was identified, confirming that this mutation is a rare cause of PD in the Finnish population. To search for a possible common origin of the p.Ala53Glu mutation, haplotype analysis was conducted in 2 families and in a patient from a third family (6 affected subjects) using both STR markers and a genome-wide SNP array. The results show that patients with the p.Ala53Glu mutation share a haplotype spanning a minimum of 5.7 Mb suggesting a common founder.

The Variant p.(Arg183Trp) in SPTLC2 Causes Late-Onset Hereditary Sensory Neuropathy.

Hereditary sensory and autonomic neuropathy 1 (HSAN1) is an autosomal dominant disorder that can be caused by variants in SPTLC1 or SPTLC2, encoding subunits of serine palmitoyl-CoA transferase. Disease variants alter the enzyme's substrate specificity and lead to accumulation of neurotoxic 1-deoxysphingolipids. We describe two families with autosomal dominant HSAN1C caused by a new variant in SPTLC2, c.547C>T, p.(Arg183Trp). The variant changed a conserved amino acid and was not found in public variant databases. All patients had a relatively mild progressive distal sensory impairment, with onset after age 50. Small fibers were affected early, leading to abnormalities on quantitative sensory testing. Sural biopsy revealed a severe chronic axonal neuropathy with subtotal loss of myelinated axons, relatively preserved number of non-myelinated fibers and no signs for regeneration. Skin biopsy with PGP9.5 labeling showed lack of intraepidermal nerve endings early in the disease. Motor manifestations developed later in the disease course, but there was no evidence of autonomic involvement. Patients had elevated serum 1-deoxysphingolipids, and the variant protein produced elevated amounts of 1-deoxysphingolipids in vitro, which proved the pathogenicity of the variant. Our results expand the genetic spectrum of HSAN1C and provide further detail about the clinical characteristics. Sequencing of SPTLC2 should be considered in all patients presenting with mild late-onset sensory-predominant small or large fiber neuropathy.

Mesencephalic complex I deficiency does not correlate with parkinsonism in mitochondrial DNA maintenance disorders.

Genetic evidence from recessively inherited Parkinson's disease has indicated a clear causative role for mitochondrial dysfunction in Parkinson's disease. This role has long been discussed based on findings that toxic inhibition of mitochondrial respiratory complex I caused parkinsonism and that tissues of patients with Parkinson's disease show complex I deficiency. Disorders of mitochondrial DNA maintenance are a common cause of inherited neurodegenerative disorders, and lead to mitochondrial DNA deletions or depletion and respiratory chain defect, including complex I deficiency. However, parkinsonism associates typically with defects of catalytic domain of mitochondrial DNA polymerase gamma. Surprisingly, however, not all mutations affecting DNA polymerase gamma manifest as parkinsonism, but, for example, spacer region mutations lead to spinocerebellar ataxia and/or severe epilepsy. Furthermore, defective Twinkle helicase, a close functional companion of DNA polymerase gamma in mitochondrial DNA replication, results in infantile-onset spinocerebellar ataxia, epilepsy or adult-onset mitochondrial myopathy, but not typically parkinsonism. Here we sought for clues for this specificity in the neurological manifestations of mitochondrial DNA maintenance disorders by studying mesencephalic neuropathology of patients with DNA polymerase gamma or Twinkle defects, with or without parkinsonism. We show here that all patients with mitochondrial DNA maintenance disorders had neuronopathy in substantia nigra, most severe in DNA polymerase gamma-associated parkinsonism. The oculomotor nucleus was also affected, but less severely. In substantia nigra, all patients had a considerable decrease of respiratory chain complex I, but other respiratory chain enzymes were not affected. Complex I deficiency did not correlate with parkinsonism, age, affected gene or inheritance. We conclude that the cell number in substantia nigra correlated well with parkinsonism in DNA polymerase gamma and Twinkle defects. However, complex I defect is a general consequence of mitochondrial DNA maintenance defects, and does not explain manifestation of parkinsonism or degree of mesencephalic cell death in patients with mitochondrial DNA maintenance disorders.

Mitochondrial recessive ataxia syndrome mimicking dominant spinocerebellar ataxia.

We studied the genetic background of a family with SCA, showing dominant inheritance and anticipation. Muscle histology, POLG1 gene sequence, neuropathology and mitochondrial DNA analyses in a mother and a son showed typical findings for a mitochondrial disorder, and both were shown to be homozygous for a recessive POLG1 mutation, underlying mitochondrial recessive ataxia syndrome, MIRAS. The healthy father was a heterozygous carrier for the same mutation. Recessively inherited MIRAS mutations should be tested in dominantly inherited SCAs cases of unknown cause, as the high carrier frequency of MIRAS may result in two independent introductions of the mutant allele in the family and thereby mimic dominant inheritance.

Clustering of Alpers disease mutations and catalytic defects in biochemical variants reveal new features of molecular mechanism of the human mitochondrial replicase, Pol γ.

Mutations in Pol γ represent a major cause of human mitochondrial diseases, especially those affecting the nervous system in adults and in children. Recessive mutations in Pol γ represent nearly half of those reported to date, and they are nearly uniformly distributed along the length of the POLG1 gene (Human DNA Polymerase gamma Mutation Database); the majority of them are linked to the most severe form of POLG syndrome, Alpers-Huttenlocher syndrome. In this report, we assess the structure-function relationships for recessive disease mutations by reviewing existing biochemical data on site-directed mutagenesis of the human, Drosophila and yeast Pol γs, and their homologs from the family A DNA polymerase group. We do so in the context of a molecular model of Pol γ in complex with primer-template DNA, which we have developed based upon the recently solved crystal structure of the apoenzyme form. We present evidence that recessive mutations cluster within five distinct functional modules in the catalytic core of Pol γ. Our results suggest that cluster prediction can be used as a diagnosis-supporting tool to evaluate the pathogenic role of new Pol γ variants.

Functional analysis of H. sapiens DNA polymerase gamma spacer mutation W748S with and without common variant E1143G.

Mitochondrial DNA polymerase, POLG, is the sole DNA polymerase found in animal mitochondria. In humans, POLGalpha W748S in cis with an E1143G mutation has been linked to a new type of recessive ataxia, MIRAS, which is the most common inherited ataxia in Finland. We investigated the biochemical phenotypes of the W748S amino acid change, using recombinant human POLG. We measured processive and non-processive DNA polymerase activity, DNA binding affinity, enzyme processivity, and subunit interaction with recombinant POLGbeta. In addition, we studied the effects of the W748S and E1143G mutations in primary human cell cultures using retroviral transduction. Here, we examined cell viability, mitochondrial DNA copy number, and products of mitochondrial translation. Our results indicate that the W748S mutant POLGalpha does not exhibit a clear biochemical phenotype, making it indistinguishable from wild type POLGalpha and as such, fail to replicate previously published results. Furthermore, results from the cell models were concurrent with the findings from patients, and support our biochemical findings.