SPAST Intragenic CNVs Lead to Hereditary Spastic Paraplegia via a Haploinsufficiency Mechanism.

The most common form of hereditary spastic paraplegia (HSP), SPG4 is caused by single nucleotide variants and microrearrangements in the SPAST gene. The high percentage of multi-exonic deletions or duplications observed in SPG4 patients is predisposed by the presence of a high frequency of Alu sequences in the gene sequence. In the present study, we analyzed DNA and RNA samples collected from patients with different microrearrangements in SPAST to map gene breakpoints and evaluate the mutation mechanism. The study group consisted of 69 individuals, including 50 SPG4 patients and 19 healthy relatives from 18 families. Affected family members from 17 families carried varying ranges of microrearrangements in the SPAST gene, while one individual had a single nucleotide variant in the 5'UTR of SPAST. To detect the breakpoints of the SPAST gene, long-range PCR followed by sequencing was performed. The breakpoint sequence was detected for five different intragenic SPAST deletions and one duplication, revealing Alu-mediated microhomology at breakpoint junctions resulting from non-allelic homologous recombination in these patients. Furthermore, SPAST gene expression analysis was performed using patient RNA samples extracted from whole blood. Quantitative real-time PCR tests performed in 14 patients suggest no expression of transcripts with microrearrangements in 5 of them. The obtained data indicate that nonsense-mediated decay degradation is not the only mechanism of hereditary spastic paraplegia in patients with SPAST microrearrangements.

Assessment of serum ferritin as a biomarker in COVID-19: bystander or participant? Insights by comparison with other infectious and non-infectious diseases.

BACKGROUND: The 2019 coronavirus disease (COVID-19) caused by the SARS-CoV-2 virus has an impact on all aspects of patient care. Serum ferritin generally represents a biomarker of choice when iron deficiency is suspected. However, ferritin is also an acute-phase-protein exhibiting elevated serum concentration in various inflammatory diseases. Here we focus on the role of serum ferritin for diagnostic and clinical management of patients with COVID-19 in comparison with other infectious and non-infectious diseases. METHODS: We examined scientific articles listed in PubMed reporting on ferritin in various infectious and non-infectious diseases. We then compared these results with nine current COVID-19 ferritin reports published in 2020. RESULTS: Several non-infectious, as well as non-COVID-19 infectious diseases, are characterised by a partly dramatic elevation of serum ferritin levels. All COVID-19 studies published between February and May 2020, which documented laboratory serum ferritin, indicate ferritin as a biomarker of COVID-19 severity in hospitalised patients. CONCLUSIONS: Serum ferritin may be considered both a prognostic and stratifying biomarker that can also contribute to therapeutic decision-making concerning patients with COVID-19. It should be emphasised, however, that most scientific reports refer to cohorts in the Asian region. Further validation in other cohorts is urgently required.

"Missing mutations" in MPS I: Identification of two novel copy number variations by an IDUA-specific in house MLPA assay.

BACKGROUND: Mucopolysaccharidosis type I (MPS I) is a rare, recessively inherited lysosomal storage disorder, characterized by progressive multi-systemic disease. It is caused by a reduced or absent alpha-l iduronidase (IDUA) enzyme activity secondary to biallelic loss-of-function variants in the IDUA. Over 200 causative variants in IDUA have been identified. Nevertheless, there is a fraction of MPS I patients with only a single mutated IDUA allele detectable. METHODS: As genetic testing of MPS I is usually based on sequencing methods, copy number variations (CNVs) in IDUA can be missed and therefore presumably remain underdiagnosed. The aim of this study was the detection of CNVs using an IDUA-specific in house multiplex ligation-dependent probe amplification (MLPA) assay. RESULTS: A total of five unrelated MPS I patient samples were re-analyzed after only a single heterozygous IDUA mutation c.979G>C (p.A327P), c.1469T>C (p.L490P), c.1598C>G (p.P533R), c.1205G>A (p.W402X), c.973-7C>G (p.?) could be identified. We detected a novel splice site variant c.973-7C>G (p.?), as well as two novel CNVs, a large deletion of IDUA exon 14 and 3'UTR c.(1828 + 1_1829-1)_(*1963_?)del, and a large duplication extending from IDUA exon 2 to intron 12 c.(157 + 1_158-1)_(1727 + 1_1728-1)dup. CONCLUSION: Together with the CNVs we previously identified, a total of four pathogenic IDUA CNVs have now been reported.

Autosomal dominant mitochondrial membrane protein-associated neurodegeneration (MPAN).

BACKGROUND: Mitochondrial membrane protein-associated neurodegeneration (MPAN) is caused by pathogenic sequence variants in C19orf12. Autosomal recessive inheritance has been demonstrated. We present evidence of autosomal dominant MPAN and propose a mechanism to explain these cases. METHODS: Two large families with apparently dominant MPAN were investigated; additional singleton cases of MPAN were identified. Gene sequencing and multiplex ligation-dependent probe amplification were used to characterize the causative sequence variants in C19orf12. Post-mortem brain from affected subjects was examined. RESULTS: In two multi-generation non-consanguineous families, we identified different nonsense sequence variations in C19orf12 that segregate with the MPAN phenotype. Brain pathology was similar to that of autosomal recessive MPAN. We additionally identified a preponderance of cases with single heterozygous pathogenic sequence variants, including two with de novo changes. CONCLUSIONS: We present three lines of clinical evidence to demonstrate that MPAN can manifest as a result of only one pathogenic C19orf12 sequence variant. We propose that truncated C19orf12 proteins, resulting from nonsense variants in the final exon in our autosomal dominant cohort, impair function of the normal protein produced from the non-mutated allele via a dominant negative mechanism and cause loss of function. These findings impact the clinical diagnostic evaluation and counseling.

Development and validation of a TTR-specific copy number screening tool, and application to potentially relevant patient cohorts.

TTR amyloidosis (ATTR) is a fatal condition caused by extracellular deposits of misfolded transthyretin. Patients often present with cardiac disease, but manifestations may also involve other organs including the peripheral nervous system. ATTR is considered familial when heterozygous mutations in the TTR gene are present (ATTRmutant or ATTRm), or acquired when no TTR aberrations are detected (ATTRwildtype or ATTRwt). We hypothesized that TTR copy number variants (CNVs), which would escape the standard diagnostic approaches, contribute to ATTR-related phenotypes, and developed a multiplex ligation-dependent probe amplification-based (MLPA-based), TTR-specific copy number screening tool. High inter-sample and intra-sample homogeneity of MLPA signals and the expected drop in signal intensity for restriction digest-based positive controls validated this tool. Subsequent application to 13 patients diagnosed with ATTRwt, and to 93 patients presenting with late onset and presumably inherited polyneuropathy did not identify TTR CNVs. We discuss insufficient sensitivity of the assay as well as non-existence and non-pathogenicity of TTR CNVs as potentially underlying our negative finding, but suggest size and composition of our cohorts as more likely explanations. Our CNV-screening tool will be made available to initiatives interested in screening additional and potentially more appropriate patient samples.

A nonstop variant in REEP1 causes peripheral neuropathy by unmasking a 3'UTR-encoded, aggregation-inducing motif.

Single-nucleotide variants that abolish the stop codon ("nonstop" alterations) are a unique type of substitution in genomic DNA. Whether they confer instability of the mutant mRNA or result in expression of a C-terminally extended protein depends on the absence or presence of a downstream in-frame stop codon, respectively. Of the predicted protein extensions, only few have been functionally characterized. In a family with autosomal dominant Charcot-Marie-Tooth disease type 2, that is, an axonopathy affecting sensory neurons as well as lower motor neurons, we identified a heterozygous nonstop variant in REEP1. Mutations in this gene have classically been associated with the upper motor neuron disorder hereditary spastic paraplegia (HSP). We show that the C-terminal extension resulting from the nonstop variant triggers self-aggregation of REEP1 and of several reporters. Our findings support the recently proposed concept of 3'UTR-encoded "cryptic amyloidogenic elements." Together with a previous report on an aggregation-prone REEP1 deletion variant in distal hereditary motor neuropathy, they also suggest that toxic gain of REEP1 function, rather than loss-of-function as relevant for HSP, specifically affects lower motor neurons. A search for similar correlations between genotype, phenotype, and effect of mutant protein may help to explain the wide clinical spectra also in other genetically determined disorders.

Doublet-Mediated DNA Rearrangement-A Novel and Potentially Underestimated Mechanism for the Formation of Recurrent Pathogenic Deletions.

Deletions and duplications of genomic DNA contribute to evolution, phenotypic diversity, and human disease. The underlying mechanisms are incompletely understood. We identified deletions of exon 10 of the SPAST gene in two unrelated families with hereditary spastic paraplegia. We excluded a founder event, but observed that the breakpoints map to identical repeat regions. These regions likely represent an intragenic "doublet," that is, an enigmatic class of local duplications. The fusion sequences for both deletions are compatible with recombination-based as well as with replication-based mechanisms. Searching the literature, we identified a partial SLC24A4 deletion that involved two copies of another doublet, and was likely formed in an analogous way. Comparing the SPAST and the SLC24A4 doublets with doublets identified previously suggested that many additional doublets have a high potential for triggering rearrangements. Considering that doublets are still being formed in the human genome, and that they likely create high local instability, we suggest that a two-step mechanism consisting of doublet generation and subsequent doublet-mediated deletion/duplication may underlie certain copy-number changes for which other mechanisms are currently assumed. Further studies are necessary to delineate the significance of the thus-far understudied doublets for the formation of copy-number variation.

Maternal mosaicism for IDUA deletion clarifies recurrence risk in MPS I.

Mucopolysaccharidosis I (MPS I) is a rare autosomal recessive multisystem lysosomal storage disorder. It is caused by biallelic loss-of-function variants in IDUA, encoding alpha-l iduronidase. Here, we describe an individual affected by MPS I due to a paternally inherited deletion of IDUA exons 1 and 2, c.(?_-88)_(299+1_300-1)del and a whole-gene deletion of IDUA (?_-88?)_(*136?)del secondary to maternal somatic mosaicism. We define a previously unreported mutational mechanism for this disorder.

Novel Genetic, Clinical, and Pathomechanistic Insights into TFG-Associated Hereditary Spastic Paraplegia.

Hereditary spastic paraplegias (HSPs) are genetically and clinically heterogeneous axonopathies primarily affecting upper motor neurons and, in complex forms, additional neurons. Here, we report two families with distinct recessive mutations in TFG, previously suggested to cause HSP based on findings in a single small family with complex HSP. The first carried a homozygous c.317G>A (p.R106H) variant and presented with pure HSP. The second carried the same homozygous c.316C>T (p.R106C) variant previously reported and displayed a similarly complex phenotype including optic atrophy. Haplotyping and bisulfate sequencing revealed evidence for a c.316C>T founder allele, as well as for a c.316_317 mutation hotspot. Expression of mutant TFG proteins in cultured neurons revealed mitochondrial fragmentation, the extent of which correlated with clinical severity. Our findings confirm the causal nature of bi-allelic TFG mutations for HSP, broaden the clinical and mutational spectra, and suggest mitochondrial impairment to represent a pathomechanistic link to other neurodegenerative conditions.

A polymorphic Alu insertion that mediates distinct disease-associated deletions.

Large deletions that are associated with insertions of Alu-derived sequence represent a rare, but potentially unique class of alterations. Whether they form by a one-step mechanism or by a primary insertion step followed by an independent secondary deletion step is not clear. We resolved two disease-associated SPAST deletions, which involve distinct exons by long range PCR. Alu-derived sequence was observed between the breakpoints in both cases. The intronic regions that represent the targets of potentially involved Alu retrotransposition events overlapped. Microsatellite- and SNP-based haplotyping indicated that both deletions originated on one and the same founder allele. Our data suggest that the deletions are best explained by two-step insertion-deletion scenarios for which a single Alu retrotransposition event represents the shared primary step. This Alu then mediated one of the deletions by non-homologous end joining and the other by non-allelic homologous recombination. Our findings thus strongly argue for temporal separation of insertion and deletion in Alu insertion-associated deletions. They also suggest that certain Alu integrations confer a general increase in local genomic instability, and that this explains why they are usually not detected during the probably short time that precedes the rearrangements they mediate.

The spectrum of KIAA0196 variants, and characterization of a murine knockout: implications for the mutational mechanism in hereditary spastic paraplegia type SPG8.

BACKGROUND: The hereditary spastic paraplegias (HSPs) are rare neurodegenerative gait disorders which are genetically highly heterogeneous. For each single form, eventual consideration of therapeutic strategies requires an understanding of the mechanism by which mutations confer pathogenicity. SPG8 is a dominantly inherited HSP, and associated with rather early onset and rapid progression. A total of nine mutations in KIAA0196, which encodes the WASH regulatory complex (SHRC) member strumpellin, have been reported in SPG8 patients so far. Based on biochemical and cell biological approaches, they have been suggested to act via loss of function-mediated haploinsufficiency. METHODS: We generated a deletion-based knockout allele for E430025E21Rik, i.e. the murine homologue of KIAA0196. The consequences on mRNA and protein levels were analyzed by qPCR and Western-blotting, respectively. Motor performance was evaluated by the foot-base angle paradigm. Axon outgrowth and relevant organelle compartments were investigated in primary neuron cultures and primary fibroblast cultures, respectively. A homemade multiplex ligation-dependent probe amplification assay enabling identification of large inactivating KIAA0196 deletion alleles was applied to DNA from 240 HSP index patients. RESULTS: Homozygous but not heterozygous mice showed early embryonic lethality. No transcripts from the knockout allele were detected, and the previously suggested compensation by the wild-type allele upon heterozygosity was disproven. mRNA expression of genes encoding other SHRC members was unaltered, while there was evidence for reduced SHRC abundance at protein level. We did, however, neither observe HSP-related in vivo and ex vivo phenotypes, nor alterations affecting endosomal, lysosomal, or autophagic compartments. KIAA0196 copy number screening excluded large inactivating deletion mutations in HSP patients. The consequences of monoallelic KIAA0196/E430025E21Rik activation thus differ from those observed for dominant HSP genes for which a loss-of-function mechanism is well established. CONCLUSIONS: Our data do not support the current view that heterozygous loss of strumpellin/SHRC function leads to haploinsufficiency and, in turn, to HSP. The lethality of homozygous knockout mice, i.e. the effect of complete loss of function, also argues against a dominant negative effect of mutant on wild-type strumpellin in patients. Toxic gain-of-function represents a potential alternative explanation. Confirmation of this therapeutically relevant hypothesis in vivo, however, will require availability of appropriate knockin models.

A novel strumpellin mutation and potential pitfalls in the molecular diagnosis of hereditary spastic paraplegia type SPG8.

Hereditary spastic paraplegia (HSP) is a clinically and genetically heterogeneous, neurodegenerative movement disorder. A total of eight KIAA0196/strumpellin variants have thus far been associated with SPG8, a rare dominant HSP. We present a novel strumpellin alteration in a small family with clinically pure HSP. We corroborated its causality by comparing it to rare benign variants at several levels, and, along this line, also re-considered previous genetic reports on SPG8. These analyses identified significant challenges in the interpretation of strumpellin alterations, and suggested that at least two of the few families claimed to suffer from SPG8 may have been genetically misdiagnosed.

A hereditary spastic paraplegia mouse model supports a role of ZFYVE26/SPASTIZIN for the endolysosomal system.

Hereditary spastic paraplegias (HSPs) are characterized by progressive weakness and spasticity of the legs because of the degeneration of cortical motoneuron axons. SPG15 is a recessively inherited HSP variant caused by mutations in the ZFYVE26 gene and is additionally characterized by cerebellar ataxia, mental decline, and progressive thinning of the corpus callosum. ZFYVE26 encodes the FYVE domain-containing protein ZFYVE26/SPASTIZIN, which has been suggested to be associated with the newly discovered adaptor protein 5 (AP5) complex. We show that Zfyve26 is broadly expressed in neurons, associates with intracellular vesicles immunopositive for the early endosomal marker EEA1, and co-fractionates with a component of the AP5 complex. As the function of ZFYVE26 in neurons was largely unknown, we disrupted Zfyve26 in mice. Zfyve26 knockout mice do not show developmental defects but develop late-onset spastic paraplegia with cerebellar ataxia confirming that SPG15 is caused by ZFYVE26 deficiency. The morphological analysis reveals axon degeneration and progressive loss of both cortical motoneurons and Purkinje cells in the cerebellum. Importantly, neuron loss is preceded by accumulation of large intraneuronal deposits of membrane-surrounded material, which co-stains with the lysosomal marker Lamp1. A density gradient analysis of brain lysates shows an increase of Lamp1-positive membrane compartments with higher densities in Zfyve26 knockout mice. Increased levels of lysosomal enzymes in brains of aged knockout mice further support an alteration of the lysosomal compartment upon disruption of Zfyve26. We propose that SPG15 is caused by an endolysosomal membrane trafficking defect, which results in endolysosomal dysfunction. This appears to be particularly relevant in neurons with highly specialized neurites such as cortical motoneurons and Purkinje cells.

MLPA-based evidence for sequence gain: pitfalls in confirmation and necessity for exclusion of false positives.

Multiplex ligation-dependent probe amplification (MLPA) has become a standard method for identifying copy number mutations in diagnostic and research settings. The occurrence of false-positive deletion findings and the underlying causes are well recognized, whereas false-positive duplication/amplification findings have not been appreciated so far. We here present three pertinent cases which were only identified on extended, nonstandard secondary analyses. We also offer and experimentally validate a potential explanation. Our findings imply that MLPA data indicating gain of genomic sequence require validation on an independent sample or by an independent method.