LRSAM1 and the RING domain: Charcot-Marie-Tooth disease and beyond.

In the past decade, mutations in LRSAM1 were identified as the genetic cause of both dominant and recessive forms of axonal CMT type 2P (CMT2P). Despite demonstrating different inheritance patterns, dominant CMT2P is usually characterized by relatively mild, slowly progressive axonal neuropathy, mainly involving lower limbs, with age of onset between the second and fifth decades of life. Asymptomatic individuals were identified in several pedigrees exemplifying the strong phenotypic variability of these patients requiring serial clinical evaluation to establish correct diagnosis; in this respect, magnetic resonance imaging of lower-limb musculature showing fatty atrophy might be helpful in detecting subclinical gene mutation carriers. LRSAM1 is a universally expressed RING-type E3 ubiquitin protein ligase catalysing the final step in the ubiquitination cascade. Strikingly, TSG101 remains the only known ubiquitination target hampering our mechanistic understanding of the role of LRSAM1 in the cell. The recessive CMT mutations lead to complete loss of LRSAM1, contrary to the heterozygous dominant variants. These tightly cluster in the C-terminal RING domain highlighting its importance in governing the CMT disease. The domain is crucial for the ubiquitination function of LRSAM1 and CMT mutations disrupt its function, however it remains unknown how this leads to the peripheral neuropathy. Additionally, recent studies have linked LRSAM1 with other neurodegenerative diseases of peripheral and central nervous systems. In this review we share our experience with the challenging clinical diagnosis of CMT2P and summarize the mechanistic insights about the LRSAM1 dysfunction that might be helpful for the neurodegenerative field at large.

Peripheral myelin protein 2 - a novel cluster of mutations causing Charcot-Marie-Tooth neuropathy.

BACKGROUND: Charcot-Marie-Tooth (CMT) disease is the most common inherited neuromuscular disorder characterized by wide clinical, genetic and pathomechanistic heterogeneity. Recently, the gene encoding peripheral myelin protein 2 (PMP2) was identified as a novel cause for CMT neuropathy with three mutations that structurally cluster together (p.Ile43Asn, p.Thr51Pro, p.Ile52Thr) reported in five families. RESULTS: Using whole exome sequencing and cohort screening we identified two novel missense substitutions in PMP2 in Bulgarian (p.Met114Thr, c.341C > T) and German (p.Val115Ala, c.344 T > C) families. The mutations affect adjacent and highly conserved amino acid residues outside of the known mutation-rich region in the protein. Crystal structure analysis positions the affected residues within a cluster of highly conserved fatty acid coordinating residues implying their functional significance. The clinical, electrophysiological and imaging features in both families were consistent with a childhood onset polyneuropathy with variable patterns of demyelination, slow to very slow progression, and most severe involvement of the peroneal muscles. CONCLUSIONS: We expand the genetic and phenotypic spectrum of PMP2-related peripheral neuropathy. Our findings reveal a second mutational cluster in the protein.

Charcot-Marie-Tooth disease type 2G redefined by a novel mutation in LRSAM1.

OBJECTIVE: To identify the unknown genetic cause in a large pedigree previously classified with a distinct form of axonal Charcot-Marie-Tooth disease type 2G (CMT2G) and to explore its transcriptional consequences. METHODS: Clinical reevaluation of the pedigree was performed, followed by linkage analysis with the redefined disease statuses, and whole genome and exome sequencing. The impact of the mutation was investigated by immunoblotting and transcriptome sequencing. RESULTS: Thirteen affected individuals over 3 generations displayed mild and quiescent lower-limb axonal sensorimotor neuropathy. Magnetic resonance imaging (MRI) of lower-limb musculature systematically showed fatty atrophy in clinical and subclinical mutation carriers. We redefined the disease-linked region to chr9q31.3-q34.2 and subsequently identified a novel missense variant in the E3 ubiquitin-protein ligase LRSAM1 (p.Cys694Tyr). Unlike previous reports, we demonstrated in patients' lymphoblasts that the mutation does not influence overall protein levels of LRSAM1, nor of its ubiquitylation target TSG101. The mutation is associated with several transcriptional changes, including a significant upregulation of another E3 ubiquitin-protein ligase, NEDD4L, and of TNFRSF21, a key regulator of axonal degeneration. INTERPRETATION: Our findings demonstrate that the isolated genetic entity CMT2G is caused by a missense mutation in LRSAM1 and should be reclassified as CMT2P. MRI of lower-limb musculature can be used to detect minimal signs of the disease. Transcriptome analysis of patients' cells highlights novel molecular players associated with LRSAM1 dysfunction, and reveals pathways and therapeutic targets shared with amyotrophic lateral sclerosis and Alzheimer disease. Ann Neurol 2016;80:823-833.

De novo PMP2 mutations in families with type 1 Charcot-Marie-Tooth disease.

We performed whole exome sequencing on a patient with Charcot-Marie-Tooth disease type 1 and identified a de novo mutation in PMP2, the gene that encodes the myelin P2 protein. This mutation (p.Ile52Thr) was passed from the proband to his one affected son, and segregates with clinical and electrophysiological evidence of demyelinating neuropathy. We then screened a cohort of 136 European probands with uncharacterized genetic cause of Charcot-Marie-Tooth disease and identified another family with Charcot-Marie-Tooth disease type 1 that has a mutation affecting an adjacent amino acid (p.Thr51Pro), which segregates with disease. Our genetic and clinical findings in these kindred demonstrate that dominant PMP2 mutations cause Charcot-Marie-Tooth disease type 1.

Transcriptome analysis of the filamentous fungus Aspergillus nidulans directed to the global identification of promoters.

BACKGROUND: The filamentous fungus Aspergillus nidulans has been a tractable model organism for cell biology and genetics for over 60 years. It is among a large number of Aspergilli whose genomes have been sequenced since 2005, including medically and industrially important species. In order to advance our knowledge of its biology and increase its utility as a genetic model by improving gene annotation we sequenced the transcriptome of A. nidulans with a focus on 5' end analysis. RESULTS: Strand-specific whole transcriptome sequencing showed that 80-95% of annotated genes appear to be expressed across the conditions tested. We estimate that the total gene number should be increased by approximately 1000, to 11,800. With respect to splicing 8.3% of genes had multiple alternative transcripts, but alternative splicing by exon-skipping was very rare. 75% of annotated genes showed some level of antisense transcription and for one gene, meaB, we demonstrated the antisense transcript has a regulatory role. Specific sequencing of the 5' ends of transcripts was used for genome wide mapping of transcription start sites, allowing us to interrogate over 7000 promoters and 5' untranslated regions. CONCLUSIONS: Our data has revealed the complexity of the A. nidulans transcriptome and contributed to improved genome annotation. The data can be viewed on the AspGD genome browser.