Mary M. Reilly

Extended phenotypic spectrum of KIF5A mutations From spastic paraplegia to axonal neuropathy

Objective: To establish the phenotypic spectrum of KIF5A mutations and to investigate whether KIF5A mutations cause axonal neuropathy associated with hereditary spastic paraplegia (HSP) or typical Charcot-Marie-Tooth disease type 2 (CMT2). Methods: KIF5A sequencing of the motor-domain coding exons was performed in 186 patients with the clinical diagnosis of HSP and in 215 patients with typical CMT2. Another 66 patients with HSP or CMT2 with pyramidal signs were sequenced for all exons of KIF5A by targeted resequencing. One additional patient was genetically diagnosed by whole-exome sequencing. Results: Five KIF5A mutations were identified in 6 unrelated patients: R204W and D232N were novel mutations; R204Q, R280C, and R280H have been previously reported. Three patients had CMT2 as the predominant and presenting phenotype; 2 of them also had pyramidal signs. The other 3 patients presented with HSP but also had significant axonal neuropathy or other additional features. Conclusion: This is currently the largest study investigating KIF5A mutations. By combining next-generation sequencing and conventional sequencing, we confirm that KIF5A mutations can cause variable phenotypes ranging from HSP to CMT2. The identification of mutations in CMT2 broadens the phenotypic spectrum and underlines the importance of KIF5A mutations, which involve degeneration of both the central and peripheral nervous systems and should be tested in HSP and CMT2.

A Man with Episodes of Shoulder Pain and a Weak Arm

A 69 year old man presents with a history of recurrent episodes of brachial neuritis. The symptoms started at the age of 35 and the episodes were usually preceded by flu-like illnesses or infection. His son had an episode of brachial neuritis at the age of 17. The clinical picture is consistent with hereditary neuralgic amyotrophy (HNA), however genetic testing does not reveal mutations in the Septin 9 (SEPT9) gene. As only ~ half of the HNA families studied had a defect in the SEPT9 gene, mutations in other genes that have not yet been identified may be responsible for HNA.

A Man with a Pacemaker Develops Difficulty Walking

A 66 year old man presents with a two year history of gradually progressive lower limb weakness. The presentation was preceded by symptoms compatible with bilateral carpal tunnel syndrome. Four years before the development of neurological symptoms he had a permanent pacemaker inserted. Amyloid deposits were detected on sural nerve biopsy. A mutation in the transthyretin gene (Val30Met) confirms the diagnosis of Familial amyloid polyneuropathy.

A Woman with Burning Hands

A 59 year old woman presents with a 3 year history of burning sensations in the hands and feet. There is neither limb weakness nor autonomic symptoms. Nerve conduction studies and thermal thresholds are normal. Her father was reported to have Fabry disease. Genetic testing confirms the presence of a heterozygous mutation in the alpha-galactosidase gene.

A Man with Painful Feet and Hand Ulcers

A 49 year old man presents with a 20 year history of progressive numbness, shooting pain and weakness affecting the upper and lower limbs. He also has recurrent ulcers. His mother had a painful sensory neuropathy. Nerve conduction studies are compatible with a sensory and motor axonal neuropathy. Genetic testing reveals a mutation in the SPTLC1 gene confirming the diagnosis of Hereditary Sensory and Autonomic Neuropathy (HSAN) type 1A.

A Woman Who Could Not Wear High Heels

A 44 year old woman with a positive family history for neuromuscular disease, presents with a slowly progressive peripheral neuropathy, found to be demyelinating on neurophysiology. Genetic testing confirms the diagnosis.

Progressive Motor Weakness in a Somalian Man

A 43 year old man presents with a 20 year history of progressive lower limb weakness. He has no sensory symptoms. There is no family history of neurological disease. Nerve conduction studies reveal normal sensory responses with reduced or absent motor responses in the upper and lower limbs. Genetic testing reveals a heterozygous mutation in the Heat Shock Protein B1 (HSPB1) gene confirming the diagnosis of distal hereditary motor neuropathy.

A Medical Student with Episodes of Weakness and Sensory Disturbance

A 25 year old woman presents with transient tingling in both hands after prolonged writing. She had an episode of left foot drop at the age of 24 without a clear cause. Nerve conduction studies reveal bilateral median and ulnar neuropathy and compression of the left peroneal nerve at the fibular head. Genetic testing reveals chromosome 17 deletion confirming the diagnosis of Hereditary neuropathy with liability to pressure palsy.

RARE CODING VARIANTS IN THE MME GENE, ENCODING THE METALLOPROTEASE NEPRILYSIN, ARE LINKED TO LATE-ONSET AXONAL NEUROPATHIES

Rare coding variants in the mme gene, encoding the metalloprotease neprilysin, are linked to late-onset axonal neuropathiesBackground: Spinal muscular atrophy with lower extremity predominance (SMA-LED) is an autosomal dominant congenital motor neuron disease. The condition presents with distal limb weakness and muscle atrophy, further compounded with intellectual disability. The most common cause are mutations in dynein cytoplasmic 1 heavy chain 1 (DYNC1H1; OMIM:600112), which encodes the largest subunit of cytoplasmic dynein 1. Dynein is defined by its role as a retrogradely oriented molecular motor but it is also fundamental to other cellular processes including growth cone dynamics and regulation of the Golgi apparatus. Moreover, mutations in dynactin 1 (DCTN1; OMIM: 601143) encoding p150 (Glued) subunit of the dynactin complex, which regulates cytoplasmic dynein function, cause autosomal dominant distal hereditary motor neuronopathy. Objective: To dissect common molecular mechanisms underlying motor neuron degeneration caused by R399G and D338N mutations in DYNC1H1. Methods: Immunofluorescence was performed on patient fibroblasts harbouring the R399G or D338N DYNC1H1 mutation to assess the integrity of the Golgi apparatus and the localization of dynein to the organelle. Modifications of microtubules and the interaction of dynein with golgin-160 were investigated using biochemical analysis. Results: Decreased a-tubulin acetylation was a common molecular phenotype in patient fibroblasts harbouring the R399G (p50.05, N=3) or D338N (p50.01, N=5) mutation in comparison to wild-type fibroblasts (N=3 and N=5, respectively). However, only the R399G mutant fibroblasts (N=20) exhibited a significant (p50.0001) decrease of dynein at the Golgi apparatus in comparison to wild-type cells (N=21). Uniquely, the R399G mutation also caused a significant and inherent fragmentation of the Golgi apparatus, which correlated with the zygosity of the mutation (+/R339G p50.01 N=4, R399G/R399G p50.0001 N=4). A consequent compensational response was measured as an increased interaction between the dynein intermediate chain and golgin-160 in the R399G mutant cells. Excitingly, the treatment of R399G mutant fibroblasts with tubacin (N=32), an HDAC6 inhibitor, caused a striking statistically significant (p50.0001) amelioration of the Golgi apparatus integrity by increasing microtubule acetylation in comparison to untreated R399G mutant fibroblasts (N=33). Discussion and conclusions: Using DYNC1H1 mutations we illustrate a dynein-dependent acetylation of the microtubule network, which if aberrant and compounded by a decrease in the amount of dynein present on the Golgi membranes results in the fragmentation of the organelle. Intriguingly, a-tubulin acetylation, is significantly reduced in motor neurons harbouring ALS associated mutant TUBA4A (OMIM: 191110). These data suggest a tentative link between genetic variations in DYNC1H1 and the microtubule cytoskeleton, which could contribute to aberrant tubulin modification, Golgi integrity, and axonal transport and consequently susceptibility to ALS. Importantly, we show that ameliorating the microtubule acetylation is sufficient to rescue the Golgi integrity, thereby providing a potential therapeutic target for this pathology.