John K. Fink

Mutations in a newly identified GTPase gene cause autosomal dominant hereditary spastic paraplegia

The hereditary spastic paraplegias (HSPs; Strümpell-Lorrain syndrome, MIM number 18260) are a diverse class of disorders characterized by insidiously progressive lower-extremity spastic weakness (reviewed in refs. 1–3). Eight autosomal dominant HSP (ADHSP) loci have been identified, the most frequent of which is that linked to the SPG4 locus on chromosome 2p22 (found in ∼42%), followed by that linked to the SPG3A locus on chromosome 14q11–q21 (in ∼9%). Only SPG4 has been identified as a causative gene in ADHSP. Its protein (spastin) is predicted to participate in the assembly or function of nuclear protein complexes. Here we report the identification of mutations in a newly identified GTPase gene, SPG3A, in ADHSP affected individuals.

Whole-Genome Sequencing for Optimized Patient Management

A disease mutation identified by whole-genome sequencing of twins with dystonia allowed optimization of treatment, resulting in clinical improvements. Guiding Treatment with Genomics Whole-genome sequencing of DNA from patients with different diseases is proving useful for identifying new disease-causing mutations, but can it help physicians make better decisions about treatment options for these patients? A new study by Bainbridge and colleagues suggests that it can. Bainbridge et al. sequenced the complete genomes of a male and female fraternal twin pair, who had been diagnosed 9 years earlier with the movement disorder dopa (3,4-dihydroxyphenylalanine)–responsive dystonia (DRD). This complex disorder is difficult to diagnose and may be mistaken for other movement disorders involving loss of the neurotransmitter dopamine. The standard treatment for DRD is to replace dopamine by providing a dopamine precursor called l-dopa, the drug that is also used to treat the common movement disorder Parkinson disease. When the twins were diagnosed with DRD, they seemed to fit the classic description of DRD and were given l-dopa, which did help to alleviate many of their symptoms. When Bainbridge and colleagues analyzed the full genome sequences of the twins, they were surprised to discover no mutations in the two genes most commonly mutated in DRD. Instead, they pinpointed a mutation in the SPR gene encoding sepiapterin reductase, which synthesizes a cofactor needed for the action of enzymes that make not only dopamine but also the neurotransmitter serotonin. This finding suggested to the authors that supplementing the twin’s current l-dopa treatment with a serotonin precursor, 5-hydroxytryptophan, might provide further improvement in their symptoms. Sure enough, when the twins were given both l-dopa and 5-hydroxytryptophan instead of l-dopa alone, they showed improvement in their symptoms after 1 to 2 weeks, including greater attention in school, better motion and coordination, and reduced hand tremor as evidenced by more legible handwriting. Although this study involved only one twin pair, it does demonstrate how whole-genome sequencing could be applied to glean more detailed information about a patient’s disease, leading to more optimized treatment and a better outcome. Whole-genome sequencing of patient DNA can facilitate diagnosis of a disease, but its potential for guiding treatment has been under-realized. We interrogated the complete genome sequences of a 14-year-old fraternal twin pair diagnosed with dopa (3,4-dihydroxyphenylalanine)–responsive dystonia (DRD; Mendelian Inheritance in Man #128230). DRD is a genetically heterogeneous and clinically complex movement disorder that is usually treated with l-dopa, a precursor of the neurotransmitter dopamine. Whole-genome sequencing identified compound heterozygous mutations in the SPR gene encoding sepiapterin reductase. Disruption of SPR causes a decrease in tetrahydrobiopterin, a cofactor required for the hydroxylase enzymes that synthesize the neurotransmitters dopamine and serotonin. Supplementation of l-dopa therapy with 5-hydroxytryptophan, a serotonin precursor, resulted in clinical improvements in both twins.

Treatment of Wilson Disease With Ammonium Tetrathiomolybdate: III. Initial Therapy in a Total of 55 Neurologically Affected Patients and Follow-up With Zinc Therapy

BACKGROUND It is unclear what anticopper drug to use for patients with Wilson disease who present with neurologic manifestations because penicillamine often makes them neurologically worse and zinc is slow acting. OBJECTIVE To evaluate the frequency of neurologic worsening and drug adverse effects with ammonium tetrathiomolybdate. DESIGN Open-label study of 55 untreated patients (22 of them new) presenting with neurologic Wilson disease treated with tetrathiomolybdate varying from 120 to 410 mg/d for 8 weeks and then followed up for 3 years. Neurologic function was assessed with scored neurologic and speech tests. SETTING A university hospital referral setting. PATIENTS All untreated, newly diagnosed patients with neurologic Wilson disease. INTERVENTION Treatment with tetrathiomolybdate. MAIN OUTCOME MEASURES Neurologic function was evaluated by neurologic and speech examinations. Drug adverse effects were evaluated by complete blood cell counts and biochemical measures. RESULTS Only 2 (4%) of 55 patients treated with tetrathiomolybdate showed neurologic deterioration, compared with an estimated 50% of penicillamine-treated patients. Five of the 22 new patients exhibited bone marrow suppression and 3 had aminotransferase elevations. These numbers are higher than in the original 33 patients and appear to be due primarily to a more rapid dose escalation. CONCLUSIONS Tetrathiomolybdate shows excellent efficacy in patients with Wilson disease who present with neurologic manifestations. With rapid escalation of dose, adverse effects from bone marrow suppression or aminotransferase elevations can occur.

Treatment of Wilson's disease with zinc: XV long-term follow-up studies

Wilsons disease is an inherited disease of copper accumulation caused by a failure of biliary excretion of excess copper. Accumulated copper causes liver disease in these patients, and in perhaps two thirds of patients, it causes brain damage leading to clinical neurologic or psychiatric dysfunction. Maintenance treatment involves reversing the positive copper balance. The earliest approaches have used chelators, such as penicillamine or trientine, which increase the urinary excretion of copper. A more recent approach has used zinc, which blocks the absorption of copper and increases copper excretion in the stool. Because of the high level of endogenously secreted copper in alimentary secretions, the reabsorption of which is partially blocked by zinc therapy, zinc acts to remove accumulated copper from the body as well as prevent its reaccumulation. In the present article we present data on the long-term follow-up (up to 10 years) of maintenance zinc treatment of 141 patients with Wilsons disease. The data presented document that zinc is effective as a sole therapy in the long-term maintenance treatment of Wilsons disease and that it has a low toxicity. The results demonstrate the efficacy of zinc therapy in treating the presymptomatic patient from the beginning of therapy. We also present limited data on the use of zinc in the treatment of pregnant patients and children who have Wilsons disease; these data also indicate efficacy and low toxicity. The median follow-up period for the group as a whole is 4.8 years; for the presymptomatic patients it is 6.5 years; for the children it is 3.6 years.

Hereditary spastic paraplegia

The hereditary spastic paraplegias (HSPs) comprise a large group of inherited neurologic disorders. HSP is classified according to the mode of inheritance, the HSP locus when known, and whether the spastic paraplegia syndrome occurs alone or is accompanied by additional neurologic or systemic abnormalities. Analysis of 11 recently discovered HSP genes provides insight into HSP pathogenesis. Hereditary spastic paraplegia is a clinical diagnosis for which laboratory confirmation is sometimes possible, and careful exclusion of alternate and co-existing disorders is an important element in HSP diagnosis. Treatment for HSP is presently limited to symptomatic reduction of muscle spasticity, reduction in urinary urgency, and strength and gait improvement through physical therapy. Prenatal genetic testing in HSP is possible for some individuals with the increasing availability of HSP gene analysis.

Hereditary spastic paraplegia: clinico-pathologic features and emerging molecular mechanisms.

Hereditary spastic paraplegia (HSP) is a syndrome designation describing inherited disorders in which lower extremity weakness and spasticity are the predominant symptoms. There are more than 50 genetic types of HSP. HSP affects individuals of diverse ethnic groups with prevalence estimates ranging from 1.2 to 9.6 per 100,000. Symptoms may begin at any age. Gait impairment that begins after childhood usually worsens very slowly over many years. Gait impairment that begins in infancy and early childhood may not worsen significantly. Postmortem studies consistently identify degeneration of corticospinal tract axons (maximal in the thoracic spinal cord) and degeneration of fasciculus gracilis fibers (maximal in the cervico-medullary region). HSP syndromes thus appear to involve motor-sensory axon degeneration affecting predominantly (but not exclusively) the distal ends of long central nervous system (CNS) axons. In general, proteins encoded by HSP genes have diverse functions including (1) axon transport (e.g. SPG30/KIF1A, SPG10/KIF5A and possibly SPG4/Spastin); (2) endoplasmic reticulum morphology (e.g. SPG3A/Atlastin, SPG4/Spastin, SPG12/reticulon 2, and SPG31/REEP1, all of which interact); (3) mitochondrial function (e.g. SPG13/chaperonin 60/heat-shock protein 60, SPG7/paraplegin; and mitochondrial ATP6); (4) myelin formation (e.g. SPG2/Proteolipid protein and SPG42/Connexin 47); (5) protein folding and ER-stress response (SPG6/NIPA1, SPG8/K1AA0196 (Strumpellin), SGP17/BSCL2 (Seipin), “mutilating sensory neuropathy with spastic paraplegia” owing to CcT5 mutation and presumably SPG18/ERLIN2); (6) corticospinal tract and other neurodevelopment (e.g. SPG1/L1 cell adhesion molecule and SPG22/thyroid transporter MCT8); (7) fatty acid and phospholipid metabolism (e.g. SPG28/DDHD1, SPG35/FA2H, SPG39/NTE, SPG54/DDHD2, and SPG56/CYP2U1); and (8) endosome membrane trafficking and vesicle formation (e.g. SPG47/AP4B1, SPG48/KIAA0415, SPG50/AP4M1, SPG51/AP4E, SPG52/AP4S1, and VSPG53/VPS37A). The availability of animal models (including bovine, murine, zebrafish, Drosophila, and C. elegans) for many types of HSP permits exploration of disease mechanisms and potential treatments. This review highlights emerging concepts of this large group of clinically similar disorders.

Hereditary spastic paraplegia: Advances in genetic research

Hereditary spastic paraplegia (HSP) is a diverse group of inherited disorders characterized by progressive lower-extremity spasticity and weakness. Insight into the genetic basis of these disorders is expanding rapidly. Uncomplicated autosomal dominant, autosomal recessive, and X-linked HSP are genetically heterogeneous: different genes cause clinically indistinguishable disorders. A locus for autosomal recessive HSP is on chromosome 8q. Loci for autosomal dominant HSP have been identified on chromosomes 2p, 14q, and 15q. One locus (Xq22) has been identified for X-linked, uncomplicated HSP and shown to be due to a proteolipoprotein gene mutation in one family. The existence of HSP families for whom these loci are excluded indicates the existence of additional, as yet unidentified HSP loci. There is marked clinical similarity among HSP families linked to each of these loci, suggesting that gene products from HSP loci may participate in a common biochemical cascade, which, if disturbed, results in axonal degeneration that is maximal at the ends of the longest CNS axons. Identifying the single gene defects that cause HSPs distal axonopathy may provide insight into factors responsible for development and maintenance of axonal integrity. We review clinical, genetic, and pathologic features of HSP and present differential diagnosis and diagnostic criteria of this important group of disorders. We discuss polymorphic microsatellite markers useful for genetic linkage analysis and genetic counseling in HSP. NEUROLOGY 1996;46: 1507-1514

Neuropathy Target Esterase Gene Mutations Cause Motor Neuron Disease

The possibility that organophosphorus (OP) compounds contribute to motor neuron disease (MND) is supported by association of paraoxonase 1 polymorphisms with amyotrophic lateral sclerosis (ALS) and the occurrence of MND in OP compound-induced delayed neuropathy (OPIDN), in which neuropathy target esterase (NTE) is inhibited by organophosphorylation. We evaluated a consanguineous kindred and a genetically unrelated nonconsanguineous kindred in which affected subjects exhibited progressive spastic paraplegia and distal muscle wasting. Affected subjects resembled those with OPIDN and those with Troyer Syndrome due to SPG20/spartin gene mutation (excluded by genetic linkage and SPG20/spartin sequence analysis). Genome-wide analysis suggested linkage to a 22 cM homozygous locus (D19S565 to D19S884, maximum multipoint LOD score 3.28) on chromosome 19p13 to which NTE had been mapped (GenBank AJ004832). NTE was a candidate because of its role in OPIDN and the similarity of our patients to those with OPIDN. Affected subjects in the consanguineous kindred were homozygous for disease-specific NTE mutation c.3034A-->G that disrupted an interspecies conserved residue (M1012V) in NTEs catalytic domain. Affected subjects in the nonconsanguineous family were compound heterozygotes: one allele had c.2669G-->A mutation, which disrupts an interspecies conserved residue in NTEs catalytic domain (R890H), and the other allele had an insertion (c.2946_2947insCAGC) causing frameshift and protein truncation (p.S982fs1019). Disease-specific, nonconserved NTE mutations in unrelated MND patients indicates NTEs importance in maintaining axonal integrity, raises the possibility that NTE pathway disturbances contribute to other MNDs including ALS, and supports the role of NTE abnormalities in axonopathy produced by neuropathic OP compounds.

NIPA1 Gene Mutations Cause Autosomal Dominant Hereditary Spastic Paraplegia (SPG6)

The hereditary spastic paraplegias (HSPs) are genetically heterogeneous disorders characterized by progressive lower-extremity weakness and spasticity. The molecular pathogenesis is poorly understood. We report discovery of a dominant negative mutation in the NIPA1 gene in a kindred with autosomal dominant HSP (ADHSP), linked to chromosome 15q11-q13 (SPG6 locus); and precisely the same mutation in an unrelated kindred with ADHSP that was too small for meaningful linkage analysis. NIPA1 is highly expressed in neuronal tissues and encodes a putative membrane transporter or receptor. Identification of the NIPA1 function and ligand will aid an understanding of axonal neurodegeneration in HSP and may have important therapeutic implications.

Novel Locus for Autosomal Dominant Hereditary Spastic Paraplegia, on Chromosome 8q

Hereditary spastic paraplegia (HSP) is a clinically and genetically heterogeneous group of disorders characterized by insidiously progressive spastic weakness in the legs. Genetic loci for autosomal dominant HSP exist on chromosomes 2p, 14q, and 15q. These loci are excluded in 45% of autosomal dominant HSP kindreds, indicating the presence of additional loci for autosomal dominant HSP. We analyzed a Caucasian kindred with autosomal dominant HSP and identified tight linkage between the disorder and microsatellite markers on chromosome 8q (maximum two-point LOD score 5.51 at recombination fraction 0). Our results clearly establish the existence of a locus for autosomal dominant HSP on chromosome 8q23-24. Currently this locus spans 6.2 cM between D8S1804 and D8S1774 and includes several potential candidate genes. Identifying this novel HSP locus on chromosome 8q23-24 will facilitate discovery of this HSP gene, improve genetic counseling for families with linkage to this locus, and extend our ability to correlate clinical features with different HSP loci.