Hereditary spastic paraplegia (HSP) is a clinically and genetically heterogeneous group of neurodegenerative diseases that predominantly involves the lower extremities. The essential features are insidiously progressive spastic leg weakness associated with corticospinal tract and dorsal column degeneration [1]. HSP is classified according to the mode of inheritance (autosomal dominant, autosomal recessive, or X-linked), clinical symptoms ("pure" or "complicated"), and specific gene locus ("SPG1" through "SPG17") [1]. Autosomal dominant HSP (AD-HSP) is the most prevalent mode and includes approximately 70% of cases. Approximately 40-45% of AD-HSP patients have pathogenic mutations in the spastin gene (SPG4 or SPAST gene), which causes predominantly "pure" HSP [2]. Pure HSP is characterized by severe progressive spasticity limited to both legs, which is eventually accompanied by urinary urgency (reported in up to 50% of cases in later disease stages) and mildly impaired vibration sense resulting from "dying back" degeneration affecting axonal transport of the longest fibers that innervate the lower extremities [2]. Pure HSP can be confused with other diseases involving the corticospinal tract and dorsal column. Therefore, clinicians should consider this when they encounter patients who do not show improvement in gait disturbance, although they receive appropriate treatment. Here, we report the case of a patient who was initially misdiagnosed with subacute combined degeneration (SCD), but in fact had HSP with a novel SPAST mutation.

A 58-year-old man with gait disturbance came to our hospital on Oct 30, 2011. He did not complain of lower extremity weakness or leg pain. He was unable to regulate his steps by himself, and his acquaintances reported he walked with a slight limp. The impaired gait began 3 years after he had undergone subtotal gastrectomy and chemotherapy for 6 months. Thereafter, he felt tingling sensations in the hands and feet and acquired gait difficulties. He denied having a family history of abnormal gait or developmental problems. He had quit smoking and drinking alcohol many years prior to his diagnosis.

Upon neurological examination, his muscle strength was normal. The limbs did not reveal any spasticity or rigidity. He responded normally to pin prick and thermal stimulation. However, there was a severely decreased response to vibration in both big toes, particularly in the left. The ankle jerk reflexes were bilaterally decreased, whereas the reflexes of other joints were normal. Both toes showed extensor toe signs. Other pathologic reflexes were not observed. Cranial nerve and cerebellar functions were normal. He did not exhibit scissoring or spastic gait, but rather a limping gait. Thus, he had mixed neurological signs that involved the upper motor neuron and peripheral nerves.

Considering his previous medical history, we investigated his brain, spinal cord, meninges, and peripheral nerve by using magnetic resonance imaging (MRI), cerebrospinal fluid (CSF) analysis, blood tests, and electrodiagnostic studies. Notable results included the lower-normal limit of serum vitamin B12 (363 pg/ml; reference range 200-1,000 pg/ml) and generalized sensory dominant polyneuropathy with axonal involvement. Blood tests, including those for syphilis, HIV, diabetes, connective tissue disease, vasculitis, and renal and liver function, were normal. CSF chemistry and cell counts were also in the reference range. Mild small vessel disease and a small amount of chronic subdural hematoma were observed in the left cerebral convexity. Whole-spine MRI revealed normal signal intensity in the cord, except for disc extrusion with mild cord indentation, which did not seem relevant to the exhibited neurological symptoms. Nerve conduction studies showed mild sensory dominant peripheral polyneuropathy with axonal involvement.

We diagnosed him with SCD. He was intramuscularly administered cyanocobalamin regularly for 6 months. However, there was no improvement in his condition. After reconsidering his abnormal neurological signs and laboratory tests, we examined HSP mutations in his genomic DNA. As a result, the SPAST mutation was detected from the gene sample of the proband (Fig. 1). The mutation, a novel splicing mutation, is c.870+1delG (IVS5; heterozygote). The deletion of 188 bp of exon 5 was confirmed by RT-PCR. We compared this result with the mutation database (http://hgmd.cf.ac.uk/ac/index.php), and there was no previous information of this polymorphism or mutation at this locus.

We brought his children into the clinic for neurological examination. His 32-year-old son complained of subjective gait difficulty, even though there was no abnormality in his walking. However, he showed hyper-reflexic knee jerks, ankle clonus, and bilateral extensor toe signs. His 29-year-old daughter with an intellectual disability of unknown origin did not have any abnormalities upon examination. Genetic tests revealed that his son had the same SPAST mutation. The symptoms and signs of the patient and his son were different and are described in Table 1. Other relatives were not available to perform genetic analyses.

As described, we misdiagnosed this patient with SCD, which is distinct from HSP, because of atypical neurological signs and previous medical history. A genetic test revealed a novel splicing mutation in the SPAST gene that confirmed HSP. Genetic analysis of his son indicated the same result.

Genetically, HSP is classified according to the mode of inheritance and various gene loci. Among the several autosomal dominant HSP loci, mutations in SPAST are responsible for the largest share (about 40%) [2]. Since the first mutation in the SPAST gene was identified in 1999 [3], at least 300 different pathogenic mutations have been detected. SPAST is composed of 17 exons, encodes the spastin protein, and belongs to the ATPases Associated with various cellular Activities (AAA) family [2]. In the SPAST gene, the AAA domain in the C-terminal region (amino acids 342 to 599) and the microtubule interacting and trafficking (MIT) domain in the N-terminal region play important roles [2]. A large number of mutations are located in the AAA domain and are related with microtubule-severing and membrane-binding properties. It is thought that the AAA domain directly affects axonal and other intracellular transport. Furthermore, the MIT domain is involved in microtubule dynamics, such as intracellular organelle trafficking and endocytosis [4]. All pathogenic mutations appear to act via loss of function (haploinsufficiency) [4].

In Korea, there are 4 previous reports of novel SPAST mutations: One report involved a large genetic study of SPAST and ATL1 mutations among several families. Of 8 different SPAST mutations, 7 novel mutations were identified in this study. Most mutations are located between exons 7-17, in the AAA domain, and 1 mutation is located in intron 11. Only one was a splicing mutation; the others were missense mutations, in-frame deletions, and frame-shift mutations [5]. Unlike in our patient, the clinical manifestations in the patients in these reports typically corresponded to pure AD-HSP. It is to be remained whether our HSP patient should be classified as having a rare "complicated" AD-HSP with a SPAST gene mutation or "pure" AD-HSP, combined with nutritional-deficiency peripheral neuropathy.

In conclusion, we report a novel splicing mutation, c.870+1delG (IVS5), in a Korean family with autosomal dominant-inherited HSP. We confirmed the deletion of exon 5 by using RT-PCR and segregation of the SPAST gene among family members by direct sequencing.

Table 1. Clinical findings of HSP family