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What is Hereditary spastic paraplegia and explain in detail?
Hereditary spastic paraplegia (HSP) is a group of inherited diseases whose main feature is a progressive gait disorder. The disease presents with progressive stiffness (spasticity)  and contraction in the lower limbs. HSP is also known as hereditary spastic paraparesis, familial spastic paraplegia, French settlement disease, Strumpell disease, or Strumpell-Lorrain disease.  The symptoms are a result of dysfunction of  long axons in the spinal cord. The affected cells are the primary motor neurons; therefore, the disease is an upper motor neuron disease. HSP is not a form of cerebral palsy even though it physically may appear and behave much the same as spastic diplegia. The origin of HSP  is different from cerebral palsy. Despite this, some of the same anti-spasticity medications used in spastic cerebral palsy are sometimes used to  treat HSP symptoms.
HSP is caused by defects in  transport of  proteins, structural proteins, cell-maintaining proteins, lipids, and other substances through the cell. Long nerve fibers (axons) are affected because  long distances make nerve cells particularly sensitive to defects in these mentioned mechanisms.The disease was first described in 1880 by the German neurologist Adolph Strümpell. It was described more extensively in 1888 by Maurice Lorrain, a French physician.  Due to their contribution in describing the disease, it is still called Strümpell-Lorrain disease in French-speaking countries. The term hereditary spastic paraplegia was coined by Anita Harding in 1983.

Signs and symptoms
Symptoms depend on the type of HSP inherited. The main feature of the disease is progressive spasticity in the lower limbs due to pyramidal tract dysfunction. This also results in brisk reflexes, extensor plantar reflexes, muscle weakness, and variable bladder disturbances. Furthermore, among the core symptoms of HSP are also included abnormal gait and difficulty in walking, decreased vibratory sense at the ankles, and paresthesia.
Individuals with HSP can experience extreme fatigue associated with central nervous system and neuromuscular disorders, which can be disabling. Initial symptoms are typically difficulty with balance, stubbing the toe or stumbling. Symptoms of HSP may begin at any age, from infancy to older than 60 years. If symptoms begin during the teenage years or later, then spastic gait disturbance usually progresses over many years. Canes, walkers, and wheelchairs may eventually be required, although some people never require assistance devices. Disability has been described as progressing more rapidly in adult onset forms.More specifically, patients with the autosomal dominant pure form of HSP reveal normal facial and extraocular movement. Although jaw jerk may be brisk in older subjects, there is no speech disturbance or difficulty of swallowing. Upper extremity muscle tone and strength are normal. In the lower extremities, muscle tone is increased at the hamstrings, quadriceps and ankles. Weakness is most notable at the iliopsoas, tibialis anterior, and to a lesser extent, hamstring muscles.
In the complex form of the disorder, additional symptoms are present. These include: peripheral neuropathy, amyotrophy, ataxia, intellectual disability, ichthyosis, epilepsy, optic neuropathy, dementia, deafness, or problems with speech, swallowing or breathing.Anita Harding classified the HSP in a pure and complicated form. Pure HSP presents with spasticity in the lower limbs, associated with neurogenic bladder disturbance as well as lack of vibration sensitivity (pallhypesthesia). On the other hand, HSP is classified as complex when lower limb spasticity is combined with any additional neurological symptom.This classification is subjective and patients with complex HSPs are sometimes diagnosed as having cerebellar ataxia with spasticity, intellectual disability (with spasticity), or  leukodystrophy. Some of the genes listed below have been described in other diseases than HSP before. Therefore, some key genes overlap with other disease groups.

Age of onset
In the past, HSP has been classified as early onset beginning in early childhood or later onset in adulthood. The age of onsets has two points of maximum at age 2 and around age 40. New findings propose that an earlier onset leads to a longer disease duration without loss of ambulation or the need for the use of a wheelchair. This was also described earlier, that later onset forms evolve more rapidly.

Cause
HSP is a group of genetic disorders. It follows general inheritance rules and can be inherited in an autosomal dominant, autosomal recessive or X-linked recessive manner. The mode of inheritance involved has a direct impact on the chances of inheriting the disorder. Over 70 genotypes had been described, and over 50 genetic loci have been linked to this condition. Ten genes have been identified with autosomal dominant inheritance. One of these, SPG4, accounts for ~50% of all genetically solved cases, or approximately 25% of all HSP cases. Twelve genes are known to be inherited in an autosomal recessive fashion. Collectively this latter group account for ~1/3 cases.Most altered genes have known function, but for some the function havent been identified yet. All of them are listed in the gene list below, including their mode of inheritance.  Some examples are spastin (SPG4) and paraplegin (SPG7) are both AAA ATPases.

Genotypes
The genes are designated SPG (Spastic gait gene). The gene locations are in the format: chromosome - arm (short or p: long or q) - band number. These designations are for the human genes only. The locations may (and probably will) vary in other organisms.
Despite the number of genes known to be involved in this condition ~40% of cases have yet to have their cause identified. In the table below SPG? is used to indicate a gene that has been associated with HSP but has not yet received an official HSP gene designation.

Pathophysiology
The major feature of HSP is a length-dependent axonal degeneration. These include the crossed and uncrossed corticospinal tracts to the legs and fasciculus gracilis. The spinocerebellar tract is involved to a lesser extent. Neuronal cell bodies of degenerating axons are preserved and there is no evidence of primary demyelination. Loss of anterior horn cells of the spinal cord are observed in some cases. Dorsal root ganglia, posterior roots and peripheral nerves are not directly affected.HSP affects several pathways in motor neurons. Many genes were identified and linked to HSP. It remains a challenge to accurately define the key players in each of the affected pathways, mainly because many genes have multiple functions and are involved in more than one pathway.

Axon pathfinding
Pathfinding is important for axon growth to the right destination (e.g. another nerve cell or a muscle). Significant for this mechanism is the L1CAM gene, a cell surface glycoprotein of the immunoglobulin superfamily. Mutations leading to a loss-of-function in L1CAM are also found in other X-linked syndromes. All of these disorders display corticospinal tract impairment (a hallmark feature of HSP). L1CAM participates in a set of interactions, binding other L1CAM molecules as well as extracellular cell adhesion molecules, integrins, and proteoglycans or intracellular proteins like ankyrins.The pathfinding defect occurs via the association of L1CAM with neuropilin-1. Neuropilin-1 interacts with Plexin-A proteins to form the Semaphorin-3A receptor complex. Semaphorin-a3A is then released in the ventral spinal cord to steer corticospinal neurons away from the midline spinal cord / medullary junction. If L1CAM does not work correctly due to a mutation, the cortiocospinal neurons are not directed to the correct position and the impairment occurs.

Lipid metabolism
Axons in the central and peripheral nervous system are coated with an insulation, the myelin layer, to increase the speed of action potential propagation. Abnormal myelination in the CNS is detected in some forms of hsp HSP. Several genes were linked to myelin malformation, namely PLP1, GFC2 and FA2H. The mutations alter myelin composition, thickness and integrity.Endoplasmic reticulum (ER) is the main organelle for lipid synthesis. Mutations in genes encoding proteins that have a role in shaping ER morphology and lipid metabolism were linked to HSP. Mutations in ATL1, BSCL2 and ERLIN2 alter ER structure, specifically the tubular network and the formation of three-way junctions in ER tubules. Many mutated genes are linked to abnormal lipid metabolism. The most prevalent effect is on arachidonic acid (CYP2U1) and cholesterol (CYP7B1) metabolism, phospholipase activity (DDHD1 and DDHD2), ganglioside formation (B4GALNT-1) and the balance between carbohydrate and fat metabolism (SLV33A1).

Endosomal trafficking
Neurons take in substances from their surrounding by endocytosis. Endocytic vesicles fuse to endosomes in order to release their content. There are three main compartments that have endosome trafficking: Golgi to/from endosomes; plasma membrane to/from early endosomes (via recycling endosomes) and late endosomes to lysosomes. Dysfunction of endosomal trafficking can have severe consequences in motor neurons with long axons, as reported in HSP. Mutations in AP4B1 and KIAA0415 are linked to disturbance in vesicle formation and membrane trafficking including selective uptake of proteins into vesicles. Both genes encode proteins that interact with several other proteins and disrupt the secretory and endocytic pathways.

Mitochondrial function
Mitochondrial dysfunctions have been connected with developmental and degenerative neurological disorders. Only a few HSP genes encode mitochondrial proteins. Two mitochondrial resident proteins are mutated in HSP: paraplegin and chaperonin 60. Paraplegin is a m-AAA metalloprotease of the inner mitochondrial membrane. It functions in ribosomal assembly and protein quality control. The impaired chaperonin 60 activity leads to impaired mitochondrial quality control. Two genes DDHD1 and CYP2U1 have shown alteration of mitochondrial architecture in patient fibroblasts. These genes encode enzymes involved in fatty-acid metabolism.

Diagnosis
Initial diagnosis of HSPs relies upon family history, the presence or absence of additional signs and the exclusion of other nongenetic causes of spasticity, the latter being particular important in sporadic cases.Cerebral and spinal MRI is an important procedure performed in order to rule out other frequent neurological conditions, such as multiple sclerosis, but also to detect associated abnormalities such as cerebellar or corpus callosum atrophy as well as white matter abnormalities. Differential diagnosis of HSP should also exclude spastic diplegia which presents with nearly identical day-to-day effects and even is treatable with similar medicines such as baclofen and orthopedic surgery; at times, these two conditions may look and feel so similar that the only perceived difference may be HSPs hereditary nature versus the explicitly non-hereditary nature of spastic diplegia (however, unlike spastic diplegia and other forms of spastic cerebral palsy, HSP cannot be reliably treated with selective dorsal rhizotomy).Ultimate confirmation of HSP diagnosis can only be provided by carrying out genetic tests targeted towards known genetic mutations.

Classification
Hereditary spastic paraplegias can be classified based on the symptoms; mode of inheritance; the patients age at onset; the affected genes; and biochemical pathways involved.

Treatment
No specific treatment is known that would prevent, slow, or reverse HSP. Available therapies mainly consist of symptomatic medical management and promoting physical and emotional well-being. Therapeutics offered to HSP patients include:

Baclofen – a voluntary muscle relaxant to relax muscles and reduce tone. This can be administered orally or intrathecally