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.