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.
Mutant PANK2 gene coded proteins are often caused by null or missense mutations most notably a 7bp deletion in the PANK2 gene coding sequence.This disorder has been reported in specific communities based on intra-community marriages where both parents of the child are carrying the same mutation. One of the communities reported is Agrawal (Agarwal) Community mainly based in Northern Part of India. The known mutation in Agarwal community is pathogenic mutation 1c.215_216insA in PANK2 gene. This is also coded as chr20:3870292-3870293insA by some labs. It results in a frameshift and premature truncation of the protein 47 amino acids downstream to codon 183 (p.Arg183GlufsTer47; ENST00000316562). Diagnosis
A neurological examination would show evidence of muscle rigidity; weakness; and abnormal postures, movements, and tremors. If other family members are also affected, this may help determine the diagnosis. Genetic tests can confirm an abnormal gene causing the disease. However, this test is not yet widely available. Other movement disorders and diseases must be ruled out. Individuals exhibiting any of the above listed symptoms are often tested using MRI (Magnetic Resonance Imaging) for a number of neuro-related disorders. An MRI usually shows iron deposits in the basal ganglia. Development of diagnostic criteria continues in the hope of further separating PKAN from other forms of neurodegenerative diseases featuring NBIA.