+Classification of the trinucleotide repeat, and resulting disease status, depends on the number of CAG repeats in Huntington's disease Repeat count Classification Disease status 40 Full-penetrance Affected<unk> s generally show genetic anticipation: their severity increases with each successive generation that inherits them. This is likely explained by the addition of CAG repeats in the affected gene as the gene is transmitted from parent to child. For example, Huntington's disease occurs when there are more than 35 CAG repeats on the gene coding for the protein HTT. A parent with 35 repeats would be considered normal and would not exhibit any symptoms of the disease. However, that parent's offspring would be at an increased risk of developing Huntington's compared to the general population, as it would take only the addition of one more CAG codon to cause the production of mHTT (mutant HTT), the protein responsible for disease. Huntington's very rarely occurs spontaneously; it is almost always the result of inheriting the defective gene from an affected parent. However, sporadic cases of Huntington's in individuals who have no history of the disease in their families do occur. Among these sporadic cases, there is a higher frequency of individuals with a parent who already has a significant number of CAG repeats in their HTT gene, especially those whose repeats approach the number (36) required for the disease to manifest. Each successive generation in a Huntington's-affected family may add additional CAG repeats, and the higher the number of repeats, the more severe the disease and the earlier its onset. As a result, families that have suffered from Huntington's for many generations show an earlier age of disease onset and faster disease progression. Non-trinucleotide expansions The majority of diseases caused by expansions of simple DNA repeats involve trinucleotide repeats, but tetra-, penta- and dodecanucleotide repeat expansions are also known that cause disease. For any specific hereditary<unk> , only one repeat expands in a particular gene.
Trinucleotide repeat disorder
Lysosomal storage disease
Ribosomopathy
Lipid storage disorder
Trinucleotide repeat expansion
Creatine transporter defect
Cerebral creatine deficiency
Peroxisomal disorder