The TPP1 gene provides instructions for making an enzyme called tripeptidyl peptidase 1. This enzyme is produced as an inactive enzyme, called a proenzyme, which has an extra segment attached. This segment must be removed, followed by additional processing steps, for the enzyme to become active. The active tripeptidyl peptidase 1 enzyme is found in cell structures called lysosomes, which digest and recycle different types of molecules. Tripeptidyl peptidase 1 acts as a peptidase, which means that it breaks down protein fragments, known as peptides, into their individual building blocks (amino acids). Specifically, tripeptidyl peptidase 1 cuts (cleaves) peptides into groups of three amino acids.
Health Conditions Related to Genetic Changes
At least 115 mutations in the TPP1 gene have been found to cause CLN2 disease. This condition impairs motor and mental development, typically starting in early childhood, causing gradually worsening movement disorders and a decline in intellectual function. In addition, affected children often develop recurrent seizures (epilepsy) and vision impairment. In some cases, signs and symptoms of CLN2 disease do not appear until later in childhood, usually after age 4.
Most of the TPP1 gene mutations that cause CLN2 disease change single amino acids in tripeptidyl peptidase 1, resulting in a severe decrease in enzyme activity. A reduction in functional enzyme results in the incomplete breakdown of certain peptides. CLN2 disease is characterized by the accumulation of proteins or peptides and other substances in lysosomes. These accumulations occur in cells throughout the body; however, nerve cells seem to be particularly vulnerable to their effects. The accumulations can cause cell damage leading to cell death. The progressive death of nerve cells in the brain and other tissues leads to the signs and symptoms of CLN2 disease.
Individuals who are diagnosed with CLN2 disease later in childhood likely have TPP1 gene mutations that result in the production of an enzyme with a small amount of normal function. Protein function in these individuals is higher than in those who have the condition beginning earlier in childhood. As a result, it takes longer for peptides and other substances to accumulate in the lysosomes and damage nerve cells.More About This Health Condition
Mutations in the TPP1 gene have also been found to cause spinocerebellar ataxia, autosomal recessive 7 (SCAR7), which is a condition characterized by progressive problems with movement. During childhood, individuals with SCAR7 develop walking difficulties; impaired speech (dysarthria); and eye movement problems, such as involuntary movement of the eyes (nystagmus), rapid eye movements (saccades), and trouble moving the eyes side-to-side (oculomotor apraxia). People with SCAR7 have progressive loss of cells (atrophy) of various parts of the brain, particularly within the cerebellum, which is the area of the brain involved in coordinating movements.
Compared to individuals with CLN2 disease (described above), individuals with SCAR7 likely have a higher level of normally functioning tripeptidyl peptidase 1. As a result, SCAR7 is associated with milder signs and symptoms than CLN2 disease and tends to develop in late childhood or adolescence. When examined, cells from some individuals with SCAR7 showed lysosomal accumulations while cells from other affected individuals did not.
Other Names for This Gene
- cell growth-inhibiting gene 1 protein
- growth-inhibiting protein 1
- lysosomal pepstatin insensitive protease
- tripeptidyl aminopeptidase
- tripeptidyl peptidase I
- tripeptidyl-peptidase 1
- tripeptidyl-peptidase 1 preproprotein
Additional Information & Resources
Tests Listed in the Genetic Testing Registry
Scientific Articles on PubMed
- Elleder M, Dvorakova L, Stolnaja L, Vlaskova H, Hulkova H, Druga R, Poupetova H, Kostalova E, Mikulastik J. Atypical CLN2 with later onset and prolonged course: a neuropathologic study showing different sensitivity of neuronal subpopulations to TPP1 deficiency. Acta Neuropathol. 2008 Jul;116(1):119-24. doi: 10.1007/s00401-008-0349-3. Epub 2008 Feb 19. Citation on PubMed or Free article on PubMed Central
- Fietz M, AlSayed M, Burke D, Cohen-Pfeffer J, Cooper JD, Dvorakova L, Giugliani R, Izzo E, Jahnova H, Lukacs Z, Mole SE, Noher de Halac I, Pearce DA, Poupetova H, Schulz A, Specchio N, Xin W, Miller N. Diagnosis of neuronal ceroid lipofuscinosis type 2 (CLN2 disease): Expert recommendations for early detection and laboratory diagnosis. Mol Genet Metab. 2016 Sep;119(1-2):160-7. doi: 10.1016/j.ymgme.2016.07.011. Epub 2016 Jul 25. Citation on PubMed
- Guhaniyogi J, Sohar I, Das K, Stock AM, Lobel P. Crystal structure and autoactivation pathway of the precursor form of human tripeptidyl-peptidase 1, the enzyme deficient in late infantile ceroid lipofuscinosis. J Biol Chem. 2009 Feb 6;284(6):3985-97. doi: 10.1074/jbc.M806943200. Epub 2008 Nov 26. Citation on PubMed or Free article on PubMed Central
- Kohan R, Carabelos MN, Xin W, Sims K, Guelbert N, Cismondi IA, Pons P, Alonso GI, Troncoso M, Witting S, Pearce DA, Dodelson de Kremer R, Oller-Ramirez AM, Noher de Halac I. Neuronal ceroid lipofuscinosis type CLN2: a new rationale for the construction of phenotypic subgroups based on a survey of 25 cases in South America. Gene. 2013 Mar 1;516(1):114-21. doi: 10.1016/j.gene.2012.12.058. Epub 2012 Dec 22. Citation on PubMed or Free article on PubMed Central
- Pal A, Kraetzner R, Gruene T, Grapp M, Schreiber K, Gronborg M, Urlaub H, Becker S, Asif AR, Gartner J, Sheldrick GM, Steinfeld R. Structure of tripeptidyl-peptidase I provides insight into the molecular basis of late infantile neuronal ceroid lipofuscinosis. J Biol Chem. 2009 Feb 6;284(6):3976-84. doi: 10.1074/jbc.M806947200. Epub 2008 Nov 26. Citation on PubMed
- Sun Y, Almomani R, Breedveld GJ, Santen GW, Aten E, Lefeber DJ, Hoff JI, Brusse E, Verheijen FW, Verdijk RM, Kriek M, Oostra B, Breuning MH, Losekoot M, den Dunnen JT, van de Warrenburg BP, Maat-Kievit AJ. Autosomal recessive spinocerebellar ataxia 7 (SCAR7) is caused by variants in TPP1, the gene involved in classic late-infantile neuronal ceroid lipofuscinosis 2 disease (CLN2 disease). Hum Mutat. 2013 May;34(5):706-13. doi: 10.1002/humu.22292. Epub 2013 Mar 11. Citation on PubMed