The juvenile Batten disease gene

Juvenile Batten disease results from mutations (mistakes) in the CLN3 gene (blueprint) responsible for making CLN3 protein. More than Sixty-seven (67) different mutations in the CLN3 gene have been shown to cause juvenile Batten disease(Mole). However, most children with the disease are missing a string of 966 DNA building blocks in one or both of their mutated CLN3 genes. This is known as the “~1kb deletion.” The specific effects of the ~1kb deletion or other 66 disease-causing mutations are not well-understood. However, it is believed that cells with mutations in both of their CLN3 genes do not make fully functional CLN3 protein. (Katz, Gao et al. 1997)

Where is CLN3 protein supposed to be?

cellCLN3 protein is found in the membranes (walls) of various cellular compartments (organelles). Much like a bustling city with departments for utilities and transportation, organelles have different functions working together to support the overall health of the cell. There are power plants (mitochondria), processing and packaging centers (Golgi apparatus), transit centers (endoplasmic reticulum), and the sanitation department (lysosomes)(Jarvela, Sainio et al. 1998, Luiro, Kopra et al. 2001, Luiro, Kopra et al. 2006)(Kyttala, Ihrke et al. 2004)(Katz, Gao et al. 1997, Kremmidiotis, Lensink et al. 1999, Pearce 2000, Persaud-Sawin, McNamara et al. 2004, Phillips, Benedict et al. 2005)CLN3 protein has been found in the membranes of all of these organelles as well as the nuclear envelope, responsible for regulating what enters and leaves the nucleus(Luiro, Kopra et al. 2001, Luiro, Kopra et al. 2006). In neurons, CLN3 protein can be found in specialized compartments called synaptosomes which are important for communication between nerve cells. CLN3 protein may have the same function in each of these organelles or different functions depending on the cell’s needs. Although we don’t know the true function(s) of the protein, there are several clues.

What is CLN3 protein doing in all of these places?

The accumulation of undigested autofluorescent waste material in the garbage disposal and recycling center (lysosomes) of cells is the clinical hallmark of juvenile Batten disease suggesting that CLN3 protein plays a critical role in lysosomal function. The presence of CLN3 within other organelles could represent interactions between lysosomes and those organelles and/or it is possible that CLN3 plays a minor role in non-lysosomal sites.

In addition to waste removal and recycling, lysosomes are important for sensing and maintaining a cell’s nutrient status, cell-to-cell communication, membrane repair, and protein transport system(Luiro, Kopra et al. 2001, Luiro, Yliannala et al. 2004, Luiro, Kopra et al. 2006, Gavin, Wen et al. 2013)(Narayan, Rakheja et al. 2006, Maxfield FR 2016). Studies have also shown that disturbances in these activities have a profound impact on the health of neurons in more well-known neurodegenerative diseases like Alzheimer’s, Parkinson’s, and Huntington’s diseases(Andrews 2000, Andrews 2005, Metcalf, Calvi et al. 2008, Appelqvist, Waster et al. 2013). Altogether, these results suggest that brain cells (neurons) are especially sensitive to disturbances or changes in lysosomal processes and missing normal CLN3 protein makes them even more vulnerable. They can compensate for the loss of healthy CLN3 for a short time but eventually, the scales tip too far and brain cells begin to die.

We know that CLN3 is a transmembrane protein. A transmembrane protein is an integral protein that spans organelle membranes in a single or multiple pass similar to fabric stitches. Many transmembrane proteins function as gateways permitting specific substances to enter or leave the organelle. Below is a short list of known transmembrane protein functions. We do know which, if any, of these functions can be attributed to CLN3 protein and several laboratories are reporting progress in this area.

How do we figure out what CLN3 protein does? More importantly, how do we fix the problems that occur when functional CLN3 protein is missing?

University-based researchers are hard at work using a variety of state-of-the-art techniques and animal models to 1) pinpoint the function(s) of CLN3 protein, 2) prioritize which CLN3 protein functions are most critical to the cell’s overall health and proper function, and 3) pair their findings with strategies to prevent the earliest and most damaging effects of the loss of CLN3 protein.

At the same time, scientists practicing clinical and translational research in the pharmaceutical sector are working to transform discoveries made using cell cultures and animal models into safe and effective treatments for children with Batten disease. It is estimated that it takes an average of $1.3 billion and 12 years to turn a discovery into medicine to treat a disease(DiMasi JA 2003, Dickson and Gagnon 2004, HG 2007). To see an animation of our strategic path towards a cure, click here.  To see a list of current treatment strategies under investigation, click here.

 

References

  1. Andrews NW. Membrane repair and immunological danger. EMBO Rep. 2005. 6826–6830.
  2. Andrews NW. Regulated secretion of conventional lysosomes. Trends Cell Biol. 2000. 10, 316–321.
  3. Appelqvist H, Wäster P, Kågedal K, et al. The lysosome: from waste bag to potential therapeutic target. J Mol Cell Biol. 2013 Aug;5(4):214-226.
  4. Dickson M and Gagnon JP. Key Factors in the Rising Cost of New Discovery and Development. Nat Rev Drug Disc (May 2004): 417-429.
  5. DiMasi JA, Hansen RW and Grabowski HG. The price of innovation: new estimates of drug development costs. J Health Econ 22(2003):151-185.
  6. DiMasi JA and Grabowski HG. The cost of Biopharmaceutical R&D: Is Biotech Different? Managerial and Decision Economics 28 (2007): 469-479.
  7. Gavin M, Wen GY, Messing J, et al. Substrate Reduction Therapy in Four Patients with Milder CLN1 Mutations and Juvenile-Onset Batten Disease Using Cysteamine Bitartrate. JIMD Rep. 2013;11:87-92.
  8. Jarvela I, Saino M, Rantamaki T, et al. Biosynthesis and intracellular targeting of the CLN3 protein defective in Batten disease [J]. Hum Mol Genet. 1998;7:85-90.
  9. Katz ML, Gao CL, Prabhakaram M. et al. Immunochemical localization of the Batten disease (CLN3) protein in retina [J]. Invest Opthamol Vis Sci. 1997;38:2375-2386.
  10. Kremmidiotis G, Lensink IL, Bilton RL et al. The Batten disease gene product (CLN3p) is a Golgi integral membrane. Hum Mol Genet Mar. 1999;8(3):523-531.
  11. Kyttala A, Ihrke G, Vesa J, et al. Two motifs target Batten disease protein CLN3 to lysosomes in transfected nonneuronal and neuronal cells. Mol Biol Cell. 2004 Mar;15(3):1313-1323.
  12. Luiro K, Kopra O, Blom T, et al. Batten disease (JNCL) is linked to disturbances in mitochondrial, cytoskeletal, and synaptic compartments. J. Neurosci Res. 2006;84: 1124–1138.
  13. Luiro K, Kopra O, Lehtovirta M et al. CLN3 protein is targeted to neuronal synapses but excluded from synaptic vesicles: new clues to Batten disease. Hum Mol Genet. 2001 Sep 15;10(19):2123-131.
  14. Luiro K, Yliannala K, Ahtiainen L, et al. Interconnections of CLN3, Hook1 and Rab proteins link Batten disease to defects in the endocytic pathway. Hum Mol Genet. 2004;13:3017–3027.
  15. Maxfield, Frederick R., Willard, James M. and Lu, Shuyan. Lysosomes: Biology, Diseases and Therapeutics, First Edition. Hoboken, New Jersey: John Wiley & Sons, Inc. 2016. Print.
  16. Metcalf DJ, Calvi AA, Seaman MNj, et al. Loss of the Batten disease gene CLN3 prevents exit from the TGN of the mannose 6-phosphate receptor. Traffic. 2008;11: 1905–1914.
  17. Narayan SB, Rakheja D, Pastor JV, et al. Over-expression of CLN3P, the Batten disease protein, inhibits PANDER-induced apoptosis in neuroblastoma cells: further evidence that CLN3P has anti-apoptotic properties. Mol Genet Metab. 2006;88:178–183.
  18. NCL Resource: A Gateway for Batten Disease [Internet]. London: University College London; Available from: http://www.ucl.ac.uk/ncl/batten.html
  19. Pearce DA. Localization and processing of CLN3, the protein associated to Batten disease: where is it and what does it do? J Neurosci Res. 2000;59:19–23.
  20. Persaud-Sawin DA, McNamara JO 2nd, Rylova S, et al. A galactosylceramide binding domain is involved in trafficking of CLN3 from Golgi to rafts via recycling endosomes. Pediatr Res. 2004 Sep;56(3):449-463.
  21. Phillips SN, Benedict JW, Weimer JM, et al. CLN3, the protein associated with Batten disease: structure, function and localization. Neurosci Res. 2005;79: 573–583