Meta-Biol

• Pathways

The human gene SLC22A18 (aka TSSC5) encodes organic cation transporter-like protein 2 (ORCTL2). It is expressed at high levels in kidney, liver and colon and at lower levels in heart, brain and lung. ORCTL2 can transport organic cations such as chloroquine and quinidine with the antiport of protons.

The human chromosome region 11p15.5 is linked with Beckwith-Wiedemann syndrome (associated with susceptibility to Wilms' tumor, rhabdomyosarcoma and hepatoblastoma). SLC22A18 is located in this region (Cooper et al. 1998, Lee et al. 1998). Mutations and/or reduced expression of SLC22A18 have been found in certain tumors such as lung cancer (LNCR; MIM:211980) (Lee et al. 1998) and embryonal rhabdomyosarcoma 1 (RMSE1; MIM:268210) (Schwienbacher et al. 1998). How SLC22A18 might be involved in growth regulation is poorly understood. There is speculation that it may be involved in resistance to chemotherapy drugs and/or in the export of genotoxic substances whose retention may increase the risk of tumor formation.

Proteins with transporting functions can be roughly classified into 3 categories: ATP hydrolysis-coupled pumps, ion channels, and transporters. Pumps utilize the energy released by ATP hydrolysis to power the movement of substrates across the membrane against their electrochemical gradient. Channels in their open state can transfer substrates (ions or water) down their electrochemical gradient at an extremely high efficiency (up to 108 s-1). Transporters facilitate the movement of a specific substrate either against or with their concentration gradient at a lower speed (about 102 -104 s-1); as generally believed, conformational change of the transporter protein is involved in the transfer process. Diseases caused by defects in these transporter proteins are detailed in this section. Disorders associated with ABC transporters and SLC transporters are annotated here (Dean 2005).

Biological processes are captured in Reactome by identifying the molecules (DNA, RNA, protein, small molecules) involved in them and describing the details of their interactions. From this molecular viewpoint, human disease pathways have three mechanistic causes: the inclusion of microbially-expressed proteins, altered functions of human proteins, or changed expression levels of otherwise functionally normal human proteins.

The first group encompasses the infectious diseases such as influenza, tuberculosis and HIV infection. The second group involves human proteins modified either by a mutation or by an abnormal post-translational event that produces an aberrant protein with a novel function. Examples include somatic mutations of EGFR and FGFR (epidermal and fibroblast growth factor receptor) genes, which encode constitutively active receptors that signal even in the absence of their ligands, or the somatic mutation of IDH1 (isocitrate dehydrogenase 1) that leads to an enzyme active on 2-oxoglutarate rather than isocitrate, or the abnormal protein aggregations of amyloidosis which lead to diseases such as Alzheimer's.

Infectious diseases are represented in Reactome as microbial-human protein interactions and the consequent events. The existence of variant proteins and their association with disease-specific biological processes is represented by inclusion of the modified protein in a new or variant reaction, an extension to the 'normal' pathway. Diseases which result from proteins performing their normal functions but at abnormal rates can also be captured, though less directly. Many mutant alleles encode proteins that retain their normal functions but have abnormal stabilities or catalytic efficiencies, leading to normal reactions that proceed to abnormal extents. The phenotypes of such diseases can be revealed when pathway annotations are combined with expression or rate data from other sources.

Depending on the biological pathway/process immediately affected by disease-causing gene variants, non-infectious diseases in Reactome are organized into diseases of signal transduction by growth factore receptors and second messengers, diseases of mitotic cell cycle, diseases of cellular response to stress, diseases of programmed cell death, diseases of DNA repair, disorders of transmembrane transporters, diseases of metabolism, diseases of immune system, diseases of neuronal system, disorders of developmental biology, disorders of extracellular matrix organization, and diseases of hemostatis.