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Cell Biology: Piet Borst


Piet Borst, Ph.D. professor, staff member and director emeritus

About Piet Borst

Experimental work in the Borst lab stopped
The experimental work in my lab ended in 2015. Since the ongoing projects were rather successful, they are being continued by former collaborators. I remain involved as adviser and, on the side, I remain active as reviewer, mentor, and lecturer on bio-ethics.

Mechanisms of anti-cancer drug resistance in mouse tumors
In collaboration with Jos Jonkers (NKI-AVL), we have studied resistance mechanisms in 'spontaneous' breast tumors arising in mice, conditionally defective in p53 and Brca1. This project was started in my lab by post-doc Sven Rottenberg and is continued by him (as PI) in Bern (Switzerland).

Recent developments in this project have led to two discoveries:

  • The Volume-Regulated Anion Channel (VRAC) acts as an entry port of cisplatin and carboplatin into the cell. (In collaboration with Thomas Jentsch, Berlin; and Thijn Brummelkamp, NKI). Loss of functional VRAC leads to modest platinum-drug resistance.
  • Continuing work on resistance to PARP inhibitors, led to the discovery that loss of PAR glycohydrolase (PARG) can cause PARG resistance. This mechanism can also occur in cells deficient in BRCA2, which are unable to (partially) restore homologous recombination, a resistance mechanism prevalent in cells deficient in BRCA1.

Physiological functions of ABC transporters
We are interested in mechanisms of drug resistance in cancer cells and have focused on resistance caused by increased ATP-dependent transport of drug out of the cell, mediated by ATP-binding cassette (ABC) transporters. We have isolated genes for these transporters and characterized their substrate specificity and sensitivity to inhibitors in transfected cells. To study the physiological function of these transporters in metabolism and defense of the body against drugs and xenotoxins , we have inactivated genes for several drug transporters by targeted gene disruption in mice. Initially we looked at P-glycoproteins (ABCB1 and ABCB4); most recently we have studied the Multidrug Resistance-associated Protein (ABCC) family members MRP2, 3, 4, 5 and 6. MRPs are known to transport organic anions out of cells and these are often produced by conjugation of toxic compounds to hydrophilic organic anions, such as glucuronic acid. Although many substrates of MRPs are known, the list is incomplete. For some MRPs there is no idea yet of their physiological function.

Senior post-doc Koen van de Wetering therefore initiated a systematic search for compounds conjugated to glucuronide or sulphate that are transported by MRPs by comparing the derivatives in plasma/urine of WT and KO mice using Mass Spectrometry. We have identified several glucuronidated and sulphated phyto-estrogens, derived from food, as novel substrates of MRP2 (ABCC2), MRP3 (ABCC3) and BCRP (ABCG2). More recently we have studied MRP5 and MRP6 by this approach.

In 2000 we generated a mouse KO of the Mrp5 gene. Although this gene is expressed in most mouse tissues, the KO mice had no phenotype (Wijnholds et al., PNAS, 97 (2000) 7476). MRP5 was found to transport some base and nucleotide analogs and later also some other drugs and cAMP and cGMP. A role for MRP5 in drug resistance or cyclic nucleotide metabolism in intact mice has not been demonstrated, however. We have therefore reinvestigated MRP5 and the Mrp5 KO mice using metabolomics. Two new classes of substrates were identified: glutamate-conjugates, some of which are related to neurotransmitters; and a new class of compounds not previously known to exist in mammals, lactoyl-aminoacids.

MRP6 (ABCC6) and PXE
Pseudoxanthoma elasticum (PXE) is an autosomal recessive disease characterized by a progressive mineralization of connective tissue, resulting in skin, arterial and eye disease. Classical PXE is caused by mutations in the MRP6 (ABCC6) gene. Studies by Uitto et al. with Abcc6-/- mice have shown that the absence of ABCC6 in the liver is crucial for PXE and have confirmed the "metabolic disease hypothesis" for PXE, which states that tissue calcification is due to the absence of a plasma factor X secreted from the basolateral hepatocyte membrane.

Using a concerted metabolomics approach Robert Jansen and Koen van de Wetering have shown that X is ATP. ATP is rapidly converted in the circulation into pyrophosphate (PPi) and AMP. PPi is a known inhibitor of tissue calcification and indeed PXE patients have only about 40% of normal PPi levels in their plasma. This discovery has inspired new treatment attempts for PXE. Our clinical colleagues in Utrecht have tested bisphosphonates with modestly positive results. As an independent PI in Philadelphia, Koen van de Wetering has contributed to a study by our long-time collaborator Andras Varadi (Budapest) showing that large doses of oral PPi can counteract ectopic calcification in Abcc6 KO mice. Varadi has shown that oral PPi is also taken up in humans and trials with oral PPi in PXE patients are in progress.

DNA base J
This project is an offshoot of our long-standing interest in the mechanisms of antigenic variation in African trypanosomes. Base J (β-glucosyl-hydroxymethyluracil), which we discovered in African trypanosomes in 1993 (Cell 1993; 75: 1129-1136), is a base present in kinetoplastid flagellates and Euglena. It replaces 1% of thymine in nuclear DNA and is predominantly located in repetitive sequences, such as telomeric repeats. We have shown that the initial step of base J synthesis, the conversion of a T-residue in DNA into hydroxymethyluracil, is catalysed by 2 enzymes belonging to the TET/JBP family of oxygenases (hydroxylases) that require Fe2+ and 2-oxoglutarate as cofactors. More recently we have shown that J is essential in Leishmania for the proper termination of transcription. Loss of J results in massive read-through of transcriptional stops and in death of the parasite (Cell 2012; 150: 909-921). This project was discontinued in Amsterdam in 2012 and transferred to Peter Myler in Seattle.

I remain involved as advisor in the structural work by Anastassis Perrakis (NKI) on the enzymes involved in J biosynthesis, J-binding proteins 1 and 2 (JBP1 and 2). We have shown in recent years that JBP1 binds with high affinity to J in DNA and that it can then hydroxylate a T 13 bp downstream in the other strand. By a variety of structural approaches Perrakis is trying to elucidate the structural features of JBP1 that allow this remarkable feat.




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Van Luenen, Henri

Henri van Luenen

Manager Research


I studied Biology at the Free University in Amsterdam.  As part of this study I did a rotation project in the lab of Piet Borst at the NKI and I stayed at the NKI ever since. Ronald Plasterk offered me a PhD student position at the NKI which I gladly accepted. After first studying adipogenesis in response to DNA hypomethylation, I switched to study the transposition mechanism of the Tc3 transposons in C. elegans. After my PhD I continued my research in the group of Piet Borst, first on the transferrin receptor in T. brucei and later on base J in Leishmania. This unusual base plays an important role in transcription termination in this parasite.

Besides my research I am also involved in the general operations of the NKI.


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Key publications View All Publications

  • Transportomics: screening for substrates of ABC transporters in body fluids using vesicular transport assays

    FASEB J. 2012;26(2):738-47. Epub 2011 Oct 27

    Krumpochova P, Sapthu S, Brouwers J, De Haas M, De Vos R, Borst P, Van de Wetering K.

    Link to PubMed
  • Glucosylated hydroxymethyluracil (DNA base J) prevents transcriptional read-through in Leishmania.

    Cell, 2012;150(5):909-21

    Van Luenen H, Farris C,  Jan S, Genest PA, Tripathi P, Velds A, Kerkhoven RM, Nieuwland M, Haydock A, Ramasamy G, Vainio S, Heidebrecht T, Perrakis A, Pagie L, Van Steensel B, Myler P, Borst P.

    link to PubMed

Recent publications View All Publications



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    Elise Marseille

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    +31 20 512 2015



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