XIAP and Nic Volker: a Molecular Story From the CBM

Nic Volker and XIAP - The Impact of a Single Nucleotide The Flow of Genetic Information Conservation of Amino Acids

What impact can genome sequencing have on your health? For Nic Information contained within the DNA provides instructions on how to create necessary RNA and The amino acid that is mutated in Nic’s sequence (Cys203) is highly Volker, it meant the difference between life and death. protein products. DNA is transcribed to create RNA, which is then processed to messenger RNA to be conserved amongst species. In the figure below, every organism from translated by ribosomes to generate proteins. This is called the “Flow of Genetic Information”, the house fly to human has a cysteine at this position. Only Nic has a By the time Nic was 4 years old, he had already undergone over 100 commonly referred to as the “Central Dogma of Biology. Variations at the DNA level may encode for tyrosine at this position. The highly conservative nature of this amino surgeries and had his colon removed. Nic was unable to eat anything mutations at the protein level. acid suggests its importance in biological function. without getting sick. His parents continued to seek treatment and they finally found a physician at the Medical In Nic’s case, the G A change in the XIAP gene results in an amino acid substitution in the XIAP College of Wisconsin (MCW) who ordered protein. In a normal XIAP protein, there is a cysteine in position 203, but in Nic’s version of the protein, Nic’s exome (the part of the genome that there is a tyrosine. codes for proteins) to be sequenced. The DNA RNA Protein research team at MCW analyzed the sequence data to reveal a single mutation in Nic’s XIAP gene.

XIAP, or X-linked inhibitor of apoptosis, is a protein that has multiple functions in the body. In addition to regulating apoptosis, or cell death, XIAP plays an important role in the immune system, fighting off infections. Tyrosine replaces cysteine at residue 203 of XIAP Structure Affects Function

Changing the sequence of amino acids can impact the structure of the Sequencing Nic’s Exome XIAP - Based on 1i3o.pdb protein, which could in turn affect the function. A “zinc finger” is a common Nic’s DNA was sequenced using NextGen DNA sequencing technologies, in structural fold in which several amino which multiple strands of single stranded DNA are affixed to small beads Cys203 acids bind to a zinc ion, thus and DNA replication proceeds, incorporating new nucleotides, yielding stabilizing the structure. Nic’s flashes of light captured by the computer. Each nucleotide is a different Nic Volker’s mutation affects an amino acid within color, allowing a sequence to be read by following the flashes of light. mutation this zinc ion binding region of XIAP. In Through this technique, researchers located 16,124 variations in Nic’s results in a functioning XIAP, there are three genome, as compared to the human reference sequence. a tyrosine cysteines and 1 histidine that bind to a zinc ion. In Nic’s version of at position XIAP, one of these cysteines has been changed to a tyrosine, a much Using a computer program 203 larger amino acid. This change may disrupt the zinc finger motif, developed by researchers at affecting the stability of the protein. MCW, the researchers were able to narrow the field of variations to a few candidates. Further analysis suggested that the variant located in the Sequencing Informs Diagnosis - and Treatment XIAP gene, a G A substitution, was the People who do not have functioning disease-causing mutation. XIAP protein can develop hemophagocytic lymphohistiocytosis (HLH), a fatal blood disorder. Since the doctors knew that Nic could potentially General Protein Structure: die from this disorder, they performed a References - Select amino acids are displayed in spacefill and colored CPK. cord blood transplant to replace Nic’s - A salt bridge is formed by the acidic amino acid glutamic acid and the Nic Volker’s Mutation Site: Mayer et al. (2011). A Timely Arrival for Genomic Medicine. Genetics in basic amino acid arginine. defective XIAP with a functioning version - The zinc finger motif is of XIAP. While this transplant was done Medicine. 13(3):195-196. - The protein folds to expose hydrophilic amino acids illustrated by demonstrated by three threonine and asparagine. cysteines (200,203,227) and to prevent Nic from developing HLH, the Worthey et al. (2011). Making a Definitive Diagnosis: Successful Clinical - The protein folds to bury hydrophobic amino acids illustrated by leucine. one histidine (220). treatment appears to have cured his Application of Whole Exome Sequencing in a Child with Intractable - The C-terminus is colored red while the N-terminus is colored blue. - Zinc is displayed in spacefill bowel disorder. Nic is still dealing with Inflammatory Bowel Disease. Genetics in Medicine 13(3):255-262. and is colored dark red. some health issues related to the Secondary Structures: - Cysteine 203 is mutated to transplant. Additional materials avaialable at: - Alpha helices are highlighted in salmon and beta sheets are highlighted tyrosine in Nic Volker. http://cbm.msoe.edu/scienceOlympiad/module2012/xiapHome.html in yellow.