The Nic Volker Story - XIAP
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Nic Volker and XIAP - The Impact of a Single Nucleotide

What impact can genome sequencing have on your health? For Nic Volker, it meant the difference between life and death.

Nic Volker

By the time Nic was 4 years old, he already undergone over 100 surgeries and had his colon removed. Nic was unable to eat anything without getting sick. His parents continued to seek treatment and they finally found a physician at the Medical College of Wisconsin (MCW) who ordered Nic's exome (the part of the genome that codes for proteins) to be sequenced. The 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.

Sequencing Nic's Exome

Nic Volker

Nic's DNA was sequenced using "NextGen" sequencing technologies, in which multiple strands of single stranded DNA are affixed to small beads and DNA replication proceeds, incorporating new nucleotides, yielding flashes of light captured by the computer. Each nucleotide is a different color, allowing a sequence to be read by following the flashes of light. Through this technique, researchers located 16,124 variations in Nic's genome, as compared to the human reference sequence.

Using a computer program developed by researchers at MCW, the researchers were able to narrow the field of variations to a few candidates. Further analysis suggested that the variant located in the XIAP gene, a G to A substitution, was the disease-causing mutation.

The Flow of Genetic Information

Information contained within the DNA provides instructions on how to create necessary RNA and protein products. DNA is transcribed to create RNA, which is then processed to messenger RNA to be translated by ribosomes to generate proteins. This is called the "Flow of Genetic Information", commonly referred to as the "Central Dogma of Biology. Variations at the DNA level may encode for mutations at the protein level.

In Nic's case, the G to A change in the XIAP gene results in an amino acid substitution in the XIAP protein. In a normal XIAP protein, there is a cysteine in position 203, but in Nic's version of the protein, there is a tyrosine.

Nic Volker

Conservation of Amino Acids

The amino acid that is mutated in Nic's sequence (Cys208) is highly conserved amongst species. In the figure below, every organism from the house fly to human has a cysteine at this position. Only Nic has a tyrosine at this position. The highly conservative nature of this amino acid suggests its importance in biological function.

Nic Volker

Structure Affects Function

Changing the sequence of amino acids can impact the structure of the protein, which could in turn affect the function. A "zinc finger" is a common structural fold in which several amino acids bind to a zinc ion, thus stabilizing the structure. Nic's mutation affects an amino acid within this zinc ion binding region of XIAP. In a functioning XIAP, there are three cysteines and 1 histidine that bind to a zinc ion. In Nic's version of XIAP, one of these cysteines has been changed to a tyrosine, a much larger amino acid. This change may disrupt the zinc finger motif, affecting the stability of the protein.

Sequencing Informs Diagnosis - and Treatment

Nic Volker

People who do not have functioning XIAP protein can develop hemophagocytic lymphohistiocytosis (HLH), a fatal blood disorder. Since the doctors knew that Nic could potentially die from this disorder, they performed a cord blood transplant to replace Nic's defective XIAP with a functioning version of XIAP. While this transplant was done to prevent Nic from developing HLH, the treatment also cured his bowel disorder. Although still dealing with some health issues related to the transplant, Nic is now able to attend school and to live his life like other little boys.

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