The Race to Find the Tangier Disease Gene

For this article, Brendan A. Maher interviewed Michael R. Hayden, director and senior scientist, Centre for Molecular Medicine and Therapeutics, University of British Columbia, Canada; Stephan Rust, cholesterol metabolism group leader at the Institut für Arterioskleroseforschung an der Westfälischen Wilhelms-Universistät; and Gerd Schmitz, a physician and director of the Institut for Clinical Chemistry and Laboratory Medicine, University Hospital, Regensburg. Data from the Web of Science (ISI, Philadelphia,) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age. M. Bodzioch, E. Orsóo, J. Klucken, T. Langmann, A. Böttcher, W. Diederich, W. Drobnik, S. Barlage, C. Büchler, M. Porsch-Özcürümez, W. E. Kaminski, H. W. Hahmann, K. Oette, G. Rothe, C. Aslandis, K. Lackner, G. Schmitz, "The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier disease," Nature Genetics, 22; 347-351, August 1999. (Cited in 198 papers) S. Rust, M. Rosier, H. Funke, J. Real, Z. Amoura, J. Piette, J. Deleuze, H. B. Brewer, N. Duverger, P. Denèfle, G. Assmann, "Tangier disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1," Nature Genetics, 22; 352-355, August 1999. (Cited in 193 papers) A. Brooks-Wilson, M. Marcil, S. M. Clee, L. Zhang, K. Roomp, M. van Dam, L. Yu, C. Brewer, J. A. Collins, H.O.F. Molhuizen, O. Loubser, B.F. F. Ouelette, K. Fichter, K.J.D. Ashbourne-Excoffon, C.W. Sensen, S. Scherer, S. Mott, M. Denis, D. Martindale, J. Frolich, K. Morgan, B. Koop, S. Pimstone, J.J.P. Kastelein, J. Genest Jr., M. R. Hayden, "Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency," Nature Genetics, 22; 336-344, August 1999. (Cited in 196 papers) Courtesy of Stephan RustStephan Rust Donald S. Fredrickson first described Tangier disease 40 years ago after finding it in two siblings, inhabitants of the tiny Chesapeake Bay island from which the disease gets its name.1 To date, barely 50 cases of this genetic disorder have been found worldwide, but most are on Tangier Island, where a founder population dates to the 17th century. The discovery by Fredrickson, a former National Institutes of Health director, waved the green flag for scientists to find Tangier's cause. The interest was not just to help those afflicted with this orphan disease, but to gain insight into another pressing health issue. One of Tangier's most striking characteristics is the dramatic deficiency of high-density lipoprotein (HDL)--the so-called good cholesterol that is believed to transport excessive cholesterol from the bloodstream and transfer it to the liver where it can be expelled. The disease--also characterized by enlarged liver and spleen, peripheral neuropathy, lipid deposits in various body parts, and premature coronary artery disease--was seen as a perfect dysfunctional model for the elusive molecular basis of reverse cholesterol transport. Further, the connection with coronary artery disease (CAD), and correlation between low HDL levels and risk for CAD meant to researchers that a better understanding of Tangier could aid in fighting heart disease, a leading killer in the civilized world. In the late 1990s, as new methods and advanced technologies in genetics became available, researchers broke into the final stretch. A linkage study mapped the gene to 9q31 in 19982 and in June of that year, three papers--two from Germany and one from Canada--were submitted to Nature Genetics within an eight-day span that definitively attached a gene to this disease. In August 1999, the magazine published the three papers linking Tangier disease to specific mutations in the gene ABC1 (later dubbed ABCA1), which encodes a protein of the ATP-binding cassette transporter (ABC) family. Gerd Assmann and Stephan Rust, principal investigator and cholesterol metabolism group leader, respectively, at the Institut für Arterioskleroseforschung an der Westfälischen Wilhelms-Universistät, were in the lead because they penned the linkage study. Moreover, Assmann had worked under Fredrickson at the NIH and had kept an interest in Tangier. In Rust's 1999 paper, analyses of families from Germany, Chile, and Spain provided enough preliminary data to select a functional gene candidate for Tangier disease. But the lead was lost. "We ordered a human cDNA clone with the name ABC1 that was in the databases ... and [mapped] to the 9q31 region by the database," Rust says. The clone produced no results through test and retest, so ABC1 was set aside. A records review revealed that the clone originally mapped to a region other than 9q31 and was not, in fact, the ABC1 they were looking for. "Thus we lost almost a year and the distance between us and our competitors ... became unfortunately small," Rust says. Had they not trusted the database, he estimates they would have published six months earlier. "Life," he laments. Gerd Schmitz, investigator on the other German paper and physician and director of the Institut for Clinical Chemistry and Laboratory Medicine, University Hospital, Regensburg, worked in Assmann's lab in the 1970s. This paper had data from five families to demonstrate numerous mutations in ABCA1 that cause Tangier disease. Some had drastic translational effects, like the base pair deletion at nucleotide 1,764, which creates a stop codon 24 base pairs downstream. This removes 75 percent of the protein product, including all 12 transmembrane regions and both ATP-binding domains. Other mutations documented by Schmitz's lab had less dramatic effects on the protein, but the phenotypic effects were similar. They found an amino acid substitution in the highly conserved Walker A motif of one ATP-binding domain. Though not affecting the residues thought to be most critical, it is nearly identical to a mutation in ABCR that is found in Stargardt macular dystrophy, a disorder caused by mutations in an ATP-binding cassette protein. Schmitz, who continues to work on the molecular mechanics of reverse cholesterol transport, says, "Our approach gave not only genetic data, but [it] also provided us with a lot of functional insight." Schmitz fully characterized five different mutations in the families he observed, and in almost all cases where there was a heterozygote for a dysfunctional allele, HDL levels as recorded were nearly half that of normal. While the discovery pertains to a relatively rare metabolic disorder, the implications could be significant. Schmitz says, "I think after the LDL [low density lipoprotein] receptor, the HDL pathway is one of the major miracles in cardiovascular diseases." Michael R. Hayden, director and senior scientist, Centre for Molecular Medicine and Therapeutics, University of British Columbia, Canada, was the investigator for the third paper describing the mutations in ABC1 that cause Tangier disease. "I think," he says, "that everybody recognized that understanding the genetic defect in this gene would be very important. And, there were actually probably 10 to 15 groups around the world that were trying to clone this particular gene." The paper from Hayden's group clearly draws ties between Tangier disease and what they call familial HDL deficiency (FHA). Studying two families with a history of Tangier, and two French Canadian families with a history of FHA, their lab found that the two independent genetic disorders mapped to the same gene and the same mutations. Tangier occurs in the homozygous state, and FHA occurs in the heterozygous state, says Hayden. Articles from other labs, however, including Assmann's, use the two terms interchangeably prior to 1999. Some disagreement over whether there are discernible differences between Tangier and FHA remains. Disparities aside, all three papers represent a remarkable step forward in understanding reverse cholesterol transport. Hayden says, "Now we have a target that can have big effects on HDL levels in humans and the potential for reducing the [most common] cause for morbidity and mortality throughout the world has now at least been offered by the cloning of this gene." He adds that some researchers believe ABCA1 is involved in regulating cell survival, apoptosis, fertility, and brain function. The end of the race for the gene represents the beginning of an even longer journey--that of discovering the mechanisms by which ATP-binding cassettes work. Rust explains, "The problem may be compared to that of cystic fibrosis," which is also linked to defects in ABC transporter genes. While some mechanistic features have been elucidated, Rust says, "this took a long time and still there are many points not well understood." Brendan A. Maher can be contacted at bmaher@the-scientist.com. References 1 www.faseb.org/genetics/ashg99/f172.htm 2. S. Rust et al., "Assignment of Tangier disease to chromosome 9q31 by a graphical linkage exclusion strategy," Nature Genetics, 20: 96-8, 1998.

The Race to Find the Tangier Disease Gene

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