The Uncertain Future of Genetic Diagnosis of Human Behavior

    By Kevin E. Noonan --

Sickle cell anemia was the first known human disease associated with a genetic polymorphism (an A→T single nucleotide polymorphism, or SNP), resulting in the substitution of a valine residue (encoded by a GTG codon) for a glutamic acid residue (encoded by a GAG codon) in the beta chain of human hemoglobin.  The work, by Linus Pauling and colleagues at Cal Tech was published in the November 25, 1949 issue of the journal Science, about three and a half years before Watson and Crick deciphered the structure of DNA.  Today, sixty years of molecular biology, including the Human Genome Project, has produced the promise of identifying genetic markers for many human diseases, to be used for molecular diagnosis and particularized treatment going under the generic term of "personalized medicine."

Although monogenic traits have proven amenable to currently-available diagnostic methods (for diseases including cystic fibrosis, Tay-Sachs disease, Huntington's disease, and Duchenne muscular dystrophy), complex traits, particularly psychiatric and other behavioral disorders have proven more resistant to these methodologies.  Thus, it was a particularly exciting development six years ago when Dr. Avshalom Caspi and his colleagues reported in a landmark study in the journal Science that individuals bearing at least one copy of a specific genetic polymorphism were more susceptible to depression as a consequence of environmental (life) stressors (see Caspi et al., 2003, "Influence of Life Stress on Depression:  Moderation by a Polymorphism in the 5-HTT Gene," Science 301: 386-89).  The polymorphism occurs in the promoter region of the serotonin transporter gene (5-HTT), and the alleles are distinguished by the presence of 14 (the "short" allele) or 16 (the "long" allele) copies of a 20-23 basepair repeat motif.  Individuals with the homozygous long/long ("l/l") genotype were shown to express 1.4-1.7 fold greater amounts of the transporter gene mRNA, to have 2-fold higher levels of labeled serotonin uptake and to bind 30-40% more serotonin than the cells of individuals with the heterozygous short/long ("s/l") or homozygous short/short ("s/s") genotypes.

This study was performed on a cohort of 847 non-Maori children in Dunedin, New Zealand, who were followed from ages 3 to 26 (specifically, at ages 3, 5, 7, 9, 11, 13, 15, 18, 21 and 26).  Stressors examined in the study included problems in employment, finances, housing, health, and relationships, as well as childhood maltreatment.  In the study, males experienced more stressors than females; overall, 30% of study participants experienced no stressful life events, 25% experienced one, 20% experienced two, 11% experienced three, and 15% experienced four or more life stressful events.  These results themselves did not have any statistically-significant relationship to the 5-HTT allele genotypes.  17% of study participants were classified as having experienced depression, a level comparable to the results seen in the general population in the U.S.  When these results were compared with the genotypes of the study participants, a statistically-significant correlation (P = 0.02) was found between depression and the presence of the "s" allele, i.e., individuals bearing at least one "s" allele ("s/l" and "s/s" individuals) were more likely to be depressed as a consequence of equivalent numbers of life stressful events.  The authors were careful to emphasize that this correlation did not show a conclusive association between the "s" allele and a predisposition to depression, however.

This study was widely hailed as providing new insight into predicting depression, at least in young adulthood.  The study garnered accolades such as "absolutely spectacular" work, and Steven Pinker opined that the study provided "successful documentation of the elusive phenomena known as 'gene-environment interactions,' [which] are like the weather, according to Mark Twain:  Everyone talks about them, nobody does anything about them -- until now."  The study results also seemed consistent with what was known about neurobiology, in that the serotonin transporter is the target for antidepressants like Prozac®, and was also consistent with other research results that showed more intense brain reactions to fearful stimuli in individuals bearing the "s" allele.

However, the correlation between stress-induced depression and the short promoter of the human HTT gene has not held up.  As widely reported on Tuesday, Neil Risch at the University of California, San Francisco and colleagues at the National Institutes of Mental Health, Johns Hopkins University, Yale University School of Medicine, the Rockefeller University, the University of Pittsburgh School of Medicine, and Virginia Commonwealth University showed no statistically-significant link between depression and the short HTT allele.  The study, published yesterday in the Journal of the American Medical Association (see "Interaction Between the Serotonin Transporter Gene (5-HTTLPR), Stressful Life Events, and Risk of Depression") was a meta-analysis of data accumulated for 14,250 participants of 14 separate studies from the medical literature and a second meta-analysis of 10,943 participants from 10 of these studies.  These statistical analyses by Risch and his colleagues failed to show any increased susceptibility to depression as a result of environmental stressors as reported by the Caspi study for individuals bearing the short promoter variant of the HTT gene in either homozygous (s/s) or heterozygous (s/l) genotypes.  Although these studies are not without their own acknowledged shortcomings, they cast doubt on the validity of the correlations reported six years ago from a much smaller sample size (albeit the earlier study presented more in-depth assessments on the existence, frequency, and significance of the environmental stressors).

These results could not have come at a worse time for Dr. Caspi and his collaborators.  These scientists filed a patent application on their predictive method, published as U.S. Publication No. 2005/0037405.  On May 19th, the U.S. Patent and Trademark Office mailed a Notice of Allowance to the inventors, for claims including the following:

1.    A method for assessing predisposition to depression in a human subject who has experiences a stressful life event, the method comprising the steps of
    determining whether the subject carries one or more copies of a short promoter allele of a 5-HTT gene; and
    concluding that the subject is predisposed to depression if the subject carries the short promoter allele.

At a minimum, the Risch study may have raised a Rule 56 obligation on the inventors, their counsel, and anyone else involved in prosecuting the application that could forestall allowance of this claim.  The results also may raise validity issues of operability and enablement.

There has been justifiable skepticism by some about the capacity for genetic analyses to predict complex behaviors (see, for example, Hamer, 2002, "Rethinking Behavior Genetics," Science 298: 71-72).  These most recent results illustrate once again how difficult it will be to translate the promise of personalized medicine into clinically-validated diagnostic tests, particularly for complex or multigenic traits and diseases.  Of course, these problems make patent protection even more important to support investment in the uncertain promise of personalized medicine.

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[Big Don]

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