A Darwinian Approach to Huntington's Disease
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Medical Hypotheses (2007) 69, 1183–1189 http://intl.elsevierhealth.com/journals/mehy A Darwinian approach to Huntington’s disease: Subtle health benefits of a neurological disorder Benjamin R. Eskenazi, Noah S. Wilson-Rich, Philip T. Starks * Department of Biology, Tufts University, Medford, MA 02155, USA Received 20 February 2007; accepted 22 February 2007 Summary Huntington’s disease (HD) is a neurodegenerative disorder that, unlike most autosomal dominant disorders, is not being selected against. One explanation for the maintenance of the mutant HD allele is that it is transparent to natural selection because disease symptoms typically occur subsequent to an individual’s peak reproductive years. While true, this observation does not explain the population-level increase in HD. The increase in HD is at least partly the result of enhanced fitness: HD+ individuals have more offspring than unaffected relatives. This phenomenon has previously been explained as the result of elevated promiscuity of HD+ individuals. For this to be true, disease symptoms must be expressed during the otherwise asymptomatic peak reproductive years and promiscuity must increase offspring production; however, neither prediction is supported by data. Instead, new data suggest that the mutant HD allele bestows health benefits on its carriers. HD+ individuals show elevated levels of the tumor suppressor protein p53 and experience significantly less cancer than unaffected siblings. We hypothesize that the mutant HD allele elevates carriers’ immune activity and thus HD+ individuals are, on average, healthier than HDÀ individuals during reproductive years. As health and reproductive output are positively related, data suggest a counterintuitive relationship: health benefits may lead to an increased prevalence of Huntington’s disease. c 2007 Elsevier Ltd. All rights reserved. Background function deficit, psychological problems, and cog- nitive breakdown [3–6]. HD is a late onset disease Huntington’s disease (HD) is a debilitating neurode- with an average clinical manifestation at age 41.5 generative disorder caused by an abnormal allele years (SD, ±12 years), an age range of 8–83 years on chromosome four. The symptoms of the disease and an average survival of 20 years post diagnosis include physical and psychological changes, begin- [7,8]. Disease progression is well defined and often ning with the degeneration of the neostriatum, a divided into stages of physiological and functional vital part of the brain with roles in both motor con- deterioration for classification purposes [7]. trol and cognitive processes [1,2]. This eventually The initial study of HD began in the 19th century leads to the main diagnostic symptoms of motor with the work of George Huntington, but only re- cently have scientists been able to understand the molecular basis of the disease. The gene asso- * Corresponding author. Tel.: +1 617 627 4849; fax: +1 617 627 3805. ciated with HD, IT-15 on chromosome four, is com- E-mail address: [email protected] (P.T. Starks). posed of two adjacent trinucleotide repeats of 0306-9877/$ - see front matter c 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2007.02.046 1184 Eskenazi et al. either CAG or CCG. d2642, an IT-15 polymorphism, rate may be as great as 8% of new cases [20].HD may also contain a GAG trinucleotide repeat and allele frequency and distribution is predicted to may be attributed to HD causality [9]. A wealth be an ongoing process of gradual expansion and of evidence indicates a strong association between ever-increasing disease incidence [17]. Spontane- CAG repeat length and the expression of HD, while ous mutation is not sufficient to explain all new the association between abnormal CCG and GAG cases. HD incidence may be increasing because repeats and the disease is less certain and fre- HD+ individuals bearing more offspring than non- quently debated [10,11]. affected individuals. HD is classified as a polymorphic triplet disease Several studies comparing HD+ individuals to because its primary mutation at the CAG microsat- unaffected siblings indicate that affected individu- ellite of IT-15, exon 1, is characterized by a marked als have between 1.14 and 1.34 children for every expansion in length over normal alleles. There is an one that an unaffected sibling has [21–25]. Accord- inverse correlation between the number of repeats ingly, it appears that there may be a fitness payoff and the age of onset of HD [12]. Although HD+ indi- associated with carrying the HD+ allele that is pos- viduals are expected to faithfully pass their allele itively promoted via natural selection. This obser- to affected offspring, studies have found that many vation is not novel, and a mechanism leading to HD+ children have longer CAG tracts than do their heightened reproductive output has previously parents [13], primarily because the CAG allele is been suggested. This elevated fertility has been transmitted unstably between generations with a attributed to heightened promiscuity of HD+ indi- tendency towards expansion [14]. There is a sex viduals as a result of the psychological manifesta- bias in HD allele stability, with paternal transmis- tions of the disease [26]. A current challenge is sion prone to expansion, while maternal transmis- determining why HD+ individuals have more chil- sion tends to be more faithful. Indeed, one study dren than do the HDÀ population. reports that 31% of the gametes of HD+ males con- tained expanded CAG alleles [15]. A system of disease classification, proposed by HD and promiscuity the Huntington disease Genetic Testing Working Group [16], allows for the prediction of a patient’s A hypothesis for explaining the fertility gap be- phenotype corresponding to microsatellite repeat tween affected and unaffected individuals posits length. Individuals with 26 or less CAG repeats that promiscuity is the culprit. Dewhurst and col- are considered normal. Those with 27–35 repeats leagues [26] proposed that hypersexuality, com- will not be expected to exhibit the disease pheno- bined with a lack of inhibition and a surplus of type, but their allele may be mutable. Individuals irresponsible behavior, was leading to increased with 36–39 repeats fall into the intermediate offspring production. Others latched onto this idea, range and may display the HD phenotype and re- and the hypothesis evolved into the notion that the duced allele penetrance. Those with 40 or more re- lack of motor control and psychological deteriora- peats are HD+ [16]. The disease has been increasing tion that accompanies the onset of Huntington’s in frequency from an ancestral state of 7–12 re- translates into a lack of sexual control and a pat- peats in a variety of primate species (no less than tern of indiscriminant mating [27,28]. To the best eight repeats have been documented in humans), of our knowledge, this hypothesis has never been and is predicted to continue spreading in the future critically tested, and the observations on which [17]. Yet, as an autosomal dominant disorder, HD this claim rests remain unsupported. should generally be selected against [18]. The increased promiscuity hypothesis makes at least two critical predictions: first, the HD+ driven behavioral change should occur during peak repro- HD, natural selection, and fitness ductive years and, as such, should occur during the generally asymptomatic period. This prediction One obvious reason for the maintenance of Hun- fails because the cognitive and psychological tington’s disease is its late onset, which affords deterioration that may lead to promiscuity is not carriers the opportunity to reproduce before the a function of the mutant allele, but of disease disease unmasks itself. In addition, the tendency progression and the associated neuronal break- towards CAG expansion over contraction has led down. Although deteriorations of the basal ganglia to numerous events of spontaneous HD in previ- – one of the first signs of cognitive decline – have ously unaffected family lines. An estimated 3% of been noted in HD+ individuals prior to clinical pre- HD cases arise via this route [19], though regional sentation [29], there are no studies linking sexual studies have shown that the potential mutation promiscuity to degeneration in this area of the A Darwinian approach to Huntington’s disease 1185 brain. While HD+ individuals do frequently lose the immune surveillance and vigor, and these correlate ability to discriminate between a good and bad with overall health, then the elevated fitness of choice, this change in behavior does not manifest HD+ individuals can be explained by health benefits until other aspects of the disease are present. of possessing the mutant allele. Studies show that the risky behavior observed in these individuals is linked to the personality changes that take place after the onset of the dis- HD, p53 and htt ease, not during the asymptomatic period [30], during which time executive functions do not ap- For the elevated health hypothesis to be sup- pear to be significantly affected [31]. Because the ported there must be a mechanistic link driving disease is typically expressed later in life, these the relationship. We suggest that p53 is a factor behavioral changes are not likely to impact in this relationship. The substance p53 is a tumor reproduction. suppressor protein whose main role in the body is Second, increased promiscuity must lead to in- the maintenance of normal cell growth and dis- creased offspring production. This prediction fails ruption of the cell cycle in damaged cells [41]. because promiscuity does not necessarily increase In recent years, research into cancer pathogene- fitness in mammals [32]. In humans, the use of con- sis has concluded that this protein is one of the traception, the absence of a temporal breeding most important for the regulation of normal cell season, and the limited reproductive period of fe- growth and the prevention of unchecked cellular males provide even more challenges to conceive division.