HIV-1 protecting CCR5-Δ32 allele in medieval Poland

Abstract

CCR5-Δ32 is the mutation in the chemokine receptor CCR5 that gives its homozygous carriers nearly complete protections from HIV-1 infection. Restricted almost exclusively to Europe, the mutation is thought to have originated in the continent and risen in frequency to the present-day value of approximately 10% due to a selective advantage it gave its carriers. The mutation bearing allele was initially calculated to be ∼1000 years old and pandemic diseases, such as Bubonic Plague or smallpox were postulated to have selected it. However, new reports appear, that question these hypotheses. Data from ancient DNA (aDNA) studies prove the mutation to be much older, as suggested by calculations based on newer genetic maps. In order to investigate if the plagues of the last millennium selected the allele, and add to the discussion on CCR5-Δ32 origin and age, we searched for the mutation in aDNA isolated from individuals whose skeletal remains were collected at archaeological sites in Poland, dated back to 11–14th centuries. The calculated mean frequency of the allele in medieval Poland (5.06% as compared to contemporary 10.26%), implies its longer than previously believed presence in European populations, and suggests that historic pandemics had little effect on its present-day frequency.

Introduction

Among many receptor molecules, C–C chemokine receptor 5 (CCR5) arouse particular interest, as it was proven that, when defective, it confers resistance to most of HIV-1 strains (Liu et al., 1996, Samson et al., 1996, Dean et al., 1996). Most of the many mutations in CCR5 gene, located in chromosome 3, region p21.3, are function altering, which may suggest their adaptive role in response to past selective pressures (Ansari-Lari et al., 1997, Carrington et al., 1997). A 32 base pair deletion within the second extracellular loop makes the protein shorter and prevents it from fusing with the cell membrane, which results in lack of the receptor expression on the surface of leukocytes (Blanpain et al., 2002). Homozygous carriers of CCR5-Δ32 exhibit no CCR5, whereas in heterozygotes, the amount of the receptor is on average lower. Because CCR5 is required for HIV-1 entry into leukocytes, this makes the first group nearly completely resistant to HIV-1 infection and in the latter, progression towards AIDS is slower, CD4⁺ T cells counts higher and mortality rate lower (Dean et al., 1996, Rappaport et al., 1997, Zimmerman et al., 1997, Philpott et al., 2003). There is no known medical condition or an apparent impairment of body function that results from absence or reduced number of CCR5 (Liu et al., 1996, Zimmerman et al., 1997), which may be explained by the fact that there are a lot of other chemokine receptors of the kind (Premack and Schall, 1996, Carrington et al., 1997). The mutation is present almost exclusively in Europe and neighbouring areas of Asia and Mediterranean. Its mean frequency in the continent is around 10% and the distribution of the allele forms a north to south cline, with lowest frequency in Sardinia (4%) and highest in Finland (15.8%) (Libert et al., 1998). Its trace presence in other parts of the world results most probably from migrations and settlement of European descent. For instance, CCR5-Δ32 has not been found in indigenous Mexican populations but its frequency in Hispanic Mexicans is approximately 4.4% (in Spain it is present in 8% of the population) (Salas-Alanis et al., 1999). The mutation is absent in indigenous populations of Asian and Pacific islands (Lu et al., 1999), South America (Leboute et al., 1999) and Africa (Samson et al., 1996). Although carriers of the mutation have been found in non-Caucasian populations, e.g. Indian (Husain et al., 1998) and Chinese (Zhang et al., 2002), it might result from gene inflow to ancestral populations (Martinson et al., 1997).

Because of the characteristic distribution of the mutation carrying allele in Europe and the whole world, there are two main theories concerning its place of origin. Libert et al. (1998) suggested, it first appeared in North-Eastern Europe and Balanovsky et al. (2005) seek its roots in a Uralic population. Alternatively, the mutation event may have taken place in a Scandinavian population and was disseminated in 8–10th centuries by Vikings (Lucotte and Mercier, 1998, Lucotte and Dieterlen, 2003). The age of the mutation was first estimated (using haplotype analysis and coalescence theory) by Stephens et al. (1998) to be approximately 700 years. The authors also suggested that to reach the present-day frequency, the allele must have been subjected to strong selective pressure imposed by a factor that operated only in Europe. It would have given an advantage of some kind to its carriers and thus quickly rose in frequency. The most obvious candidate seemed to be the Black Death, the disease that claimed millions during the last centuries in the continent. The bubonic plague of 14th century appeared as an especially suitable candidate for a selective factor since mortality rate was the highest among all epidemics (McEvedy, 1988), the pandemic route was similar to the contemporary gradient of CCR5-Δ32 in Europe, Yersinia pestis (the plague agent) infects CCR5 carrying macrophages and, above all, because of the time of the outbreak, which matched the calculations (Stephens et al., 1998). However, a different set of chromosome markers used by Libert et al. (1998) allowed the group to estimate the time of the allele appearance to be between 3400 and 1400 years before present. Still, they argue, the allele had a single origin and it was under a selective pressure. Plenty of pathogens were speculated to be involved in selecting CCR5-Δ32. These include: Shigella, Salmonella, Mycobacterium (which all target macrophages) (Stephens et al., 1998) and Variola major (the smallpox virus)—which infects cells in a similar manner as HIV-1 does and claimed a huge number of deaths cumulatively throughout the last millennium (Klitz et al., 2001, Galvani and Slatkin, 2003). Other suspected diseases were: viral haemorrhagic fever, which may have been responsible for the deadliest epidemics (Duncan and Scott, 2005) and anthrax (Winkler et al., 2004). Alternatively, the frequency of CCR5-Δ32 may have been influenced by some climatic or geographical factors (Limborska et al., 2002, Balanovsky et al., 2005).

However, new reports appear that question the initial assumption regarding the allele's age and discuss the need of any selection to account for the contemporary frequency and distribution of CCR5-Δ32. In order to add to the data and discussion on origin, dispersal and possible impact of the historically most deadly infectious diseases on the frequency of the allele, we decided to take advantage of ancient DNA (aDNA) methodology and search for the mutation in a medieval population that predates the biggest epidemics of 14th and 17th centuries. Comparison of medieval frequency of the allele with present-day values would shed light on the putative influence of pandemics on CCR5-Δ32 presence in Europe. Had the mutation been selected by any of the pathogens plaguing the European population during the last millennium, its frequency would have been much lower in the middle ages. Poland appears to be a very suitable region for such comparison as the population has always been uniform with very little ethnic minorities and no major migration or immigrant influx, which is reflected for example in the language, that is deprived of any major variability throughout the whole country. This ensures relative continuity between the medieval and contemporary populations.