In the current study, the frequency of the less-common T variant of MTP gene at −493 locus in the overall population studied is around 0.12, similar to that of other Chinese healthy populations (Shanghai Hans, 0.123 ; Hong Konger, 0.126 ) and Japanese population (0.17 ), but dramatically lower than that of Caucasians (Swedish, 0.25 ; Austrian, 0.255 ) and African Americans (0.27 ), demonstrating a huge frequency gap across races on MTP -493T which might attribute to profoundly different evolutionary pressures at this locus between the East and the West over thousands of years. Intriguingly, however, it is worth noting that this frequency is less than half that of Hei Yi Zhuang (0.26) , a Zhuang branch (also termed as Minz) perching on the Sino-Vietnam boundary in Napo County we reported more recently . We have no exact explanations for this great diversity between the two branches basing on the currently available evidence, but factors such as dietary structure and marriage custom might have contributed. On one hand, according to cultural anthropology, the origin of Minzs can at least be dated back to nearly one thousand years ago when Zhuangs immigrated southward to Laos and Thailand [24, 37]. Unlike Bama and Pingguo Zhuangs who have more choice on daily staple foods (rice and maize), Minzs have been only growing and living on maize due to adverse mountainous and droughty environments. On the other hand, social isolation, smaller population size (51 800 ) and stricter inbreeding of Minzs have resulted in both cultural and genetic homogeneity of this branch relative to other ethnic groups, although signatures of gene mixture with others were also observed . These two crucial selective forces, therefore, may partially account for the higher T allelic frequency of the MTP -493G/T polymorphism in Minz as compared to branches living in Bama and Pingguo area. Alternatively, we could not rule out putative contributions of genetic drift or founder effect on the evolution of this variant in Minz.
Accordingly, owing to relatively lower T allele frequency, MTP TT homogenous individuals were rarely present versus GG and GT in our studying populations. Intriguingly, homozygous TT genotype tended to over-represent in long-lived families both in Bama and Pingguo areas. Furthermore, TT carriers exhibited lower TC and LDL-C concentrations than those with GG genotype, in accordance with data from several previous studies [9–13] but not in others [14–17]. This correlation between genotype and lipoproteins was further supported by subsequent multiple linear regression analysis in some groups (P < 0.05) but not in all.
It is clear that the key role of MTP is to transfer lipid to a nascent apoB molecule when it enters the lumen of the ER, mutation in the coding region of the MTP gene leads to undetectable MTP activity and abetalipoproteinemia, a rare autosomal recessive disease with only trace levels of apoB-containing lipoproteins . Moreover, inhibition of MTP by a synthetic MTP-inhibitor lowers atherogenic apoB-containing lipoproteins in patients with homozygous familial hypercholesterolemia . In light of this logic, it is plausible therefore to assume that an increase of MTP gene activity may confer higher levels of apoB-containing lipoproteins. This hypothesis had obtained support in a longitudinal study of young Afro-American men by Juo et al. . However, we observed a reduction rather than an elevation of LDL-C and TC in MTP TT individuals, in agreement with the currently leading results firstly described by Karpe et al. who simultaneously reported that the T allele had an almost 2-fold higher transcriptional activity than did the G allele in vitro . They explained that T allele had fewer but more lipid-rich particles, an increased expression of the MTP gene would lead to a more efficient lipidation of immature VLDL particles. As large VLDLs are not direct precursors of LDL, the input from VLDL to the LDL fraction will decrease and this, in turn, will lower the level of LDL-C in subjects homozygous for the MTP −493 T allele [7, 9]. It is also likely that the availability of apoB polypeptides in the ER could restrict the production of apoB-containing lipoprotein, although the -493T up-regulated MTP expression by 2-fold in transfected cells, hence the increased levels of plasma apoB-containing lipoprotein could be disproportional to the increased levels of MTP . More work will be required to discern which of the above explanations is more reasonable. Alternatively, some potentially important factors such as sample sizes, race, age, BMI, and diet should be taken into account, because the interaction between the apoE gene, diet, genetic background, and other covariates has been well documented [2, 3, 14, 40]. Indeed, although our studying cohorts were at large healthy, family-based, well-organized, large sample sizes, and genetically homogeneous, we could not exclude the putative effects of diet on lipid variables, which was an unexpected finding in the current study.
As can be seen in Tables 1, 2, and 4, serum TC, TG and LDL-C levels and the prevalence of dyslipidemia were higher in Bama populations including nonagenarian subgroup as compared to non-local controls, Pingguo populations, albeit a few heterogeneities were also observed among Bama subgroups. This is somewhat beyond our theoretical predictions because almost all previous studies by other investigators had addressed that Bama long-lived individuals exhibited superior lipid profiles, lower morbility and better survivorship relative to general populations inside or outside Bama area [41, 42]. Our subsequent nutrition survey indicated that inhabitants in both Bama and Pingguo areas have experienced radical changes in diet and lifestyle in the past decade due to rapidly socioeconomic improvement (data not shown). We noted, in particular, that individuals from Bama area tend to intake more fats, more proteins, more carbohydrates, and even more alcoholic beverages, which generate more calories than did before (data not shown). Furthermore, Bama families usually consume cheap fat pork and cook foods using animal oil (lard oil), while Pingguo families, with better socioeconomic status, usually consume expensive lean pork and vegetable oil (mainly peanut and camellia oil). It is widely recognized that there are great heterogeneities in composition, especially fatty acid between vegetable oil and lard oil and the former contains more unsaturated fatty acids which are favorable for health. Alternatively, a diet rich in saturated fat and cholesterol was found to promote a high-absorption status in GTP TT French women recently . Therefore, these radical dietary changes and potential gene-diet interaction may play critical role in shaping the current lipoprotein profiles in Bama cohorts. Further studies are necessary to clarify this point.
Geesaman and colleagues were the first to link MTP gene with exceptional human longevity. They found that a two-marker haplotype composed of the rs2866164-G/MTP-Q95 (rs61733139) was significantly underrepresented in a sample of 190 long-lived Americans of European extraction, and suggested that this risk haplotype confers a higher mortality earlier in life and consequently an extended life expectancy . However, these findings have not since been validated in other long-lived cohorts from French , German , Danish , Dutch , British , and Italian populations , leading several to conclude that the findings of Geesaman and colleagues were confounded by population stratification in their controls [19, 22]. Conversely, Huffman et al.  uncovered most recently that MTP rs2866164-CC instead of -GT/GG served as a deleterious but not a longevity genotype. It underrepresented with aging before 85 years old but sharply enriched in nonagenarians and centenarians wherein its unfavorable effects might be buffered by other longevity genotypes such as CETP (rs5882), ApoC3 (rs2542052), AdipoQ (rs56354395). MTP -493G/T (rs1800591) had been found to be in perfect linkage disequilibrium with rs2866164 [3, 19], and the minor T of -493G/T described in this article corresponds with the minor allele G of rs28166164 [3, 20]. Here, the allelic and genotypic frequencies of MTP -493G/T did not differ significantly between BLF and other tested cohorts, suggesting a lack of association between this polymorphism and human longevity in Bama area, lending further support to the leading hypothesis on this point. However, the TT genotype tended to overrepresent in the three long-lived families as compared to their geographic-matched controls. Together with the finding that TT individuals exhibited lower LDL and TC levels than did G carriers, it appears that T variant plays a favorable role in the lipid modulation in our tested cohorts. However, we could not infact rule out the possibility that T is a risk allele as proposed by Geesaman et al. . Given this is true, the enrichment of TT in long-lived families could be buffered by unknown longevity genotypes. To clarify these points, further works, e.g. functional analyses, buffering gene determinations, longitudinal studies, are necessary.