The role of cigarette smoking and statins in the development of postmenopausal osteoporosis: a pilot study utilizing the Marshfield Clinic Personalized Medicine Cohort

SUMMARY

A cohort of postmenopausal osteoporotic females and controls with normal bone mineral density, the interleukin 6 (IL6) -634G > C (rs1800796) C allele of the promoter region showed association with osteoporosis. The lipoprotein receptor-related protein 5 (LRP5) gene showed association between C135242T C/T alleles and osteoporosis only in smokers, suggesting a role for environmental interaction.

INTRODUCTION

A nested case-control study within a population-based cohort was undertaken to assess the relative impact of cigarette smoking, statin use, genetic polymorphisms, and one-way interaction of these factors on development of osteoporosis in postmenopausal women.

METHODS

Genotyping of 14 single-nucleotide polymorphisms (SNPs) corresponding to vitamin D receptor gene, estrogen receptor 1, collagen type 1 alpha 1, IL6, transcription growth factor beta, apolipoprotein E, and LRP5 genes was performed in cases (n = 309) with osteoporosis and controls (n = 293) with normal bone mineral density drawn from a homogeneous Caucasian population. SNPs were chosen based on known functional consequences or prior evidence for association and genotyped using matrix-assisted laser desorption ionization time-of-flight technology.

RESULTS

Cases differed from controls relative to body mass index, age, and smoking but not statin use. After adjusting for age, the IL6 -634G > C (rs1800796) allele showed association with osteoporosis (odds ratio (OR) for CC + CG = 2.51, p = 0.0047)), independent of statin use or smoking status. On stratification for smoking, association with LRP5 C135242T (rs545382) and osteoporosis emerged (OR 2.8 in smokers for CT alleles, p = 0.03)), suggestive of potential environmental interaction.

CONCLUSION

Evidence suggested a role for genetic variation in IL6 and LRP5 in conferring risk for osteoporosis in Caucasian women, with the latter manifest only in smokers.

An analysis of PAX1 in the development of vertebral malformations

An analysis of PAX1 in the development of vertebral malformations. Due to the sporadic occurrence of congenital vertebral malformations, traditional linkage approaches to identify genes associated with human vertebral development are not possible. We therefore identified PAX1 as a candidate gene in vertebral malformations and congenital scoliosis due to its mutation in the undulated mouse. We performed DNA sequence analysis of the PAX1 gene in a series of 48 patients with congenital vertebral malformations, collectively spanning the entire vertebral column length. DNA sequence coding variants were identified in the heterozygous state in exon 4 in two male patients with thoracic vertebral malformations. One patient had T9 hypoplasia, T12 hemivertebrae and absent T10 pedicle, incomplete fusion of T7 posterior elements, ventricular septal defect, and polydactyly. This patient had a CCC (Pro)-->CTC (Leu) change at amino acid 410. This variant was not observed in 180 chromosomes tested in the National Institute of Environmental Health Sciences (NIEHS) single nucleotide polymorphism (SNP) database and occurred at a frequency of 0.3% in a diversity panel of 1066 human samples. The second patient had a T11 wedge vertebra and a missense mutation at amino acid 413 corresponding to CCA (Pro)-->CTA (Leu). This particular variant has been reported to occur in one of 164 chromosomes in the NIEHS SNP database and was found to occur with a similar frequency of 0.8% in a diversity panel of 1066 human samples. Although each patient's mother was clinically asymptomatic and heterozygous for the respective variant allele, the possibility that these sequence variants have clinical significance is not excluded.

Comparison of human genetic and sequence-based physical maps

Recombination is the exchange of information between two homologous chromosomes during meiosis. The rate of recombination per nucleotide, which profoundly affects the evolution of chromosomal segments, is calculated by comparing genetic and physical maps. Human physical maps have been constructed using cytogenetics, overlapping DNA clones and radiation hybrids; but the ultimate and by far the most accurate physical map is the actual nucleotide sequence. The completion of the draft human genomic sequence provides us with the best opportunity yet to compare the genetic and physical maps. Here we describe our estimates of female, male and sex-average recombination rates for about 60% of the genome. Recombination rates varied greatly along each chromosome, from 0 to at least 9 centiMorgans per megabase (cM Mb(-1)). Among several sequence and marker parameters tested, only relative marker position along the metacentric chromosomes in males correlated strongly with recombination rate. We identified several chromosomal regions up to 6 Mb in length with particularly low (deserts) or high (jungles) recombination rates. Linkage disequilibrium was much more common and extended for greater distances in the deserts than in the jungles.

Mouse models in tumor suppression

Genetic lesions found in tumors are often targeted to the negative growth regulatory tumor suppressor genes. Much of our understanding of tumor suppressor gene function is derived from experimental manipulations in cultured cells. Recently, however, the generation of mice with germ line tumor suppressor gene mutations through gene targeting in embryonic stem cells has provided another dimension by allowing experimental studies of tumor suppressor function in an organismal context. Novel insights into the role of tumor suppressors in development, differentiation, cell cycle control, and tumor suppression have been obtained from the studies on these 'knockout' mice. In addition, such mice may serve as disease models for humans with inherited cancer predisposition syndromes. Perhaps the greatest advantage of many of the mouse tumor suppressor models is that they facilitate study of the roles of tumor suppressor gene loss in tumor initiation and progression in vivo. Moreover, derivation of primary cells from tumor suppressor-deficient mice has provided an important resource for in vitro studies on the role of targeted genes in cell cycle regulation, DNA damage response, regulation of apoptotic pathways, and preservation of genomic stability. In this review, we discuss some of the mechanistic insights provided by tumor suppressor-deficient mice and their utility as models for human cancer syndromes.

The mouse equivalent of the human p53ser249 mutation p53ser246 enhances aflatoxin hepatocarcinogenesis in hepatitis B surface antigen transgenic and p53 heterozygous null mice

The relative contribution to development of hepatocellular carcinoma of the mouse equivalent to the human p53ser249 mutation, found in human hepatocellular carcinoma associated with aflatoxin (AFB1) exposure, is compared with other major risk factors in a transgenic mouse model. Transgenic p53ser246 mice, expressing the mutant protein gene under the control of a truncated albumin promoter, were bred to mice lacking p53 (p53-/-) and to transgenic mice expressing hepatitis B surface antigen (HBsAg). AFB1 hepatocarcinogenesis was then determined in offspring with single or multiple risk factors by determination of the numbers of high-grade hepatic tumors at 13 months of age. In AFB1-treated male mice, expression of the p53ser246 mutation increases the incidence of high-grade tumors from 0% to 14% in HBsAg-negative, p53+/+ (wild-type homozygous) control mice; from 14% to 71% in HBsAg-negative, p53+/- (wild-type heterozygous) mice; and from 62% to 100% in HBsAg-positive, p53+/+ mice. Thus, whereas HBsAg expression and AFB1 together are strongly cocarcinogenic, the presence of the p53ser246 mutant not only significantly enhances this cocarcinogenic effect, it also increases tumorigenesis in AFB1-treated p53 heterozygous and homozygous mice not expressing HBsAg. The possibility that the p53ser246 mutant protein may act as a promoting agent for AFB1 hepatocarcinogenesis is discussed.

Hepatitis B injury, male gender, aflatoxin, and p53 expression each contribute to hepatocarcinogenesis in transgenic mice

The major risk factors for human liver cancer: hepatitis B virus (HBV) related liver injury, male gender, aflatoxin exposure, and p53 expression, are evaluated and compared in experimental transgenic mouse models. Transgenic mice that express hepatitis B surface antigen (HBsAg) in their liver and develop liver tumors at 18 months of age (HBV+ mice) were bred to p53 null mice (p53-/-) to produce mice p53+/-, HBV+ mice. These mice and control littermates ([p53+/+, HBV+], [p53+/-, HBV-], and [p53+/+, HBV-) were divided into groups that did or did not receive an injection of aflatoxin at 1 week of age. At sacrifice at 13 months of age, 100% (7/7) of male mice with each of the three risk factors (p53+/-, HBV+, AFB1+) developed high-grade hepatocellular carcinomas (HCC). If any one of the risk factors was absent, the incidence drops: if both p53 alleles are present, 62% (10/16); if HBsAg is not expressed, 14% (1/7); if AFB1 is not given, 25% (2/8). If only one of the risk factors is present no tumors above grade I are found. Similar results were observed in female mice except that HCC incidence in each group is less than in male mice. Some of the tumors in mice with more than one risk factor are of unusual histological types, such as hepatocholangio-carcinomas, adenocarcinomas and undifferentiated carcinomas that are not usually seen in HBV transgenic C57BL/6 mice. No loss or mutation of the p53 gene is detected in any of the tumors. Possibilities of how the four major risk factors for HCC interact to produce malignant liver tumors in these transgenic mouse models of hepatocarcinogenesis are discussed.

Control of mouse hepatocyte proliferation and ploidy by p53 and p53ser246 mutation in vivo

The effect of expression of the p53 gene, in the presence or absence of the p53ser246 mutation (p53*), on ploidization (image cytometry), proliferation (expression of proliferating cell nuclear antigen and radioactive thymidine histoautoradiography), and apoptosis (in situ detection of DNA fragments) is determined in hepatocytes of p53-null and p53*-transgenic mice. The mouse p53ser246 mutation is equivalent to the p53ser249 mutation found in human hepatomas associated with hepatitis B virus infection and aflatoxin exposure. The hepatocytes of heterozygous or homozygous p53-knockout mice (p53+/-; p53-/-), as well as knockout mice expressing one allele of p53ser246 (p53+/-, p53*; p53-/-, p53*), do not undergo normal polyploidization with aging and show an increase in the number of cycling (G1-, S-, and M-phase) cells. In addition, p53ser246-transgenic mice (p53+/+, p53*; p53+/-, p53*; and p53-/-, p53*) have a greatly increased number of hepatocytes in the G1 phase. No differences in rates of apoptotic hepatocytes are found among any of the mouse groups studied, so the increased proliferation results in a hyperplasia manifested by a increased number of small periportal cells. We conclude that loss of p53 removes blocks in the cell cycle, leading to increased proliferation, whereas expression of the p53ser246 mutation stimulates G0 to G1 and/or M to G1 transition of hepatocytes. Increased proliferation of hepatocytes, combined with no concomitant increase in apoptosis, may in part explain the enhanced development of hepatocellular carcinomas in p53-knockout and p53*-transgenic mice exposed to aflatoxin.

Developmental control of transcription of the CAT reporter gene by a truncated mouse alphafetoprotein gene regulatory region in transgenic mice

A truncated mouse alphafetoprotein (AFP) gene promoter/enhancer region was tested for its ability to regulate the expression of the Escherichia coli chloramphenicol acetyltransferase (CAT) reporter gene in the livers of transgenic mice. The AFP regulatory region lacked any AFP gene structural DNA, included one enhancer sequence together with the proximal promoter sequence, and an element believed to be responsible for the postnatal repression of AFP gene transcription. The neonatal livers of AFP/CAT transgenic mice showed a high level of CAT enzyme expression, which was dramatically reduced between 7 and 14 days after birth. The staining of liver sections with anti-CAT antibodies showed that this expression was limited to hepatocytes. In one lineage, reexpression of CAT in the adult liver could be achieved by restitutive proliferation of hepatocytes following partial hepatectomy or CCl4-induced necrosis; reexpression in young animals (3-4 weeks of age) was even greater. These studies show that a truncated AFP promoter/enhancer region functions in a tissue-specific and developmental stage-specific fashion, and may be used to control the expression of other genes in the livers of transgenic mice.

Characterization of a murine p53ser246 mutant equivalent to the human p53ser249 associated with hepatocellular carcinoma and aflatoxin exposure

A mutation in the tumor suppressor p53 gene resulting in an Arg-->Ser substitution in position 249 is found frequently in human hepatocellular carcinomas associated with hepatitis B infection and with aflatoxin exposure. To determine the significance of this mutation in an in vivo experimental model using transgenic mice, we introduced a two-nucleotide change in the mouse p53 gene at amino-acid position 246, which is equivalent to position 249 in human p53, by the recombinant polymerase chain reaction mismatched primer method. This p53 mutation resulted in the same change, an Arg-->Ser substitution, as in the human p53 gene at position 249. We now report that the protein product of this mutant mouse p53ser246 had properties similar to those of the wild-type protein when tested by binding to (i) monoclonal antibodies PAb246 and PAb240, ii) simian virus 40 large T antigen, and (iii) heat-shock protein. However, it had mutant-type transforming properties when tested for colony formation with an osteosarcoma cell line. It was not active, as is wild-type p53, in transcription activation of the muscle creatine kinase promoter. These properties are the same as those found in the p53trp248 product of the p53 mutation associated with the Li-Fraumeni syndrome. Although less is known about the human p53ser249 product associated with hepatocellular carcinoma, the mutant murine p53ser246 protein shares the known properties of the human gene product.