The results of our study showed that the LEPR Gln223Arg variant is highly polymorphic in the Nigerian population. The LEPR 223Arg allele frequency of 0.49 in our study participants is higher than figures reported in Caucasians (0.45), Pima Indians (0.32), Arabs (0.34) but much lower than figures in Asian populations (0.85) . Although a study of mixed U.S. population by Chung et al.  reported little difference in LEPR 223Arg allele frequency by racial groups, the study recruited only 26 blacks and racial admixture in the U.S. population may partly account for their finding. The International Hapmap Project reported LEPR 223Arg allele frequencies of 0.61 among Yorubas in Ibadan, Nigeria and 0.53 in African Americans. 
The evaluation of the relationship of the LEPR Gln223Arg polymorphism and breast cancer risk showed that premenopausal women carrying at least one LEPR 223Arg allele were at a modestly increased risk of breast cancer (OR = 1.8, 95% CI 1.0–3.3, p = 0.05); the risk was unaltered after adjusting for waist/hip ratio and age (OR = 1.8, 95% CI 1.0–3.2, p = 0.07). There was no association between the LEPR Gln223Arg polymorphism and breast cancer risk in postmenopausal women. Two studies [22, 23] evaluated the LEPR Gln223Arg polymorphism in relation to breast cancer risk; one  in the Tunisia population reported significantly increased risk in both premenopausal and postmenopausal women in a dose dependent manner (OR = 1.68, 95% CI 1.12–2.50 and OR = 2.26, 95% CI 1.31–3.90 for the LEPR Gln223Arg and Arg223Arg genotypes, respectively). In addition, the authors noted that the presence of the LEPR 223Arg allele was associated with poorer overall survival. The second study by Woo et al.  found no association between the polymorphism and breast cancer risk in both premenopausal and postmenopausal Korean women (OR = 0.54, 95% CI 0.19–1.81). The authors noted the rarity of the LEPR 223Gln allele in the Korean population (allele frequency, 0.09) and the small sample size of their study (45 breast cancer cases and 45 control subjects). Some investigators [16, 18, 21] have examined the relationship between variants of the LEPR Gln223Arg polymorphism and serum levels of leptin since there is some evidence associating serum leptin levels with breast cancer risk [17, 18]. In a study involving 118 college students in Greece (62 females and 56 males), Yiannkouris et al.  reported significantly higher serum leptin levels in individuals who were homozygous LEPR Arg223Arg compared with those harboring at least one LEPR 223Gln allele. Another study of 220 postmenopausal women in the U.K. found significant association between serum leptin levels and the LEPR genotype .
Although the actual mechanisms of leptin's role in breast cancer risk are not completely known, several lines of evidence provide support for leptin's broader physiological role, including the regulation of several neuroendocrine axes, some of which play a significant role in the pathogenesis of breast cancer. Leptin treatment corrects the hypogonadism of leptin-deficient ob/ob mice [30, 31] and starved normal mice , accelerates the onset of puberty in rodents and increasing leptin levels may signal the onset of puberty in boys and girls [33, 34]. In addition, leptin's pulsatile secretion is synchronized with the pulsatility of luteinizing hormone and estradiol in normal women . Leptin has been shown to regulate GH secretion  and serum leptin levels have been associated with circulating IGF-1 and IGFBP-3 levels in normal and GH-deficient humans [34, 37]. In addition, circulating leptin levels have been associated with certain life-style factors, such as smoking and alcohol intake . Interestingly, the above endocrine axes and life-style factors have also been implicated in the pathogenesis of breast cancer via leptin's interaction with IGF-I and IGFBP-3 [38, 39].
Tumor markers that are elevated in breast cancer can upregulate leptin production. These factors, including TNF-α, IL-1α, IL-1β, VEGF, and fibroblast growth factor 2 (FGF-2), raise leptin levels and promote tumor growth and differentiation [40, 41]. In experimental studies, leptin consistently stimulated human breast epithelial cell lines and breast cancer-derived cell lines resulting in increased DNA synthesis evaluated by thymidine incorporation test and increased cellular growth estimated by cellular density [9–12]. Several down-stream effects of leptin signaling leading to cellular proliferation were also observed including increased expression of phosphorylated signal transducers (STAT3), extracellular signal-regulated kinase (ERK), mitogen-activated protein kinases (MAPK) and transcript activator protein (AP-1), and also increased expression of cyclin dependent kinase-2 and cyclin D1, two cell-cycle regulating proteins [9, 10]. It has been suggested that the single amino acid change in the LEPR gene (LEPR Gln223Arg), a glutamine for an arginine with a change from neutral to positive, could affect the functionality of the receptor and alter its signaling capacity [13, 42, 43]. The finding of higher leptin binding activity (LBA) levels in homozygous carriers of the G allele (LEPR Arg223Arg) and higher levels of leptin in the LEPR Arg223Arg homozygotes and our finding of increased premenopausal breast cancer risk in women carrying the LEPR 223Arg allele provides supportive evidence for this proposition. It is possible that the leptin polymorphism is in linkage disequilibrium with other genes that play a role in breast cancer risk.
An important observation is the paucity of epidemiological literature on the role of polymorphisms in the leptin receptor (LEPR) gene in breast cancer susceptibility in most populations and its complete absence in sub-Saharan African populations. Much has been said about the mechanisms of leptin-induced carcinogenesis in animal models and correlation of serum leptin levels with breast cancer risk [17–20]. The effect of genotypes such as LEPR Gln223Arg polymorphism on breast cancer may vary from one population to the other as a result of marked differences in the distribution of the alleles in different populations. The finding of interaction between LEPR Gln223Arg genotypes and menopausal status and obesity may also partly explain the differences in reports from various populations. In the developed countries with a much higher percentage of postmenopausal breast cancer and a higher proportion of obese women, LEPR Gln223Arg polymorphism may be expected to have more impact on risk of the disease compared to the Nigerian population with a lower prevalence of obesity. Some of the limitations of this study include use of both incident and prevalent cases of breast cancer. Given the report of poorer prognosis associated with the LEPR 223Arg allele in some studies, it is possible that some patients harboring this allele might have died earlier leaving us with more patients with the LEPR 223Gln allele. However, such a bias if prevent would result in underestimation of the risk associated with the putative high risk LEPR 223Arg allele. There is currently no breast cancer screening program in Nigeria; therefore patients with early pre-clinical disease may have been missed in our study. Use of hospital controls is necessitated by the limited research infrastructure such as poor communication facilities and lack of population-based cancer registries in developing countries such as Nigeria.
Another issue of concern is the sample size of this study. In fact, we estimate that with an allele frequency of.49 in all women combined, a sample size of 139 cases and controls provides a power of 80% to detect an odds ratio (OR) of 2.0. There were a total of 209 cases and 209 controls in our study population indicating that this sample size has adequate statistical power to detect an OR of 2.0. However, when we stratify by menopausal status, the number of cases and controls become much lower, thus decreasing the statistical power of the study. For example, a sample size of 137 premenopausal cases and 137 premenopausal controls would be adequate to detect an odds ratio of 2.0 while there were 114 premenopausal cases and 121 premenopausal controls in our study. It is possible that variations in LEPR polymorphisms and other low penetrance genes may contribute to the marked worldwide variation in breast cancer incidence, with the highest age-standardized incidence rate (ASR) in North America (ASR, 99.4 per 100,000) and the lowest age-standardized incidence rates in sub-Saharan Africa (ASR, 27.8 per 100,000 in West Africa, 19.5 per 100,000 in East Africa and 16.5 per 100,000 in Central Africa) .