Segregation analysis of Parkinson disease

Parkinson disease (PD) is a prevalent movement disorder of unknown cause whose incidence rises with increasing age. Nearly 20% of PD is familial, a small subset of which exhibits autosomal dominant transmission. However, in most families, the inheritance is not clear. To determine the most likely mode of inheritance of PD, we performed complex segregation analyses using kindreds of 136 PD patients randomly ascertained from a clinic population. The hypotheses of a nontransmissible environmental factor, no major gene or type (sporadic), and all Mendelian inheritance (dominant, recessive, additive, decreasing) were rejected (P <0.001). Familial clustering of PD in this data set is best explained by a rare familial factor which a) is transmitted in a nonMendelian fashion, and b) influences the age at onset of PD. If confirmed, our results have immediate implications in gene-mapping studies which often search for genes that behave in a Mendelian fashion that affect susceptibility rather than age at onset and long term implications in understanding the pathogenesis of PD.     

Evidence for a Mendelian gene in a segregation analysis of generalized radiographic osteoarthritis: the Framingham Study

OBJECTIVE: To investigate the inheritance of generalized osteoarthritis (OA). METHODS: OA was identified on hand and knee radiographs obtained from members of the Framingham Study cohort (the parents) in 1967-1970 and 1992-1993, and from their adult children in the Framingham Offspring Study in 1993-1994. All hand and knee radiographs evaluated for OA were graded using the Kellgren and Lawrence (K/L) scale. A measure of generalized OA was defined as the count of the number of hand and knee joints affected, as determined by the proportion of joints with a K/L grade > or =2. The OA count, treated as a continuous variable, was adjusted for age, body mass index, and a measure of physical activity for each joint area (hand or knee). Calculations were made separately for each generation and each sex, and correlations were analyzed against the standardized residual of OA. Segregation analysis was used to test whether OA aggregated in families, and if its transmission fit a Mendelian pattern. RESULTS: A total of 337 nuclear families with 2 parents and at least 1 biologic offspring were studied. In parents, the mean age was 61.2 years at the time of hand radiographs and 72.8 years at the time of knee radiographs, which were mostly obtained at a later examination. The mean age at the time of radiographs in offspring was 53.9 years. Using standardized residuals, parent-offspring and sibling-sibling correlations ranged from 0.115 to 0.306. In segregation analyses, models testing the hypotheses of no familial aggregation, no familial transmission, or a Mendelian gene alone were all rejected (P < 0.001 for each of these models). The best-fitting models were mixed models with a Mendelian mode of inheritance and a residual multifactorial component. The Mendelian recessive model provided the best fit. CONCLUSION: These analyses support a significant genetic contribution to OA, with evidence for a major recessive gene and a multifactorial component, representing either polygenic or environmental factors.     

Genetic transmission of major affective disorders: Quantitative models and linkage analyses.

We comprehensively reviewed 2 types of studies aimed at specifying the mode of inheritance of major affective disorders: quantitative models and linkage analyses. Quantitative models attempt to represent the genetic mechanism responsible for the familial distribution of a disorder. Despite efforts to refine models by incorporating the bipolar–unipolar distinction or the sex effect, consistent support for a specific mode of transmission has not been found. Some mixed genetic models support single major locus inheritance, but transmission probabilities do not conform to Mendelian expectations. Linkage analysis is a more powerful technique used for testing the single gene hypothesis. Linkage results have also been inconsistent, showing moderate support for an X-linked variant of bipolar-related disorder and equivocal support for linkages to Chromosomes 6 and 11. However, relatively few genetic loci have been examined. Methodological factors, genetic heterogeneity, and phenotypic heterogeneity are discussed as potential explanations for inconsistent findings. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Complex segregation analysis in a sample of consecutive newborns with cleft lip with or without cleft palate in Italy

The mode of inheritance of cleft lip with or without cleft palate (CL/P) has been extensively investigated, but the results are controversial. We report results of complex segregation analysis performed in the families of 636 consecutive newborns with CL/P registered in the northeast Italy and Emilia Romagna congenital malformation registries to test hypotheses regarding CL/P inheritance. The programs POINTER and COMDS have been used. POINTER could not distinguish between alternative genetic models, and only the hypothesis of no familial transmission could be rejected. COMDS results, after inclusion of the severity parameter, rejected the hypotheses of a single major locus and were consistent with the two-locus model with a major dominant locus and at least one modifier locus.     

Thalidomide in the treatment of the mucocutaneous lesions of the Beh?et syndrome. A randomized, double-blind, placebo-controlled trial

Mood disorders are known to cluster within families, but the mode of transmission remains largely unknown. The purpose of our analysis was to determine whether selection of a sample that was homogeneous in its response to an antidepressant provided stronger evidence for a single major locus. Complex segregation analysis was applied to a sample of 171 Italian families of bipolar and unipolar probands that were responsive to the antidepressant fluvoxamine. We used regressive logistic analyses to determine the best fit from among environmental, arbitrary Mendelian, dominant, recessive and additive models. For the 171 affective families with probands that were responsive to the antidepressant fluvoxamine, a Mendelian model of inheritance was rejected. When considering 68 families of bipolar probands, the best fit was obtained for a Mendelian dominant model of transmission. The identification of a Mendelian mode of transmission in bipolar subjects who were selected according to their response to fluvoxamine supports the use of a pharmacological criterion as a tool for identifying true genetic disorders.     

Segregation analysis of breast cancer in a population-based sample of postmenopausal probands: The Iowa Women's Health Study

Inheritance of a major susceptibility gene for breast cancer has been primarily investigated in families with early-onset disease. However, familial clustering of late-onset breast cancer is well documented, and genetic factors may also be relevant. In the Iowa Women's Health Study, we evaluated evidence for a major gene after allowing for measured environmental risk factors. Two hundred sixty-five incident breast cancer probands were identified from a prospective cohort study of 41,837 women aged 55 to 69 years at baseline in 1986. A pedigree development form was mailed to the probands to ascertain all first-degree female relatives. A questionnaire and body measurement protocol were mailed to identified living relatives or surrogates. Segregation analyses were conducted on a total of 1,145 women in 251 families using regressive models as implemented in S.A.G.E. Mendelian codominant inheritance of an allele that produced an earlier-age-at-onset provided the best fit to the data. Incorporation of measured environmental risk factors as covariates yielded no significant improvements in the likelihoods. Approximately 50% of this population could be expected to carry a late-onset breast cancer susceptibility gene, and 23% of the population is susceptible because of the environment in which they live. Homozygous gene carriers are predicted to have a mean age-at-onset of 48 years, over 20 years earlier than heterozygotes; few cases would be expected among non-gene carriers. In conclusion, the transmission pattern of late-onset breast cancer may be determined by a common susceptibility gene.     

Hypertension: genes and environment

Hypertension can be classified as either Mendelian hypertension or essential hypertension, on the basis of the mode of inheritance. The Mendelian forms of hypertension develop as a result of a single gene defect, and as such are inherited in a simple Mendelian manner. In contrast, essential hypertension occurs as a consequence of a complex interplay of a number of genetic alterations and environmental factors, and therefore does not follow a clear pattern of inheritance, but exhibits familial aggregation of cases. In this review, we discuss recent advances in understanding the pathogenesis of both types of hypertension. We review the causal gene defects identified in several monogenic forms of hypertension, and we discuss their possible relevance to the development of essential hypertension. We describe the current approaches to identifying the genetic determinants of human essential hypertension and rat genetic models of hypertension, and summarise the results obtained to date using these methods. Finally, we discuss the significance of environmental factors, such as stress and diet, in the pathogenesis of hypertension, and we describe their interactions with specific hypertension susceptibility genes.     

p34cdc2 expression and meiotic competence in growing goat oocytes

Proper control of environmental factors can be crucial to the identification of genes that influence susceptibility to a complex trait, especially for a trait such as lung cancer, for which the environmental factor (smoking) accounts for a significant etiologic fraction of the disease. An earlier segregation analysis of 337 Louisiana families, which incorporated direct measure of tobacco consumption, provided evidence for autosomal codominant inheritance of a major gene that influenced age at onset of lung cancer. Subsequent analyses were performed in which the families were stratified into two subsets based on birth cohort of the proband; results suggested the presence of heterogeneity that were postulated to reflect the influence of cohort trends in tobacco consumption. To evaluate this hypothesis further, we simulated a population of three-generation pedigrees in which an autosomal dominant mode of susceptibility to lung cancer was transmitted, but tobacco use varied across generations corresponding to published trends in smoking. A total of 200,000 individuals in families of various sizes, ages, and cigarette smoking habits were simulated from 1900 to 1980. From this population, 324 families (2,405 individuals) with 380 cases of lung cancer were ascertained through 328 lung cancer probands. Complex segregation analysis was performed using the REGTL program of S.A.G.E. in which pack-years of tobacco exposure were incorporated directly into the likelihood calculations. Although the no major gene, environmental, and Mendelian recessive hypotheses were rejected, both dominant and codominant transmission provided a good fit to the data. Thus in a population of simulated families with autosomal dominant susceptibility to lung cancer, intergenerational differences in tobacco consumption led to the detection of autosomal codominant transmission as an acceptable hypothesis. These results underscore the potential danger of segregation analysis of complex traits in which exposure to known environmental influences may differ across generations.     

Genetic factors study in family aggregation of schizophrenia in Santiago, Chile

BACKGROUND: Genetic epidemiological studies indicate that genetic factors contribute to a familial aggregation of schizophrenia. The form of inheritance has not been elucidated but most studies have been done in Caucasian populations. AIM: To study the form of inheritance of schizophrenia in an urban population of Santiago, Chile, containing an admixture of Spanish origin individuals with Southamerican aborigines. SUBJECTS AND METHODS: Forty four randomly selected schizophrenic probands, 22 female, aged 28 to 48 years old, were studied. From them, an extensive genealogical reconstitution was performed. Probands and relatives were interviewed using the structured interview CIDI and DSM-III-R check-list. Schizophrenia was diagnosed using DSM-III-R criteria. Complex segregation analysis was done using Pointer program. RESULTS: The hypothesis of a multifactorial inheritance, without the participation of major genes, could not be rejected. Likewise, the major dominant and co-dominant gene forms of transmission could not be rejected. CONCLUSIONS: Our results show the participation of a major dominant locus and a multifactorial component in the inheritance of schizophrenia, as has been reported elsewhere.     

Complex segregation analysis of leprosy in southern Vietnam

To investigate the nature of the genetic component controlling susceptibility to leprosy and its subtypes, 402 nuclear families were ascertained through a leprosy patient followed at the Dermatology Hospital in Ho Chi Minh City, Vietnam; 285 families were of Vietnamese origin and 117 were of Chinese origin with a higher proportion of lepromatous forms among Chinese patients. Segregation analyses were conducted using the model developed by Abel and Bonney [(1990) Genet Epidemiol 7:391-407], which accounted for variable age of onset and time-dependent covariates. Three phenotypes were considered: leprosy per se (all forms of leprosy together), nonlepromatous leprosy, and lepromatous leprosy. For each of this phenotype, analyses were performed on the whole sample and separately on the Vietnamese and the Chinese families. The results showed that a single Mendelian gene could not account for the familial distributions of leprosy per se and its two subtypes in the whole sample. However, these results were different according to the ethnic origin of the families. In the Vietnamese subsample, there was evidence for a codominant major gene with residual familial dependences for the leprosy per se phenotype, and borderline rejection of the Mendelian transmission hypothesis for the nonlepromatous phenotype. In Chinese families, strong rejection of Mendelian transmission was obtained in the analysis of leprosy per se, and no evidence for a familial component in the distribution of the nonlepromatous phenotype was observed. For the lepromatous phenotype, the discrimination between models was poor, and no definitive conclusion could be reached. Referring to immunological data, we suggest that these results could be explained by a heterogeneity in the definition of the lepromatous phenotype. It is likely that progress in the understanding of the genetic components involved in the expression of leprosy will come from a better definition of the phenotype under study, and immunological studies are ongoing in this population to investigate this hypothesis.     

Pedigree investigation of familial non-insulin dependent diabetes mellitus

To discuss the inheritant mode of familial NIDDM. METHODS: According to WHO criteria of DM, 100 NIDDM Probands with family history of DM were diagnosed and 100 persons were chosen at random for controls. The survey of DM was performed in both groups, including FBG, HbAlc, FINS. Some members had insulin release test. RESULTS: The prevalance rates of DM in familial DM group were 26 times of the control group (34.3% and 1.3%). The prevalance rate of DM among first-degree relation was 18 times higher than that in general population (28.3% and 1.5%). The rate of diabetes in the siblings and in the off-springs was 44.4% and 9.7% respectively. The pedigree analyses showed that 83.9% affected families had one diabetic parent, one half siblings had DM, and there was a successive transmission of DM through at least three generations in sixteen large families. Besides, the incidence of DM was much higher in females than in males (40% and 28%). Among affected parents, diabetlic mothers were much more than diabetic fathers (50.8% and 27.6% P < 0.01). CONCLUSION: Familial NIDDM had a familial aggregation. It was inherited in the manner of Mendelian autosomal dominant inheritance. The difference between the rate in DM mothers and fathers was probably due to unequal prevalance rate in females and males.     

Major locus influencing plasma APO-A1 levels also controls plasma HDL3-C concentrations

Elevated plasma levels of apolipoprotein A1 (APO-A1) and high-density lipoprotein cholesterol (HDL-C) are important protective factors for atherosclerosis and coronary heart disease. Using the data on plasma concentrations of APO-A1, and HDL-C particles HDL2-C and HDL3-C in 970 Israeli individuals belonging to 228 pedigrees, we tested the hypothesis that a major locus influencing interindividual variation in APO-A1 levels also controls interindividual variation in HDL3-C and HDL2-C levels. Univariate and bivariate complex segregation analyses, as implemented in two statistical packages (MAN-3 and PAP-4.0) were applied to test the hypothesis. The results of the analysis clearly indicated the possibility of major gene involvement in the determination of plasma concentration variation of each of the 3 study variables. The results provide strong evidence in support of our hypothesis that HDL3-C genetic variation fully depends on the APO-A1 major locus. In particular, environmental and sporadic models were strongly rejected (P < 0.001) in bivariate analysis. The hypothesis of no pleiotropic effect of the putative APO-A1 locus on HDL3-C transmission was also unequivocally rejected (P < 0.001), while the bivariate Mendelian model was accepted (P > 0.05). The results of bivariate analysis of APO-A1 effect on HDL2-C were not clear. They indicated the possibility of the existence of slight genetic covariation between the two variables, and as yet we were unable to decipher the mode of covariation with the applied models.     

Segregation analysis reveals a major gene effect controlling systolic blood pressure and BMI in an Israeli population

It has been suggested that genetic factors control blood pressure level at all ages. However, the evidence is limited because of the composite nature of blood pressure and the heterogeneity of the studied samples. The purpose of the present study is to test for genetic influences on systolic blood pressure (SBP) level in a community-based Israeli family study. Segregation analysis was performed on 622 adults from 208 pedigrees. Age, sex, and body mass index (BMI) were significant covariates of SBP. Segregation analysis rejected the environmental transmission model but not the mixed Mendelian transmission model. The best-fitting genetic model was the mixed codominant model, with a heritability of 0.32 and an allele frequency of 0.18 for high SBP level. We further tested whether SBP and BMI shared a common major gene effect. Using bivariate segregation analysis involving two traits and a single locus, we found evidence for a single-locus pleiotropic effect on SBP and BMI. The allele frequency of this major locus was 0.24. The residual genetic correlation resulting from additive polygenes and the environmental correlation between these two traits were not different from zero after taking into account the shared major gene effect. The proportion of phenotypic variation attributable to this major gene effect increased with age for SBP but decreased with age for BMI.     

Tetracaine-lidocaine-phenylephrine topical anesthesia compared with lidocaine infiltration during repair of mucous membrane lacerations in children

When a rigorous methodological approach is utilized, a substantial majority of recent studies provide evidence for the familial transmission of schizophrenia. Although the absolute rates of schizophrenia among relatives of schizophrenics tend to be lower than those reported in the earlier studies due to the restrictiveness of contemporary definitions of schizophrenia, the risk to relatives compared to that of controls has remained quite consistent. This observation that relatives of schizophrenics have an elevated risk for schizophrenia compared to controls is consistent with theories of both genetic and environmental transmission. Twin studies of schizophrenia have consistently reported greater concordance rates for monozygotic than dizygotic twins. Although this indicates the importance of genetic factors, the less than 100% concordance for monozygotic twins observed in every study indicates that nongenetic factors also play a role in the etiology of schizophrenia. Further, adoption studies offer an opportunity to unconfound genes and environment. The findings of adoption studies confirm that there are genetic components for schizophrenia. Even though we have shown that family, twin, and adoption studies have provided strong evidence for the role of genetic factors in schizophrenia, the mode of transmission remains unclear. The results of mathematical modeling studies do not support the single gene model. There is somewhat more support for the multifactorial polygenic model, but the model has also been rejected in several studies. Thus, the pattern of inheritance of schizophrenia has eluded an unambiguous characterization. Genetic linkage analysis promised to clarify the mechanisms of transmission, but early positive reports were subsequently overturned and, to date, there are no consistently replicated positive linkage findings for schizophrenia. There is now a world-wide search for the location of the genes on specific chromosomes which are responsible for schizophrenia. The clinical implications of current work to the future of locating a schizophrenic gene or genes will be discussed.     

Screening of germline mutations in the CDKN2A and CDKN2B genes in Swedish families with hereditary cutaneous melanoma

BACKGROUND: Approximately 10% of human cutaneous melanomas occur in families in which several members are affected. The familial predisposition to this disease is often associated with dysplastic nevus syndrome, a condition in which afflicted family members have multiple dysplastic nevi (atypical moles). The chromosome region 9p21 and markers on chromosomes 1p and 6p have been linked to melanoma susceptibility. The tumor suppressor genes CDKN2A and CDKN2B have been mapped to the 9p21 region, and genetic analyses have revealed the presence of germline CDKN2A alterations in melanoma families. The reported frequencies of such alterations, however, vary among these families. PURPOSE: The present investigation was carried out to determine the frequencies of CDKN2A and CDKN2B germline gene mutations among members in a population-based cohort of Swedish melanoma families (i.e., melanoma kindreds). METHODS: DNA was prepared from blood samples obtained from 181 individuals belonging to 100 melanoma kindreds. The polymerase chain reaction (PCR) technique, followed by single-strand conformation polymorphism (SSCP) and nucleotide sequence analyses, were used to identify the types and frequencies of mutations in exons 1, 1beta, 2, and 3 of the CDKN2A gene and in exons 1 and 2 of the CDKN2B gene. RESULTS: CDKN2A gene aberrations were independently identified by both SSCP and nucleotide-sequence analyses. Nucleotide-sequence analysis identified a single point mutation leading to a substitution of leucine for proline in codon 48 of exon 1 in a family with a history of melanoma and several other cancers. A second abnormality, leading to an insertion of an extra arginine residue at codon number 113 of exon 2, was seen in four separate families. The CDKN2A exon-3 coding region had the wild-type sequence in all samples. No germline mutations were found in the alternative exon 1beta of the CDKN2A gene or in exons 1 and 2 of the CDKN2B gene. CONCLUSIONS: The present investigation demonstrates that CDKN2A germline gene mutations were observed in 7.8% of the 64 Swedish melanoma kindreds that each included at least two first-degree relatives with melanoma and dysplastic nevus syndrome. No CDKN2A exon 1beta or CDKN2B mutations were identified. The critical genes responsible for the inheritance of a susceptibility to develop melanoma among family members in this population have yet to be identified.     

Constitutional alterations in p16 in patients with uveal melanoma

Chromosome 9p21 contains a susceptibility gene for cutaneous melanoma. Recent studies suggest that the gene responsible may be CDK41, since it encodes a putative cell cycle inhibitor, p16, and is frequently lost or rearranged in melanoma cell lines. In this study we examined whether germline alterations in CDK41 could be identified in patients with melanoma of the uveal tract. From an archive of bloods collected from patients with uveal melanoma, we identified 13 samples drawn from patients with a history in a family member of uveal (n = 6) or cutaneous (n = 7) melanoma. An additional 24 'control' bloods (without melanoma or any other primary malignancy in a family member), similar to the 'cases' in age and number of first-degree relatives, were also selected for study. For each sample, DNA was extracted from the red blood cell fraction. Using the polymerase chain reaction-single strand conformation polymorphism method, we screened for alterations in p16. Specific changes were characterized by DNA sequencing. Six nucleotide changes were detected in five (13.5%) of the 37 samples examined. An altered gene was found in one (7.7%) of the 13 patients with a family history (of intra-ocular melanoma) and four (16.7%) of the 24 patients with no family history (P = 0.64) of melanoma. In this series the group with a positive family history was predominantly female and most pedigrees involved matrilineal descent. In these data prevalence of germline alteration in p16 was similar in familial and sporadic cases. The results provide evidence against a significant role for p16 in familial clustering of intra-ocular and cutaneous melanomas.     

Segregation analysis of plasminogen activator inhibitor-1 and fibrinogen levels in the NHLBI family heart study

Elevated plasminogen activator inhibitor-1 (PAI-1) and fibrinogen concentrations are risk factors for coronary heart disease. We investigated environmental, familial, and genetic influences on PAI-1 antigen and fibrinogen concentrations in 2029 adults from 512 randomly ascertained families in 4 US communities. We used maximum-likelihood segregation analysis to fit several genetic and nongenetic modes of inheritance to the data to determine whether mendelian inheritance of a major gene could best explain the familial distributions of these 2 hemostatic factors. Age- and gender-adjusted familial correlations for PAI-1 antigen level averaged 0.16 in first-degree relatives (95% CI=0.11 to 0.21); the spouse correlation was positive but not statistically significant (r=0.10, 95% CI=-0.02 to 0.23). Complex segregation analysis indicated a major gene associated with higher PAI-1 concentrations in 65% of individuals from these families. Demographic, anthropometric, lifestyle, and metabolic characteristics together explained 37% to 47% of the variation in PAI-1 antigen levels, and the inferred major gene explained an additional 17% of the variance. Positive and statistically significant age- and gender-adjusted familial correlations in first-degree relatives indicated a possible heritable component influencing plasma fibrinogen concentration (r=0. 17, 95% CI=0.13 to 0.22); however, segregation analysis did not provide statistical evidence of a major gene controlling fibrinogen level. These family data suggest that there are modest familial and genetic effects on the concentration of PAI-1.     

Genetic mapping of a major susceptibility locus for juvenile myoclonic epilepsy on chromosome 15q

The epilepsies are a group of disorders characterised by recurrent seizures caused by episodes of abnormal neuronal hyperexcitability involving the brain. Up to 60 million people are affected worldwide and genetic factors may contribute to the aetiology in up to 40% of patients. The most common human genetic epilepsies display a complex pattern of inheritance. These are categorised as idiopathic in the absence of detectable structural or metabolic abnormalities. Juvenile myoclonic epilepsy (JME) is a distinctive and common variety of familial idiopathic generalised epilepsy (IGE) with a prevalence of 0.5-1.0 per 1000 and a ratio of sibling risk to population prevalence (lambda(s)) of 42. The molecular genetic basis of these familial idiopathic epilepsies is entirely unknown, but a mutation in the gene CHRNA4, encoding the alpha4 subunit of the neuronal nicotinic acetylcholine receptor (nAChR), was recently identified in a rare Mendelian variety of idiopathic epilepsy. Chromosomal regions harbouring genes for nAChR subunits were therefore tested for linkage to the JME trait in 34 pedigrees. Significant evidence for linkage with heterogeneity was found to polymorphic loci encompassing the region in which the gene encoding the alpha7 subunit of nAChR (CHRNA7) maps on chromosome 15q14 (HLOD = 4.4 at alpha = 0.65; Z(all) = 2.94, P = 0.0005). This major locus contributes to genetic susceptibility to JME in a majority of the families studied.     

Quantitative models of the genetic transmission of schizophrenia.

Reviews research that has focused on specifying the genetic mechanisms involved in the familial transmission of vulnerability to schizophrenia. The importance of specifying the mechanism and methodological issues involved in genetic modeling are discussed. Models of genetic transmission are described and analyzed, including single major locus models, polygenic models (such as limited loci polygenic models and multifactorial polygenic models), and mixed models. Findings provide little support for the mechanism of single major locus inheritance. Although a mechanism involving 2, 3, or 4 loci cannot be ruled out, there is no compelling support for such models. The multifactorial polygenic model has received the most support and indicates that genetic factors play a greater role than cultural factors in familial transmission. At present, results neither support nor refute a mixed genetic model including both a multifactorial component and a single major locus. Implications of genetic heterogeneity and methodological deficiencies that limit the interpretability of these studies are discussed. (79 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Human malaria: segregation analysis of blood infection levels in a suburban area and a rural area in Burkina Faso

The genetic control of blood infection levels in human malaria remains unclear. Case control studies have not demonstrated a strong association between candidate genes and blood parasite densities as opposed to surveys that have focused on severe malaria. As an alternative approach, we used segregation analyses to determine the genetic control of blood parasitemia. We surveyed 509 residents (53 pedigrees) in a rural area and 389 residents (41 pedigrees) in an urban area during 18 months. Each family was visited 20 times and 28 times in the urban area and in the rural area; the mean number of parasitemia measurements per subject was 12.1 in the town and 14.9 in the village. The intensity of transmission of Plasmodium falciparum was 8-fold higher in the rural area than in the urban area. Using the class D regressive model for both populations, we found that blood parasite densities were correlated between sibs. We obtained strong evidence for a major effect, but we found that the transmission of this major effect was not compatible with a simple Mendelian model, suggesting a more complex mode of inheritance. Moreover, there was a strong interaction between major effect and age, suggesting that the influence of the putative major gene may be more prominent in children than in adults. Further nonparametric linkage studies, such as sib pair analysis, that focus on children would help us better understand the genetic control of blood infection levels.