Heritability: Twin Studies. Twin studies are often used to assess genetic effects on variation in a trait

Transcription

1 TWINS AND GENETICS

2 TWINS

3 Heritability: Twin Studies Twin studies are often used to assess genetic effects on variation in a trait Comparing MZ/DZ twins can give evidence for genetic and/or environmental influences 3

4 Understanding Genetic and Environmental Influences Using Twin Studies Monozygotic Twins 100% genes 100% home environment

5 Understanding Genetic and Environmental Influences Using Twin Studies Monozygotic Twins 100% genes 100% home environment Dizygotic Twins 50% genes 100% home environment

6 Understanding Genetic and Environmental Influences Using Twin Studies Monozygotic Twins 100% genes 100% home environment Dizygotic Twins 50% genes 100% home environment We are a combination of our genes and environment.

7 Do both twins show the same characteristic or trait? Trait Concordance rate Monozygotic Dizygotic Height 95% 52% IQ 90% 60%

8 Determining the incidence of a disease in twins helps delineate whether there are genetic and environmental components Both genetic and environmental factors important Disease Concordance Identical (MZ) Non identical (DZ) Cleft lip and palate Rheumatoid arthritis Asthma Coronary artery disease Diabetes mellitus 38% 34% 47% 19% 56% 8% 7% 24% 9% 11%

9 MZ and DZ concordance rates

10 The contributions of genetic and environmental factors to human diseases Haemophilia Osteogenesis imperfecta Peptic ulcer Diabetes Duchenne muscular dystrophy Club foot Pyloric stenosis Dislocation of hip Tuberculosis GENETIC ENVIRONMENTAL Phenylketonuria Galactosaemia Spina bifida Ischaemic heart disease Ankylosing spondylitis Scurvy Rare Genetics simple Unifactorial High recurrence rate Common Genetics complex Multifactorial Low recurrence rate

11 What are Polygenic and Multifactorial Trait? Polygenic traits are controlled by two or more genes Multifactorial traits are polygenic with an environmental component Example: Height The Genes are inherited in Mendelian fashion Interaction of genes with environment produce many different phenotypes

12 Height as a Multifactorial Trait

13 Typical Polygenic Trait in a Population Bell curve Most individuals are clustered at ~average Few individuals at extremes of the phenotype

14 Variation in Height

15 Characteristics of Multifactorial Traits Several genes control trait Not inherited as dominant or recessive Genes controlling trait contribute a small amount to phenotype Environmental factors interact with genes to produce phenotype Many phenotypic differences in trait

16 How can the probability of recurrence be determined for multifactorial disorders? The recurrence risk for multifactorial disorders has to be determined through family studies Observe the number of affected siblings in many families (empiric risk)

17 Proband Siblings of proband Families with one child with multifactorial condition 50 brothers and sisters

18 Proband Siblings of proband Families with one child with multifactoria l condition 50 brothers and sisters 2 affected with multifactorial condition

19 Proband Siblings of proband 2/50 affected with multifactorial condition Families with one child with multifactoria l condition Therefore probability of recurrence = 1/25 (4%)

20 Multifactorial: cleftlip/palatet Examples include some cases of cleft lip and palate; neural tube defects; diabetes and hypertension Caused by a combination of genetic predisposition and environmental influences Pattern more affected people in family than expected from incidence in population but doesn t fit dominant, recessive or X linked inheritance patterns

21 Family studies of the incidence of cleft lip (± cleft palate) Anomaly Risk to sibs % Bilateral cleft lip and palate 5.7 Unilateral cleft lip and palate 4.2 Unilateral cleft lip alone 2.5 The more severe the manifestation of a multifactorial condition, the greater the probability of recurrence

22 Some multifactorial conditions have an unequal sex ratio Condition Sex ratio (males to females) Pyloric stenosis 5 to 1 Hirschprung disease 3 to 1 Congenital dislocation of hip 1 to 6 Talipes 2 to 1 Rheumatoid arthritis 1 to 3 Peptic ulcer 2 to 1 For some conditions there must be a different threshold for males and females

23 Frequency of pyloric stenosis in relatives Relationship Frequency % Increase on general population risk for same sex Male relatives of a male patient Female relatives of a male patient Male relatives of a female patient Female relatives of a female patient 5 x10 2 x20 17 x35 1 x70 Pyloric stenosis occurs more commonly in boys than in girls; for a female to be affected with pyloric stenosis, she must have a particularly strong genetic susceptibility

24

25 Multifactorial inheritance: Factors increasing probability of recurrence in a particular family Close relationship to proband High heritability of disorder (more than 1family member affected) Proband of more rarely affected sex Severe or early onset disease Multiple family members affected All these suggest that the family has a higher liability to the disorder genes of higher effect or more adverse environmental influences

26 Multifactorial inheritance: Recurrence risk decreases rapidly in more remotely related individuals

27 How evidence is gathered for genetic factors in complex diseases Familial risks (what is the incidence of a disorder in relatives compared with the incidence in the general population?) Twin studies (what is the incidence in monozygotic compared with dizygotic twins?) Adoption studies (what is the incidence in adopted children of the disorders which their parent had?) Population and Migration studies (what is the incidence in people from a particular ancestry group when they move to a different geographical area?) Evidence from these types of studies can estimate the heritability of a condition the proportion of the aetiology ascribed to genetic factors rather than environmental factors

28

29 In multifactorial disorders, there is a threshold, above which a person will develop the multifactorial disorder No.of individuals in population Threshold Liability

30 Multifactorial disorders practice points Common conditions Environmental influences act with a genetic predisposition Multiple genes with individually small risks often implicated One organ system affected + environment Recurrence risks: empiric figures obtained from population studies use in population from which obtained No.of affected individuals Threshold Liability (genetic and environmental factors)

31 Major effort to identify common disease susceptibility genes underway Risk factors for coronary artery disease Uncontrollable (but identifiable) Family history (genetics) Age Male sex Potentially controllable or treatable Fatty diet Hypertension Smoking High serum cholesterol Low serum HDL High serum LDL Stress Insufficient exercise Obesity Diabetes

32 Common diseases Congenital malformations Cleft lip/palate Congenital hip dislocation Congenital heart defects Neural tube defects Pyloric stenosis Talipes Adult onset disorders Diabetes mellitus Epilepsy Glaucoma Hypertension Ischaemic heart disease Manic depression Schizophrenia

33 Victor A. McKusick Inheritance of monogenic and complex (multifactorial) disorders. In monogenic diseases, mutations in a single gene are both necessary and sufficient to produce the clinical phenotype and to cause the disease. The impact of the gene on genetic risk for the disease is the same in all families. In complex disorders with multiple causes, variations in a number of genes encoding different proteins result in a genetic predisposition to a clinical phenotype. Pedigrees reveal no Mendelian inheritance pattern, and gene mutations are often neither sufficient nor necessary to explain the disease phenotype. Environment and life-style are major contributors to the pathogenesis of complex diseases. However, between families the impact of these same genes might be totally different. In one family, a rare gene C (Family 3) might have a large impact on genetic predisposition to a disease. However, because of its rarity in the general population, the overall population effect of this gene would be small. Some genes that predispose individuals to disease might have minuscule effects in some families (gene D, Family 3).

34 Is Intelligence a Multifactorial Trait? Head size was used to determine intelligence Early 20th century, psychological rather than physical methods Intelligence quotient (IQ) assumes that intelligence is a biological property Concordance in MZ twins raised together and apart indicates genetic and environmental factors

35 Quantitative Trait Loci (QTLs) Use information from Human Genome Project Associated with reading disability (developmental dyslexia) Genes on chromosomes 6, 15, and 4 for cognitive ability Accumulated results indicate intelligence is polygenic and multifactorial trait

36 IQ and the Bell Curve

37 Human Genome and SNPs Now that the human genome is (mostly) sequenced, attention turning to the evaluation of variation Alterations in DNA involving a single base pair are called single nucleotide polymorphisms, or SNPs Map of ~1.4 million SNPs (Feb 2001) It is estimated that ~60,000 SNPs occur within exons; 85% of exons within 5 kb of nearest SNP

38 Disease Marker Association A marker locus is associated with a disease if the distribution of genotypes at the marker locus in disease affected individuals differs from the distribution in the general population A specific allele may be positively associated (over represented in affecteds) or negatively associated (under represented)

39 Examples: Alzheimer s Alzheimer s disease and ApoE E4 present E4 absent Patients Controls The E4 allele appears to be positively associated with Alzheimer s disease: Odds Ratio = (58/16)/(33/55) = 6

40 VOLUME 43 NUMBER 10 OCTOBER 2011, page 1006 Nature Genetics

41 Figure 1 Regional association plots of the eight SNPs at seven loci showing genome-wide significant association (P < ) with pulse pressure (PP) and/or mean arterial pressure (MAP). (a h) Shown is the statistical significance of each SNP on the log 10 scale as a function of chromosome position (NCBI build 36) in the meta-analysis of stage 1 only. The sentinel SNP at each locus is shown in blue; the correlations (r2) of each of the surrounding SNPs to the sentinel SNP are shown in the colors indicated in the key. The fine-scale recombination rate is shown in blue. Gene positions are indicated at the bottom.

42 Association is not a specifically genetic phenomenon. It is simply a statistical statement about the co occurence of alleles or phenotypes. Allele A is associated with disease D if people who have D also have A significantly more often (or maybe less often) than would be predicted from the individual frequencies of D and A in the population. For example, HLA DR4 is found in 36% of the general UK population but in 78% of people with rheumatoid arthritis

43 Association has many different explanations not all of them are genetic: >> direct cause: if you have allele A you are susceptible to disease D To be a carrier of allele A is not necessary nor sufficient to develop D, but it increases the likelihood >> natural selection: people who have disease D might be more likely to survive and have children if they also have allele A >> population stratification: the general population contains several sub groups, and allele A is more frequent in one of them. HLA A1 is associated to the ability to eat with chopsticks in San Francisco HLA A1 is more frequent in Chinese, who are a large sub group in San Francisco.

44 Comparison of identity by state and identity by descent. Kok H S et al. Hum. Reprod. Update 2005;11: The Author Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please journals.permissions@oupjournals.org

45 Examples of identity by descent (IBD). Kok H S et al. Hum. Reprod. Update 2005;11: The Author Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please journals.permissions@oupjournals.org

46 Affected sib pair analysis

47