Single Gene Mutations and Inheritance IIApril 4, 2008 Lisa Schimmenti, M.D.
Objectives • Understand autosomal recessive inheritance. • Know the concept of Lyonization. • Learn and understand the various forms of X linked inheritance. • Know how to calculate the risk of affected offspring given a family history and some facts about carrier frequency using the Hardy-Weinberg Calculation.
Autosomal Recessive pattern of inheritance • Males and females equally affected • Condition seen in sibs but not usually in other relatives • Risk of recurrence in sibs 25% • 2/3 of unaffected sibs are carriers • Consanguinity increases risk
Autosomal RecessiveProbabilities Mother A a AA Aa A Father 25% affected 2/3 of unaffected are carriers aa a Aa
Loss of function alleles • Likely when point mutations give the same phenotype as deletion of an allele • Inherited as recessive traits when 50% of expression is sufficient for normal phenotype • Loss of function and gain of function changes in the same gene will cause different diseases
Effect of consanguinity DD Dd Dd DD DD Dd Dd DD Dd DD dd Dd DD Consanguinity increases the risk of sharing a common ancestral mutation
Sweeping generalizations about genes inherited as AR traits • Transcription from one allele is sufficient • Severity dependent on nature and location of mutation • What we describe in inheritance pattern as homozygosity is USUALLY compound heterozygosity at molecular level. Allelic heterogeneity is COMMON
A common type of AR traitMutation in enzyme gene • Example: • degradation of mucopolysaccharides requires a series of lysosomal enzymes. • abnormalities in any of these enzymes can cause a similar phenotype: • coarse facies, enlarged organs, skeletal abnormalities
One MPS disease: a-L-iduronidase deficiency • Severe mutations = Hurler • Milder mutations = Scheie http://medgen.genetics.utah.edu/ http://www.mpssociety.ca/gallery/ReaganKnight.html Phenotype is dependent on nature of mutation in alleles
Genes in populations How recessive conditions occur in population groups.
Deafness • 1 in 1000 infants is born deaf. • Infants are screened for deafness/hearing loss at birth by automated hearing screening methods. • There are over 400 genes that cause hearing loss • Autosomal recessive mutations in GJB2, encoding Connexin 26, will be causative in nearly half of all deaf individuals. Photo credit: Josie Helmbrecht
Start codon Exon2 Exon 1 158 bp 681 bp Mutations in Connexin26 are a common cause of hearing loss Gene Protein Six connexons form a hemichannel in one cell membrane. When two cell membranes meet, a gap junction is formed.
Connexin 26 • Common deafness alleles • One method for determining carrier frequency • Screen hearing individuals for the presence of the gene mutation. • Remember, we have two of each gene • Carriers are heterozygotes • one wildtype and one mutant allele • 35delG • This is the most common allele in European populations. • How do you determine the carrier frequency without testing hundreds of individuals?
Hardy-Weinberg Law • A mathematical model for calculating allele and gene frequencies in a population • Assumes • Large population with random mating • Allele frequencies remain constant • no new mutations • no reproductive selection bias • no significant immigration for populations with different allele frequencies
Allele frequencies • p= the frequency of allele 1 • q = thefrequency of allele 2 The sum of all alleles for a population is 1 p + q =1
Hardy Weinberg Calculation • Binomial expansion (p+q)2 = p2 + 2pq +q2 p2 = the number of homozygous wildtype individuals in the population 2pq = the number heterozygous carriers in the population q2 = the number of homozygous affected individuals in the population
Using the Hardy Weinberg calculation to determine the carrier frequency in a population q2 = affected individuals in the population for Connexin 26 related deafness: q2 =1 in 2000 (an approximated number for this exercise) p2 @1 Solve for 2pq
Using the Hardy Weinberg calculation to determine the carrier frequency in a population q2 =1 in 2000 = 0.0005 q @ 0.02 2pq = 0.04 The carrier frequency is 0.04 or 1 in 25.
Hardy Weinberg calculations are used in clinic every day • Clinical situation: • A 23 year old hearing woman named Marie comes to your genetics clinic. Her family is of European descent Marie has a deaf sister, Sally, who has deafness caused by mutations in the gene encoding Connexin 26. Her sister's genotype is 35delG/35delG. • Marie would like to know her chance of having deaf children caused by mutations in Connexin 26 if the father of the child is of European descent.
How do you answer Marie's question? • What is Marie's chance of being a carrier of 35delG? • What is the chance that the father of Marie's child will be a carrier of 35delG? • What is the chance that Marie will have a child with hearing loss?
Draw a pedigree Sally Marie
Sally Marie What is Marie's chance of being a carrier of 35delG? • Marie's parents are obligate carriers. • Marie is unaffected. • The chance of being a carrier if unaffected is 2/3. Use a Punnett square if in doubt. Wt/35delG Wt/35delG 35delG/35delG
What is the chance that the father of Marie's child will be a carrier of 35delG? q2 =1 in 2000 = 0.0005 q @ 0.02 2pq = 0.04 The carrier frequency is 0.04 or 1 in 25. The father's chance of being a carrier is 1/25.
What is the chance that Marie will have a child with hearing loss? • The chance that an affected child will be born to a carrier couple is 1 in 4 with each pregnancy. • Calculation: • 2/3 x 1/25 x 1/4 = 1/150 Father of baby's chance of being a carrier Marie's chance of being a carrier
Inheritance patterns and genes • Single allele change gives disease • Genes on X (X - linked) • Genes on autosome (AD) • Mutation in both alleles required for disease • Genes on autosome (AR) • (rare) female homozygote for XL
X-linked patterns of inheritance • X-linked recessive • Primarily males affected • Females typically unaffected, but there are exceptions • X-linked dominant • Male surviving • Either sex affected, males more severely than females • Male lethal • Only females seen with disease, mosaic patterns of expression
X-linkage: already exceptions to the “rules” • The lines of dominant and recessive are blurred • X-linked conditions are sometimes called “semi-dominant” in women • Need dosage compensation
Lyonization • Only one X is active in each cell. All others are inactivated. • X inactivation is random • X inactivation is fixed • X inactivation occurs earlyindevelopment (late blastocyst) XX
XX XX XX XX XX XX XX XX XX XX X Inactivation IX IX XI IX IX X = active X I = inactive X XI XI IX IX XI Lyonization
Consequences of “Lyonization” • Females are mosaic for their X-linked gene manifestations • should be roughly equal • All are functionally hemizygous • dosage compensation: have the same number of copies as males • Inactive X may be evident in cells • The Barr body • Shifting from random distribution may result in manifestation of disease in females • skewed lyonization
X-linked recessive “rules” • Affects mainly males • Affected males are usually born to unaffected parents • Females can be affected if born to an affected father and carrier mother • Females can be affected if they have very skewed lyonization • No male to male transmission • Males born to carrier mother have 50% risk of inheriting altered gene
X-linked recessiveWhat does the pattern look like? Carrier female Affected male Normal male Normal female
X-linked recessive recurrence risks Mother X1 X2 Daughters 50% normal 50% carriers X1X1 X1X2 X1 Father Sons 50% normal 50% affected Y X1Y X2Y
Duchenne/BeckerMuscular Dystrophy • Mutations in dystrophin gene • Duchenne • Typically del/dup but frameshift • Severe • No protein product • Becker • Also del/dup in frame so milder • Shortened protein
Clinical manifestations of Duchenne MD Onset before age 5 Calf pseudohypertrophy 1/3500 males Elevated creatine kinase Severe muscle wasting Affects resp and cardiac mm Females 8-10% some weakness CK > 95%tile in 2/3 of carriers http://www.mdausa.org/publications/fa-md-9.html http://www.neuro.wustl.edu/neuromuscular/musdist/lg.html
Dystrophin gene and molecule Huge gene (2.3 Mb) 14kb transcript 3685 AAs High mutation rate (10-4) http://www.ncbi.nlm.nih.gov/cgi-bin/SCIENCE96/gene?DMD
Using dystrophin antibody normal Becker Duchenne B B n D D http://www.neuro.wustl.edu/neuromuscular/musdist/lg.html
X-linked dominant “rules” • Males and females both affected , but typically males are worse than females. • If male lethal, only affected females observed. • Affected males will only have affected daughters and unaffected sons. • No male -> male transmission seen
X-linked dominantWhat does the pattern look like? Affected female Affected male Normal male Normal female Example with male survival
X-linked dominant, male lethalWhat does the pattern look like? Affected female Normal male Normal female
X-linked dominant recurrence risks Mother X1 X2 Daughters 50% normal 50% affected X1X1 X1X2 X1 Father Sons 50% normal 50% affected Y X1Y X2Y
Incontinentia pigmentiX-linked dominant(male lethal condition) • Linear blisters in newborn girls • “Crops” followed by scarring • Small teeth • Eye abnormalities • Patchy hair loss • Only girls are affected • Males are typically not affected
Infant with IP Linear blisters on leg Linear erosions on soles http://dermatology.cdlib.org/DOJvol4num1/path/incont.html
Potentially confusing in X-linked pedigree analysis • Male lethal X-linked conditions • New mutations may be hard to recognize as X-linked • Sex-limited conditions may look X-linked • With carrier mother and affected father can see “male to male” inheritance