Single Gene Mutations and Inheritance II April 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 Recessive Probabilities Mother A a AA A Aa Father 25% affected 2/3 of unaffected are carriers a Aa 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 trait Mutation 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

How recessive conditions occur in population groups. 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

Hearing loss is the most common condition found at birth

Mutations in Connexin26 are a common cause of hearing loss Exon 1 Exon2 158 bp 681 bp Start codon 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 35delG 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

The sum of all alleles for a population is 1 Allele frequencies p = the frequency of allele 1 q = the frequency 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

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. Marie Sally 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 early in development (late blastocyst) XX

Lyonization XX XI IX X = active X I = inactive X X Inactivation

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 recessive What 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 X1 X1X2 Father Sons 50% normal 50% affected Y X1Y X2Y

Duchenne/Becker Muscular 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 dominant What does the pattern look like? Affected female Affected male Normal male Normal female Example with male survival

X-linked dominant, male lethal What 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 X1 X1X2 Father Sons 50% normal 50% affected Y X1Y X2Y

Incontinentia pigmenti X-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

Linear erosions on soles 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