Autosomal Recessive Disease

Features of Autosomal Recessive 

• seen in a single generation (horizontal pattern)

• parents and children of affected people are normally unaffected

• males and females are equally affected and equally likely to transmit

• parents of affected children must be carriers

Recurrence Rate For 2 carrier parents is..

1/4

unaffected sibling of an affected sibling has?

2/3 of being a carrier

compound heterozygotes

• individual with 2 different alleles, both of which are defective

• severity depends upon how much residual function is retained

Heterozygote Advantage 

• the relatively high frequency of disease-associated alleles causing reduced fitness on homozygotes

• heterozygotes (Aa) have a greater fitness than either of the homozygotes (AA or aa)

Gene Therapy

the use of DNA as a pharmaceutical agent to treat disease

Somatic Gene Therapy

replace defective genes in somatic (adult) cells so cells can produce the missing gene product to alleviate or eliminate the symptoms of the disorder

germline gene therapy

corrections are made to the germ cells such that inheritable genetic alterations are prohibited

Introduction of Genes into Somatic Cells | Liposomes

• small lipid membrane microspheres containing DNA → merge with the cell membrane and deliver DNA

• some nucleic acid is randomly integrated and expressed to varying degrees

• Chemical Method

Introduction of Genes into Somatic Cells | Microprojectiles

• gold/ platinum coated in DNA

• mechanically 'shot' into cells to deliver DNA

• some nucleic acid randomly integrated and expressed to varying degress

• Mechanical Method

Introduction of Genes into Somatic Cells | Viral Vectors 

• by nature, viruses introduce DNA into cells and express their genes with high efficiency

• engineer viruses to insert and express normal genes over defective genes

3 approaches to get gene to right spot | Ex Vivo

1. the target cells are removed from the body

2. cultured in the laboratory with a vector

3. re-inserted into the body

Suitability for Cystic Fibrosis? -→ not good (lungs cells hard to ex vivo)

3 approaches to get gene to right spot | In Vivo

• the vector is introduced into the body, typically via the blood stream

• the vector homes in on the cells for which it was specifically designed to alter

Suitability For Cystic Fibrosis? → best

3 approaches to get gene to right spot | In Situ

• the affected tissue is directly exposed to gene therapy vector

• transfected cells are not removed from the body

Suitability for Cystic Fibrosis → good but not great cause CF is a multisystem disorder

how can an autosomal recessive child be produced by a couple with 1 carrier?

1. germline mosaicism

2. de novo during embryogenesis

PKU → Gene and Mutation

1. PAH gene

2. p.Arg408Trp

PKU → Effect on phenotype

• loss of phenylalanine hydroxylase (PAH) enzyme

• without PAH, phenylalanine accumulates in tissues

• the amount of enzyme is the same as for normal individuals, however, activity is reduced

Most people affect with PKU are? 

compound heterozygotes

Maternal effect of PKU

homozygous women with PKU have difficulties producing healthy children

significant risk of intellectual disability

Clinical Features (phenotype) Of PKU

• vomiting

• irritability

• eczema-like rash

• mousy odour to the urine

• increased muscle tone | Nervous System

• more active muscle tendon reflexes | Nervous System 

• microcephaly

• prominent cheek and upper jaw bones

• widely spaced teeth

• poor development of tooth enamel

• decreased body growth

• cognitive impairment

• seizures

no obvious phenotype in [zygotes] despite a [change in PAH]?

1. heterozygotes

2. ~50% reduction in PAH enzyme activit

PKU → Screening Test

newborn heel prick test (assess phenylalanine levels)

PKU → Treatment

maintain blood phenylalanine within normal levels (2-10 mg/dL)

• Strict Diet 

PKU → Litestyle Changes 

avoid high protein food

PKU → Quality Of Life

normal quality of life and life span if vigilant

Thalassemia → Gene and Mutation

1. HBA1/2 gene → α-thalassemia

2. HBB gene → β-thalassemia

Thalamessia → Affect of Phenotype

• incorrect or no synthesis of hemoglobin globin chains

  • α-thalassemia if α globin chain

  • β-thalassemia if β globin chain

Thalassemia → Gene Location

1. 4 α globin genes → 2 on each copy of chromosome 16

2. 2 β globin genes → 1 on each copy of chromosome 11

Note**remember that evens are with evens (2, 16) and odds are with odds (1, 11)

Thalassemia → normal structure of haemoglobin 

• hemoglobin is a tetramer of 2 α-like and 2 β-like globin subunits

• subunits change to suit the gas-carrying needs of the embryo, fetus, and adult

alpha - thalassemia | Different Types | Silent Carrier

• 1 of 4 α-globin genes mutated

• typically asymptomatic

alpha - thalassemia | Different Types | Minor

• 2 of 4 α-globin genes mutated

• mild anaemia

alpha - thalassemia | Different Types | HBH/ Hemoglobin H

• 3 of 4 α-globin genes mutated

• mild to moderate anaemia, enlarged spleen, jaundice

Treatment → transfusions may be needed during hemolytic or aplastic crises

alpha - thalassemia | Different Types | Hb Barts

• 4 of 4 α-globin genes mutated

• severe form where excess fluid builds up in the developing baby due to severe anaemia

• baby usually does not survive long after birth

Treatment → no effective treatment

beta - thalassemia | Different Types | minor 

• 1 of 2 β-globin genes mutated

• may have lifelong mild anaemia

beta - thalassemia | Different Types | intermediate

• 2 of 2 β-globin genes mutated

• mild to moderate anaemia

• slow growth and bone changes

• symptoms may appear in early childhood or later in life

Treatment → 

• splenectomy

• sporadic blood transfusions

• folic acid supplementation and iron chelation

beta - thalassemia | Different Types | Major

• 2 of 2 β-globin genes mutated

• children develop life-threatening anaemia within the first year of life

• failure to thrive, jaundice, enlarged spleen, bone changes and developmental delay

Treatment → 

• bone marrow transplantation

• cord blood transplantation

• regular transfusions correct the anemia, suppress erythropoiesis, and inhibit $\uparrow$ iron absorption

Thalamessia | More Info

Group of heterogeneous blood disorders where one of the globin
chains of hemoglobin has not been synthesized correctly.

1. B+= reduced β chain synthesis

2. B0 = NO β chain synthesis

Thalassemia → Screening Test

• hematologic testing of RBC indices

  1. peripheral blood smear

  2. supravital stain to detect RBC inclusion bodies

  3. electrophoresis

Thalassemia → Diagnostic Test

molecular genetic testing of HBA1/2 and HBB

Thalassemia → Hard to diagnose heterozygote

most have mild symptoms and undiagnosed due to incomplete dominance

Thalassemia → Gene therapy

haemoglobin switching - partner γ globin partners with α globin in place of defective β globin

Thalassemia → Effect on life expectancy 

mild forms of thalassemia do not shorten life span

thalassemia major babies are normal at birth but become anemic between 3-6 months, why is death delayed?

switch from γ to β globin subunits

Sickle Cell Anemia → gene and mutation

1. HBB (HbA → HbS)

2. p.Glu6Val (non-conservative)

Sickle Cell Anemia → Base changes of RNA

GAG → GUG

Sickle Cell Anemia → Base changes of DNA

CTC → CAC

Sickle Cell Anemia → Phenotype

fibre formation within RBCs → distorted RBC shape

Sickle Cell Anemia → Screening Test 

• blood screening for deformed erythrocytes using a microscope

• electrophoresis

• newborn heel prick test

Sickle Cell Anemia → Management 

based on crisis prevention

• Drink plenty of fluids (pain)-

• Pain medications -

• Blood transfusions

Outlook is good if routinely
checked.

How does it protect against malaria 

malarial parasites grow poorly in HbS/HbS and HbS/Hb+ RBCs so HbS alleles are maintained at high levels in populations exposed to malaria due to selective advantage

Why is Sickle Cell is a good example of compound heterozygotes

The relatively high frequency of disease-associated alleles causing reduced
“fitness” on homozygotes (eg. sickle-cell anaemia in Africans) has been
explained by assuming that the heterozygotes (Aa) have a greater fitness than
either of the homozygotes (AA or aa).

Haemochromatosis → Gene and Mutation

1. HFE gene

2. p.Cys282Tyr → most common in Caucasians (where it is homozygous) and accounts for the most cases of Hereditary Haemochromatosis

3. p.His63Asp

Hemochromatosis → Effect on Phenotype 

mutant HFE does not bind properly to the transcriptional regulation of Hepcidin (master iron regulatory hormone)

absorb all protein-associated iron from food → stored in body tissues

Hemochromatosis → Clinical Features

• joint pain (most common)

• fatigue

• bronze pigment

• abdominal pain

• increased libido

Hemochromatosis → Symptoms presenting

30-50 years (♀) and >50 (♂)

• It is later women because removal of iron through menstruation

• many show few symptoms before irreversible organ damage commences

Haemochromatosis → Screening Test

• iron content in blood (serum transferrin saturation)

• iron content in liver (serum ferritin)

Haemochromatosis → Treatment

regular phlebotomy

Cystic Fibrosis → Gene and Mutation

1. CFTR gene

2. p.Phe508del (ΔF508) (misfolded protein does not migrate to the cell membrane)

Cystic Fibrosis → Clinical Features | Pancreas

• glands becomes clogged → formation of cysts and eventually becomes fibrous

• decreased fat digestive enzymes → undernourishment and steatorrhea

Cystic Fibrosis → Clinical Features | Lungs 

• mucus is thicker and not cleared by cilia so it accumulates in the lungs

• persistent cough and recurrent lung infections

Cystic Fibrosis → Clinical Features |  Reproduction 

• infertility due to a blocked ductus deferens

Cystic Fibrosis → Clinical Features | Sweat Glands

• no re-uptake of secreted salt→ salty residue on the skin and a salt deficit in the body

Carriers vs affected incidents in Australia

• 1/25 people unknown carriers

• 1/2,500 babies (1 every 4 days)

Cystic Fibrosis → Screening Test

newborn heel prick test (assess [pancreatic immunoreactive trypsinogen (IRT)])

• pancreatic glands produce trypsinogen → transported to the small intestine and converted into trypsin but in CF mucus blocks pancreatic ducts that lead to the small intestine

Cystic Fibrosis → Screening Test (Carriers)

12-panel mutation screen for carriers

Cystic FIbrosis → Diagonistic Testing

sweat chloride test

Medication for CF

• antibiotics (treat and prevent infections)

• mucus-thinning drugs (expel mucus via coughing)

• bronchodilators (open airways)

• oral pancreatic enzymes (improve nutrient absorption)

Gene Therapies for CF | potentiators →ivacaftor

binds to CFTR channels to help them open (enhance CFTR activity)

Gene Therapies for CF | correctors → lumacaftor

helps/ corrects processing of CFTR proteins (gets it to the membrane)

CF → Treatment/ Lifestyle Changes

• organ transplant

• chest physical therapy

• pulmonary rehabilitation

• nebulisers every morning/ night

• psychological counselling

• nutrition/ exercise

What disease does CF protect heterozygosity protect from?

cholera (fewer chloride channels in intestinal cells to prevent water loss)

De Novo Mutations

New mutations that arise (not seen in either of an individuals parents). These
can occur in somatic cells or in gametes and as such can be passed onto the next
generation.

Guthrie Test AKA Heel Prick

A newborn baby screening test that screens for serious genetic conditions. Blood
from a neonatal heel prick is collected onto pre-printed cards known as Guthrie
cards