Genetic disorders

  • Created by: amh4
  • Created on: 15-02-17 21:47

Inborn errors of metabolism

Group of rare genetic disorders, characterised by the bodies inability to metabolise food components normally. Defects in enzymes involved in biochemical pathways producing a metabolic block which may have pathologic consequences at birth or in later life.

Most are autosomal recessive, some X linked can be mitochondrial inheritance. 

Diagnosis mainly by biochemical tests - mass spectrometry by a tandem-mass spectrometer. 

Mass spectrometry - sample enters, ionisation and adsorption of excess energy, fragmentation, mass analysis, detection. 

3 types. Silent disorders - not like threatening but untreated could lead to brain damage and developmental disabilites. Example PKU. Acute metabolic crisis- life threatening in infancy, children are protected in utero. Example Urea cycle disorders. Neurological deterioration results in death by adolescene, example Tay Sachs. 

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Autosomal recessive. 

Babies born with PKU unable to break down the amino acid phenylalanine, untreated they develop serious, ireversible mental disability, build up of phenylanaline toxic to the brain. Treatment is a strictly controlled diet to prevent disability by 21 days of age. Treatment is with a diet low in foods that contain phenylalanine and special supplements

It is due to mutations in the PAH gene which results in low levels of the enzyme phenylalanine hydroxylase. This results in the build up of dietary phenylalanine to potentially toxic levels

Untreated PKU can lead to intellectual disabilityseizures, behavioral problems, and mental disorder.

Newborn screening for PKU - guthrie spot test then analysis by mass spectrometry. 

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Medium Chain acyl-CoA dehyrogenase MCAD

Autsomal recessive. 1 in 50 carrier rate. 

Babies cannot break down fat easily to make energy for the body.

25% mortility rate, mean age at first presentation is 14 months.

Treatment to avoid metabolic crisis - avoid fasting and monitor frequency of meals. Calories, milk non diet drinks and chocolate. 

MCAD is an enzyme that breaks down fatty acids in the mitochondria to generate energy. Crucial in periods of fasting. 

Vomiting, seizures, coma, cot death

Biochemical hallmark:hypoketotic hypoglycaemia. 

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Huntington Disease Symptoms

Mean age 40 years old. Neurodegenerative AD disease. 

Early symptoms - irritabilty, clumsiness, anxiety, apathy, depression.

Middle symptomss - involuntary movements (chorea), poor balance, speech difficulties, memory loss

Late symptoms - rigidity, unable to walk, unable to speak, dementia 

Lifespan usually only 15-20 years after onset of symptoms. 

Juvenile HD - onset before age 20, much quicker onset of late stage symptoms. Seizures are common. Very long repeat inserts (>50 CAG repeats)

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Genetics of Huntington Disease

HTT Huntingtin gene, codes for protein huntingtin codes for a series CAG trinucleotide repeats (glutamine) in exon 1.  A series of them results in the production of a chain of glutamine known as a polyglutamine tract. A mutated HTT gene results in an expanded CAG repeat, which when longer than 36 repeats folds abnormally and accumulates in neurons in the brain and is toxic. Toxic 'gain of function'.

<27 CAG normal allele

27-35 CAG Intermediate alleles

36-39 CAG reduced penetrance alleles

40+ definite HD

Huntingtin protein - highest expression in brain and testes, essential function in neuronal maintenance and activity, can regulate gene transcription. 

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Mitochondrial Disorders

Mitochondrial disorders may be caused by mutations, acquired or inherited, in mitochondrial DNA (mtDNA) or in nuclear genes that code for mitochondrial components. 

Mitochondria are the energy-transducing organelles of eukaryotic cells in which fuels to drive cellular metabolism are converted into ATP through the process of oxidative phosphorylation and have a central role in metabolism.

The mammalian mitochondrial genome is a closed-circular, double-stranded DNA molecule of about 16.6 kb. It has genes for two rRNAs, 14 tRNAs, and 12 polypeptides. It has a higher mutation rate that nucleur DNA. There are hundreds of genomes in each mitochondrial. 

In sexual reproduction, mitochondria are inherited exclusively from the mother; the mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. Mechanisms for this include simple dilution (an egg contains on average 200,000 mtDNA molecules, whereas a healthy human sperm was reported to contain on average 5 molecules. 

Individuals with mitochondrial disorders resulting from mutation of mtDNA may harbor a mixture of mutated and wild-type mtDNA within each cell (heteroplasmy).

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Heteroplasmy in mtDNA

Single-cell studies and cybrid-cell studies have shown that the proportion of mutated mtDNA must exceed a critical threshold level before a cell expresses a biochemical abnormality of the mitochondrial respiratory chain (the threshold effect). The percentage level of mutated mtDNA may vary among individuals within the same family, and also among organs and tissues within the same individual. This is one explanation for the varied clinical phenotype seen in individuals with disorders caused by mutation of mtDNA.

Generally effects the neurological system, eyes, muscle, liver heart, endocrine system due to high energy demands and therefore the tissue is rich in mitochondria. 

Clinical features: progressive multisystem involvement, CNS involvement almost always in late stage of disease. Usually symptoms worsen with time, family history seen. Very variable time of presentation. 

Treatment - exercise, vitamins. 

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Leigh Syndrome + Kearns Sayre Syndrome

Leigh Syndrome - Onset by age of 2 years. Presentation is non specific. Poor faltering growth, motor retardation, hyoptonia (floppy). Later signs - ventilatory distrubances, eye movement disorders, dystonia. 

Childhood neurodegenerative disease. Characteristic neuropathology.

Kearns-Sayre Syndrome - Onset <20 years. Progressive paralysis of eye muscles. Pigmentary retinopathy. Ataxia. Myopathy, heart block.

Other examples - Lebers Hereditary Optic neuroretinopathy or Barth Syndrome. 

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Transmission of mtDNA

Bottleneck effect.

A small number of mothers mitochondria selected randomly goes into each early egg cell, so variable percentages of mutant and wild type DNA can be passed down to children. This means there is variable severity in affected family members depending on the degree of heteroplasmy as well as varied clinical phenotype because of different mutation load in different organs. 

Up to 100% reuccurence risk. 

Oocyte donation - use cytoplasm from another woman, combine with mothers nucleur genome. 

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Haemoglobin is the oxygen binding protein of red blood cells, made of two different proteins. 2 alpha globin chains and 2 beta globin chains. Babies are born with fetal haemoglobin, 90% at birth and decreases to 1%. 

Haemoglobinopathies are conditions that affect the RBS and their capacity to carry oxygen around the body. 

Sickle cell anaemia, beta thalassaemia and alpha thalassaemia. All autosomal recessive. 

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Sickle Cell anaemia

Chromosome 11 HBB gene. Haemoglobin consists of two α polypeptide chains, two β polypeptide chains.

The gene defect is a known mutation of a single nucleotide (A to T) of the β-globin gene, which results in glutamic acid (E/Glu) being substituted by valine at position 6. This produces a new form of haemoglobin called HbS, which behaves very differently to regular haemoglobin (HbA). HbS causes the red blood cells to develop abnormally and become sickle-shaped (rather than the usual doughnut shape), harder and less flexible. 

Pain develops when sickle-shaped red blood cells block the flow of blood to the chest, abdomen and joints.Faster destruction of red blood cells (haemolysis)   due to the removal of the deformed cells. The resulting chronic anaemia is due to the rapid breakdown of red blood cells and not to iron deficiency as commonly believed. Sickle cells can be trapped in the spleen, jaundice, kidney, chest, risk of stroke, eye pain blurry vision, ulcers, blood in urine, painful joints, hearing loss. 

Heterozgote advantage - malaria. Treatment - hyrozyurea, blood transfusion, BM transplantation. 

Factors causing sickle crisis - reduced amounts of oxygen, dehydration, infection, temp change alchohol, smoking, stress, excessive physical exertion 

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Thalassaemia Syndromes

Alpha thalassaemia.1 abnormal alpha globin gene -no symptoms (silent). 2 abnormal alpha globin genes- smaller RBCs, mild anemia, no treatment. 3abnormal alpha globin genes - moderate to severe anaemia, transfusions. 4 abnormal globin genes - death before birth, in utero blood transfusions sometimes possible. 

Genes - HBA1 HBA2 chromosome 16

Beta thalassaemia. Minor - one abnormal betaglobin gene, minor anemaia. Major - two abnormal beta globin genes, severe bone deficiency. Intermedia - two abnormal but milder form, moderately severe anaemia.

Treatment - severe anemia, shortened life expentancy, regular transfusions needed, chelation therapy, bone marrow transplant.

Untreated thalassemia major - growth retardation, jaundice, enlarged spleen and liver, leg ulcers, skeletal deformaties, death before 20 years, damage to endocrine glands, delay in puberty, diabetes. 

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Cystic Fibrosis

AR. Carrier frequency 1 in 25.

Disorder of epithelial tissues, symptoms are persistant lung infections, meconium ilieus, failure to thrive, infertility, congenital absense of vas deference, echogenic bowel. 

.ΔF508-CFTR, occurs in >90% of patients, 1800 other mutations found. Phenotype genotype correlations. Characterised by build up of thick sticky mucus and dysfunction of several organs – Lungs, pancreas, sweat glands intestine

CFTR gene on chromosome 7. CFTR has 27 exons, encodes a protein found on the membrane of epithelial cells. Functions - Chloride ion channel transporting ions across cells. Mutant CFTR prevents ion movement leady to sticky mucus build up. 

Diagnosis - sweat test, lung function test, no cure, antibiotics. 

Newborn screening - measures immunoreactive trypsinogen in blood, trypsonogen is made in the pancreas but blockage of ducts prevents it reaching the pancreas so it raises in the blood. Higher in babies with CF - refer for genetic testing. 

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Fragile X syndrome

Triplet repeat expansion CGG in the 5’UTR of FMR1 gene on the X chromosome causes fragile X. FMR1 produces FMRP interacts with RNA- shuttles from nucleus to cytoplasm. FMRP important for synaptic function and normal neuronal development. No FMR1-mental retardation. X linked dominant. Females with full FMR1 mutations may have a milder phenotype than males due to variability in X-inactivation.

FMR1 related disorders: Premature ovarian failure, Fragile X associated tremor ataxia syndrome (FXTAS)

Symptoms: Moderate to severe developmental delay, social communication difficulties, Attention deficit hyperactive disorder (ADHD), autistic behaviours

Features-large ears and head, long face, macroorchidism, hard to recognise in children, more prominent with age, patients can be managed-special education, speech therapy

Normal 45 repeats

Intermediate (46-58) -stable transmission. Premutation (59-200)- unstable and expands when transmitted by a female

Full mutations (>200)- very unstable, in these individuals with a repeat expansion greater than 200, there is methylation of the CGG repeat expansion and FMR1 promoter, leading to the silencing of the FMR1 gene and a lack of its product.

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Replication Slippage

Replication slippage, otherwise known as slipped-strand mispairing, is a form of mutation that leads to either a trinucleotide or dinucleotide expansion or contraction during DNA replication. A slippage event normally occurs when a sequence of repetitive nucleotides (tandem repeats) are found at the site of replication. 

In the first step, DNA polymerase encounters the direct repeat during the replication process. The polymerase complex suspends replication and is temporarily released from the template strand. The newly synthesized strand then detaches from the template strand and pairs with another direct repeat upstream. DNA polymerase reassembles its position on the template strand and resumes normal replication, but during the course of reassembling, the polymerase complex backtracks and repeats the insertion of deoxyribonucleotides that were previously added. This results in some repeats found in the template strand being replicated twice into the daughter strand. This expands the replication region with newly inserted nucleotides. The template and the daughter strand can no longer pair correctly. Nucleotide excision repair proteins are mobilized to this area where one likely outcome is the expansion of nucleotides in the template strand while the other is the absence of nucleotides. Although trinucleotide contraction is possible, trinucleotide expansion occurs more frequently

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Fragile X inheritance

Expansion only in maternal alleles

Sherman paradox - the daughters of normal transmitting males are normal, the offspring of these women are often affected. 

While the location of this fragile site established that fragile X syndrome is indeed X-linked, inheritance of this disorder was clearly not typical of other X-linked disorders. Premutations must pass through females in order to transform into the full mutation.

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Duchenne Muscular Dystrophy/Beckers Muscular dystr

X linked recessive. 

Awkward manner of walking, stepping, or running – (patients tend to walk on their forefeet, because of an increased calf muscle tone. Also, toe walking is a compensatory adaptation to knee extensor weakness.) Frequent falls, Fatigue, Difficulty with motor skills (running, hopping, jumping). Muscle wasting begins in the legs and pelvis, then progresses to the muscles of the shoulders and neck, followed by loss of arm muscles and respiratory muscles. DMD is caused by a mutation of the dystrophin gene at locus Xp21, located on the short arm of the X chromosome. Dystrophin is responsible for connecting the cytoskeleton of each muscle fiber to the underlying basal lamina (extracellular matrix), through a protein complex containing many subunits. 

Becker muscular dystrophy is an X-linked recessive inherited disorder characterized by slowly progressive muscle weakness of the legs and pelvis. Less severe. 

This is essentially because the DMD mutations are frameshift deletions and the BMD ones are in-frame deletions. This results in no dystrophin in DMD and partially-functional dystrophin in BMD. As you might expect, there is a continuum of effects with severe BMD blending with mild DMD.

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X inactivation

X-inactivation (also called lyonization) is a process by which one of the copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by its being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin. Inactivation occurs on a cellular level, resulting in a mosaic expression, in which patches of cells have an inactive maternal X-chromosome, while other patches have an inactive paternal X-chromosome. For example, a female heterozygous for haemophilia (an X-linked disease) would have about half of her liver cells functioning properly, which is typically enough to ensure normal blood clotting.

Sequences at the X inactivation center (XIC), present on the X chromosome, control the silencing of the X chromosome. 

DNA packaged in heterochromatin, such as the Xi, is more condensed than DNA packaged in euchromatin, such as the Xa. The inactive X forms a discrete body within the nucleus called a Barr body. The Barr body is generally located on the periphery of the nucleus, is late replicating within the cell cycle, and, as it contains the Xi, contains heterochromatin modifications and the Xist RNA.

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Pseudoautosomal regions

The function of these pseudoautosomal regions is that they allow the X and Y chromosomes to pair and properly segregate during meiosis in males.

The pseudoautosomal regions, PAR1, PAR2,[1] and PAR3,[2] are homologous sequences of nucleotides on the X and Y chromosomes. 

The pseudoautosomal regions get their name because any genes within them (so far at least 29 have been found)[5] are inherited just like any autosomal genes.

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Turner Syndrome 45X

Short stature (~143cm) Cardiovascular malfomations (15-20%)

coarctation, VSD Renal anomalies Lymphodoema (new borns)

Gonadal dysgenesis absent 2o sex characteristics amenorrhoea

Infertility IQ 10-15 points below sibs Speech/language delay

Turner syndrome (TS), also known as 45,X or 45,X0, is a condition in which a female is partly or completely missing an X chromosome.[1] Signs and symptoms vary among those affected. Often, a short and webbed necklow-set ears, low hairline at the back of the neck, short stature, and swollen hands and feet are seen at birth. Typically, they only develop menstrual periods and breasts with hormone treatment, and are unable to have children without reproductive technologyHeart defectsdiabetes, and low thyroid hormone occur more frequently. Most people with TS have normal intelligence. Many, however, have troubles with spatial visualization that may be needed for mathematics.[2] Vision and hearing problems occur more often

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Kleinfelter Syndrome 47XXY

Verbal IQ ↓10-20 points, most in normal school, may require speech therapy

Passive behaviour Tall stature hypogonadism - testosterone replacement

Gynaecomastia ↑ breast cancer risk Infertility

Klinefelter syndrome (KS) also known as 47,XXY or XXY, is the set of symptoms that result from two or more X chromosomes in males.[1] The primary feature is sterility.[1] Often symptoms may be subtle and many people do not realize they are affected. Sometimes symptoms are more prominent and may include weaker muscles, greater height, poor coordination, less body hair, smaller genitalsbreast growth, and less interest in sex.[2] Often it is only at puberty that these symptoms are noticed.[3] Intelligence is usually normal; however, reading difficulties and problems with speech are more common. Symptoms are typically more severe if three or more X chromosomes are present.[2]

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