Autosomal Recessive Disorders Flashcards
What is a compound heterozygote?
It is a person who carries two different mutant alleles of the same gene.
Phenylketonuria (PKU)
What is the Phenotype?
-Microcephaly and profound mental retardation if untreated during infancy.
- Neurobehavioral symptoms such as seizure, tremor, and gait disorders are common.
- High phenylalanine and low tyrosine levels in the plasma because the conversion from Phe to Tyr is impaired.
- High levels of _phenylalanine metaboli_tes in urine and sweat gives a characteristic “mousy” odor.
Frequency
Disease frequency is 1/10,000 births (q2 = 1/10,000, q = 1%) among individuals of Northern European ancestry. Carrier frequency is about 1/50 (2pq ≈ 2q = 2%).
Biochemical defects
PKU is an inborn defect of phenylalanine metabolism. Most PKU cases are caused by defects in the PAH gene encoding phenylalanine hydroxylase, an enzyme produced in the liver that catalyzes the conversion of Phe to Tyr using molecular oxygen and a cofactor tetrahydrobiopterin (BH4).
A small fraction of PKU patients (1~3%) have normal PAH but are defective in genes that are needed for the synthesis or regeneration of BH4, the cofactor of PAH. BH4 is also the cofactor for two other enzymes, tyrosine hydroxylase and tryptophan hydroxylase, both of which synthesize monoamine neurotransmitters.
The high phenylalanine level in PKU damages the developing central nervous system in early childhood and interferes with the function of the mature brain, although the mechanism of damage is unclear. BH4-deficient PKU patients have problems caused by both hyperphenylalaninemia and neurotransmitter imbalance.
What is the Molecular basis of PKU?
The PAH gene is at chromosome 12q22-24. Most mutations in PAH are partial or complete loss-of-function alleles. PAH gene exhibits high allelic heterogeneity; over 400 alleles have been identified. Most PKU patients are *compound heterozygotes (i.e. having two different mutant alleles of the PAH gene). Severity of phenotype varies and probably reflects compound heterozygosity.
**What is the recommended newborn screening process?** Guthrie test (aka Guthrie bacterial inhibition assay) for PKU was developed in the mid- 1960s. This test is based on the findings that thienylalanine inhibits the growth of the bacterium Bacillus subtilis and that such inhibition can be overcome by a high level of phenylalanine in the blood sample of a PKU baby.
Mass Spectrometry is the current method of choice for many newborn screenings including PKU. A mass spectrometer simultaneously sorts many molecules in a blood specimen by weight (mass) and size, and also measures the quantity of each molecule.
Timing of Test
The sensitivity of PKU screening is influenced by the age of the newborn when the blood sample is obtained. Phenylalanine level is typically normal in PKU babies at birth because of normal PAH in maternal supply and increases progressively with the initiation of protein feedings during the first days of life. Early detection and treatment is crucial to prevent irreversible damage to the developing brain. However, if tested too early (within 1-2 days of birth), some affected children can be missed. Newborns are tested first after birth and then again at their first pediatrician’s visit days later.
What is the current treatment for PKU?
When treated early with low-phenylalanine diet, the mental retardation can be prevented. Phenylalanine is an essential amino acid and thus cannot be eliminated from the diet.
The low-phenylalanine diet should be maintained throughout childhood and school years, and preferably the patient’s whole life.
BH4-deficient PKU patients are treated with oral BH4 (for example: Kuvan), low-phenylalanine diet, and supplements (L-dopa and 5-hydroxytryptophan etc) to balance neurotransmitter levels.
Maternal PKU
It is important to maintain PKU women on a low-phenylalanine diet throughout their child- bearing years. PKU mothers who are not on a low-phenylalanine diet have a markedly increased risk of miscarriage and giving birth to children with congenital malformations, mental retardation, and growth impairment, irrespective of the genotypes of the children. These defects are caused by elevated phenylalanine levels in the maternal circulation during fetal development and thus termed maternal PKU.
a1-Antitrypsin Deficiency (ATD)
What are the Phenotypes of ATD?
ATD patients have a 20-fold increased risk of developing emphysema, with more severe symptoms among smokers. This disorder is late-onset, especially in non-smokers, but 80- 90% of deficient individuals will eventually develop disease symptoms. Many patients also develop liver cirrhosis and have increased risk of liver carcinoma due to the accumulation of a misfolded a1-AT mutant protein in the liver.
Frequency
a1-antitrypsin deficiency is a common genetic disorder among Northern European Caucasians. Disease frequency is 1/2,500, carrier frequency ~1/25. The most common normal (wild-type) allele, the M allele, occurs with a frequency of 95%; thus 90% (0.952 =0.9025) of white Europeans have M/M genotype.
Most ATD diseases are associated with two mutant alleles, the Z and S alleles.
- Individuals with Z/Z genotype have only 10-15% of normal a1-AT activity and account for most cases of the disease.
- Individuals with S/S genotype have 50-60% of normal a1-AT activity and usually do not express disease symptoms. Z/S compound heterozygotes have 30-35 % of normal a1-AT activity and may develop emphysema.
- *What is the Biochemical defect?**
- *a1-antitrypsin** (ATT or SERPINA1) is made in the liver and secreted into plasma.
SERPINA1 is a member of serpins (serine protease inhibitor), which are suicide substrates that bind and inhibit specific serine proteases. The main target of SERPINA1 is elastase, which is released by neutrophils in the lung. When left unchecked, elastase can destroy the connective tissue proteins (particularly elastin) of the lung, causing alveolar wall damage and emphysema.
Molecular basis
The SERPINA1 gene is on chromosome 14 (14q32.13). There are ~20 different mutant alleles, although the Z & S alleles account for most of the disease cases.
- The Z allele (Glu342Lys) encodes a misfolded protein that aggregates in the endoplasmic reticulum (ER) of liver cells, causing damage to the liver in addition to the lung.
- The S allele (Glu264Val) expresses an unstable protein that is less effective.
What is thes screening process?
Sequence specific oligonucleotide probes can be used to distinguish the M, Z and S alleles in a target population and provide accurate prenatal diagnosis.
- *Environmental factors (Ecogenetics)**
- **Smoking** accelerates the onset of emphysema in ATD patients. Tobacco smoke damages the lung, prompting the body to send more neutrophils to the lung for protection. More neutrophils release more elastase, causing more severe lung damage.
Treatment
Two approaches of delivering human SERPINA1 to the pulmonary epithelium are being studied: intravenous infusion and aerosol inhalation.
Tay-Sachs disease (GM2 gangliosidosis type I)
What are the Phenotypes?
T-S is an inherited disorder that progressively destroys neurons in the brain and spinal cord (CNS).
The most common form of T-S is an early-onset, fatal disorder apparent in infancy. T-S infants appear normal until the age of 3-6 months, when early symptoms such as muscle weakness, decreased attentiveness, and **increased startle response appear. As the disease progresses, the T-S children experience symptoms of neurodegeneration including seizures, vision and hearing loss, diminishing mental function, and paralysis.
-An eye abnormality called “cherry-red spot” is a characteristic of T-S. Children of T-S usually live only till 3-4 years of age.
Frequency
The Ashkenazic Jewish population is at 100-fold higher risk for T-S (~1/3,600) than the general population (~1/360,000). Other high-risk groups for T-S are certain French- Canadian communities of Quebec, the Old Order Amish community in Pennsylvania, and the Cajun population of Louisiana.
What is the Biochemical defect?
T-S is a lysosomal storage disease. Inability to degrade **GM2 ganglioside results in up to 300-fold accumulation of this sphingolipid inside swollen lysosomes in neurons of the central nervous system.
This is due to a defective hexosaminidase A (HexA) needed for in metabolizing GM2 is responsible for T-S. HexA is a heterodimer of alpha and Beta subunits, which are encoded by the HEXA and HEXB genes, respectively. Although HexA is a ubiquitous enzyme, the impact of T-S is primarily in the brain where most of GM2 ganglioside is synthesized.
*Sandhoff disease (GM2 gangliosidosis type II) presents the same neurological symptoms as T-S. Sandhoff disease patients have defects in both Hexosaminidase A and Hexosaminidase B (HexB); Hex B is a homodimer of bb.
-It is important to distinguish that T-S is caused by a defective alpha subunit; only HexA activity is affected.
Sandhoff disease is caused by a defective Beta subunit; both HexA and HexB activities are affected (since both have it). The a subunit gene HEXA (chromosome 15) and the b subunit gene HEXB (chromosome 5).
What is an AB-variant of Tay-Sachs?
Is a rare form of T-S in which both HexA and HexB are normal but GM2 accumulates due to a defect in the GM2 activator protein (GM2-AP), which facilitates interaction between the lipid substrate and the HexA enzyme (a subunit) within the cell.
Note: HexA and HexB refer to the two enzymes; HEXA and HEXB refer to two genes encoding the a and b subunit, respectively.
Molecular basis
Over 100 HEXA mutations are known. The most common mutant allele (~80%) in the Ashkenazi Jewish population is a 4 bp insertion in exon 11 of HEXA, causing a frameshift and a premature stop codon in the coding sequence of the gene (i.e. it’s a null allele).
Screening
Enzymatic activity assay: both HexA and HexB enzymes are present in the serum. Their activities can be distinguished in such assays because only **HexA is inactivated by heat.
Carrier Screening: primarily among Ashkenazi Jewish population, the enzyme test has 97% accuracy because carriers have lower HexA enzyme levels in the blood.
Prenatal screening: the enzyme test can also be performed on cultured amniotic fluid cells to detect T-S fetus when both parents are known to be carriers. Notably, this screening has reduced the number of T-S cases by about 95% over the past 30 years.
DNA testing: The tests currently available can detect about 95% of carriers in the Ashkenazi Jewish population and about 60% of carriers among non-Jewish individuals. Therefore, some carriers will be missed by DNA test alone.
