Autosomal Recessive Disorders

Question 1

Characteristics of Autosomal Recessive (AR) Disorders

Answer 1

a) Phenotype expressed only in people who have two mutant alleles of the same gene.
b) Both parents of an affected child are obligated carriers of the disease-causing allele(s).
c) Men and women are usually equally affected.
d) Horizontal pedigree (affected individuals are usually siblings).
e) Carriers are usually undetected, thus the birth of the first affected child is usually unexpected.
The recurrence risk is 1 in 4 (25%) for each unborn child of the same couple.
f) The probability of an unaffected sibling being a carrier is 2/3.
g) The majority of mutant allele(s) are present in carriers instead of patients.
h) Sometimes with a higher frequency within people of a small group (high-risk group).
i) Increased incidence of parental consanguinity for a child affected by a rare AR disorder.

Question 2

Allelic heterogeneity

Answer 2

• the existence of multiple mutant alleles of a single gene.

Question 3

Compound heterozygote

Answer 3

• one who carries two different mutant alleles of the same gene.

Question 4

Phenylketonuria (PKU) Phenotype (if untreated) (~4-7)

Answer 4

1) Microcephaly and profound mental retardation if untreated during infancy.
2) Neurobehavioral symptoms such as seizure, tremor, and gait disorders are common.
3) High phenylalanine and low tyrosine levels in the plasma because the conversion from Phe to Tyr is impaired.
4) High levels of phenylalanine metabolites in urine and sweat gives a characteristic “mousy” odor.

Question 5

Phenylketonuria (PKU) Frequency

Answer 5

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%).

Question 6

Phenylketonuria (PKU) Biochemical defects

Answer 6

PKU is an inborn defect of phenylalanine metabolism. Most PKU cases are caused by defects in the PAH gene encoding phenylalanine hydroxylase, a liver enzyme 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.

Question 7

Phenylketonuria (PKU) Genetic basis (chromosome location, allele frequency/variety)

Answer 7

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.

Question 8

Phenylketonuria (PKU) Newborn screening

Answer 8

Mass spec. High [Phe]/[Tyr] ratio is a red flag. 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.
[Guthrie test–> + growth of B. subtilis is a + result b/c Phe overcomes levels of thienylalanine]

Question 9

Phenylketonuria (PKU) Treatment

Answer 9

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, low-phenylalanine diet, and supplements (L-dopa and 5-hydroxytryptophan etc) to balance neurotransmitter levels.

Question 10

Maternal PKU

Answer 10

Irrespective of child’s genotype (most likely Rr), must keep mom on low Phe diet b/c of high rate birth defects (Phe levels in mom’s blood overwhelms baby enzymes causing toxicity)

Question 11

α1-Antitrypsin Deficiency (ATD) Phenotypes

Answer 11

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 α1-AT mutant protein in the liver.

Question 12

α1-Antitrypsin Deficiency (ATD) Frequency

Answer 12

α1-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.

Question 13

Mutant ATD alleles

Answer 13

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 α1-AT activity and account for most cases of the disease. Individuals with S/S genotype have 50-60% of normal α1-AT activity and usually do not express disease symptoms. Z/S compound heterozygotes have 30-35 % of normal α1-AT activity and may develop emphysema.

Question 14

α1-Antitrypsin Deficiency (ATD) Biochemical defects

Answer 14

α1-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.

Question 15

α1-Antitrypsin Deficiency (ATD) Genetic/chromosomal basis

Answer 15

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.

Question 16

α1-Antitrypsin Deficiency (ATD) Screening

Answer 16

Sequence specific oligonucleotide probes can be used to distinguish the M, Z and S alleles in a target population and provide accurate prenatal diagnosis.

Question 17

α1-Antitrypsin Deficiency (ATD) Environmental factors (Ecogenetics)

Answer 17

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.

Question 18

α1-Antitrypsin Deficiency (ATD) Treatment

Answer 18

Two approaches of delivering human SERPINA1 to the pulmonary epithelium are being studied: intravenous infusion and aerosol inhalation.

Question 19

Tay-Sachs disease (GM2 gangliosidosis type I) Phenotypes (3 early, 5 late) + special

Answer 19

Neurodegenerative disorder. T-S infants appear normal until the age of 3-6 months, when 
Early symptoms:
1) muscle weakness, 
2) decreased attentiveness
3) increased startle response appear. 

As the disease progresses, the T-S children experience symptoms of neurodegeneration:

1) seizures,
2) vision loss
3) hearing loss,
4) diminishing mental function
5) 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.

Question 20

Tay-Sachs disease frequency

Answer 20

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.

Question 21

Tay-Sachs disease Biochemical defects

Answer 21

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. A defective hexosaminidase A (HexA) needed for in metabolizing GM2 is responsible for T-S. HexA is a heterodimer of αβ, 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. Note: HexA and HexB refer to the two enzymes; HEXA and HEXB refer to two genes encoding the α and β subunit, respectively.

Question 22

Sandhoff disease (GM2 gangliosidosis type II)

Answer 22

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 ββ. T-S is caused by a defective α subunit; only HexA activity is affected. Sandhoff disease is caused by a defective β subunit; both HexA and HexB activities are affected. The α subunit gene HEXA and the β subunit gene HEXB reside on chromosomes #15 and #5, respectively.

Question 23

AB-variant of Tay-Sachs is:

Answer 23

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 (α subunit) within the cell.

Question 24

Tay-Sachs disease chromosomal basis

Answer 24

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).

Question 25

T-S Screening

Answer 25

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.

Question 26

Other forms of T-S

Answer 26

Juvenile and adult-onset forms of T-S are very rare with milder symptoms appearing in childhood, adolescence, or adulthood; the patients have decreased HexA activity.