X-Linked Disorders Flashcards

1
Q

The HGP sequenced and identified about __ genes in each human

A

The HGP sequenced and identified about 20,500 genes in each human

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2
Q

Each chromosome comes in __, inherited from each __

A

Each chromosome comes in pairs, inherited from each parent

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3
Q

What is the essential first step in determining inheritance patterns ?

A

Investigate pedigree

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4
Q

Define pedigree

A
  • pictorial representation of a family medical history
  • tracks inheritance pattern of disease
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5
Q

A pedigree includes

A
  • entire maternal and paternal lineage
  • minimum of 3 generations
  • illness, defects, conditions of each family member
  • age (current, at diagnosis, at death)
  • miscarriages/ stillbirths
  • adoptions
  • ethnicity
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6
Q

Diffentiate genotype vs phenotype

A

Genotype:
- actual DNA sequence at a specific locus

Phenotype:
- physical manifestations of genotype (hair color, susceptibility to a disease)

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7
Q

Allele vs wild-type vs variant

A

Allele:
- different forms of a gene

Wild-type:
- most common allele in a population

Variant:
- permanent alteration in a DNA sequence (mutation)

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8
Q

List specific info in family history that may indicate X-linked genetic disorders

A
  1. Multiple affected males in the maternal side
  2. Especially neonatal/ child deaths
  3. Mildly affected females (sisters mothers, maternal aunts)
  4. No known risk factors
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9
Q

X-linked recessive inheritance

A
  • higher expression in males > females
  • heterozygous females usually do not have phenotype
  • X-linked disorders inherited from father to all daughters
  • never transmitted from father to sons
  • affected males within same family always related through females
  • significant proportion due to new/ de novo variants in a gene on X-chromosome
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10
Q

Characteristics of X-linked dominant inheritance

A
  • more commonly expressed in females > males
  • BUT females have milder phenotypic expressions of genetic disease
  • affected males will have normal sons and affected daughters
  • affected female have 50% risk of having children with genetic disease
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11
Q

What is XIST ?

A

X-inactive specific transcript:
- non-coding untranslated RNA
- major effector of X-inactivation process
- causes chromatin condensation and inactivation = Barr body
- epigenetics change = changes gene but does not involve a base change

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12
Q

Purpose of X-chromosome inactivation in females

A
  • compensates for dosage of X-linked genes in females (XX) vs males (XY) = both males and females only have one active X-chromosome
  • X-chromosome with variant is always silenced
  • cells that transcribe normal allele compensates enough gene products for deficient variant cells
  • deficient cells divide less efficiently, and are eventually overgrown by normal cells
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13
Q

T or F: Random X-chromosome inactivation is a normal, expected process

A

TRUE; Random X-chromosome inactivation is a normal, expected process

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14
Q

How does DNA methylation of X alleles evaluate X-chromosome Inactivation ?

A
  • Human androgen receptor assay (HUMARA)
  • Targets polymorphic short tandem repeat of the Xq-
    linked androgen receptor (AR) gene
  • Methylation status of the AR on inactive X
    chromosome correlates to the whole
    X chromosome
    inactivation

Since Paternal X and maternal X have 50% probability of being
methylated and inactivated
- 1:1 ratio for X chromosome inactivation is expected
- deviations from this theoretical ratio = skewed X inactivation

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15
Q

Describe Methylation-specific PCR

A

Two-step approach:

  1. PCR with primers specific for methylated vs unmethylated DNA
  2. Chemical modification of DNA with sodium bisulfite
    - treatment converts unmethylated cytosines into uracil = amplified as Thymine
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16
Q

How does Expressed Polymorphisms of X alleles evaluate X-chromosome Inactivation ?

A
  • not often used
  • not clinically useful for blood
17
Q

Why are males hemizygous ?

A

Males have a single X chromosome

18
Q

Phenotype of MECP2 disorder

A

Progressive neurodevelopmental disorder:
- First year is normal, followed by rapid regression of development
- Repetitive, stereotypic hand movements
- Fits of screaming and inconsolable crying
- Seizures
- Acquired microcephaly

19
Q

Describe MECP2 gene and describe pathogenesis of disorder

A

“Methyl-CpG binding protein 2 gene:”
- located on Xq28
- encodes Chromatin-associated protein that activates/ represses transcription

  • neurons cannot mature in MECP2 disorders
20
Q

How are MECP2 disorders diagnosed ?

A
  • Sequencing and deletion/duplication analysis of MECP2
  • 99% are de novo (a single occurrence in a family)
21
Q

Describe DMD gene and inheritance pattern

A
  • Located on Xp21.2 - Xp.21.1
  • encodes for Dystrophin protein (large muscle protein)
  • X-linked recessive: Hemizygous; MOSTLY IN MEN or heterozygous pathogenic variants (in females) result in dystrophinopathies
22
Q

Describe DMD phenotype

A

Duchene Muscular Dystrophy:
- Delayed motor capabilities; waddling gait and difficulty doing active moments
- Wheelchair dependent by 12 years of age
- Few survive beyond 30
- Respiratory complications and progressive cardiomyopathy being common causes of death

23
Q

Describe Becker Muscular Dystrophy phenotype

A
  • Later-onset skeletal muscle weakness
  • Mean age of death = mid-40s
  • Heart failure due to cardiomyopathy is the most common cause of death
24
Q

Diagnosis of DMD

A
  • characteristic clinical findings
  • elevated [CK]
  • ID of a hemizygous pathogenic variant in DMD gene
25
Describe ABCD1 gene and pathogenesis of disorder
- Located on **Xq28** - Encodes **ATP-binding cassette subfamily D member 1** - X-linked recessive: **Hemizygous** or **heterozygous** (females) with variant ABCD1 results in X-linked **adrenoleukodystrophy** - **Failure to transport fatty acids into peroxisome** results in **accumulation of long-chain fatty acids** - Affects the nervous system, adrenal cortex, and Leydig cells of the testes
26
Describe purpose of ATP-binding cassette subfamily D member 1 protein ?
- Member of the **ATP-binding cassette (ABC) protein transporter** family - **Transports certain fatty acids into peroxisome**
27
X-linked adrenoleukodystrophy in females
- **Adrenal function usually normal** - More than 20% of female (heterozygous) carriers develop **mild-to-moderate spastic paraparesis** in middle age or later Paraparesis: inability to move legs
28
X-linked adrenoleukodystrophy in males
- Impaired adrenocortical function - Manifests between **4 to 8 years** of age - Initially resembles attention-deficit disorder/ hyperactivity and **progressively leads to cerebral disability**
29
Diagnosis of X-linked adrenoleukodystrophy
- Suggestive clinical findings - **Elevated very long chain fatty acids (VLCFA)** - **Abnormal brain MRI** in boys - ID heterozygous ABCD1 pathogenic variant in girls
30
Describe F8 gene and inheritance pattern of disorder
- Located on **Xq28** - Encodes **coagulation factor VIII** - X-linked recessive: **Hemizygous** or **heterozygous** (female) pathogenic variants in F8 cause **hemophilia A**
31
What is FVIII ?
- Large plasma glycoprotein - Cofactor in blood coagulation cascade; activates FX - Binds von Willebrand factor during transport in blood
32
Describe Hemophilia A phenotype in males
- Delayed or recurrent bleeding after injury Severe hemophilia A: - During first 2 years of age - Spontaneous joint bleeds or deep-muscle hematomas - Prolonged bleeding & excessive pain from minor injuries Mild hemophilia A: - Diagnosed later in life - Prolonged bleeding occurs from surgery or tooth extractions
33
Hemophilia A in females
~ 30% of **heterozygous females** have **reduced FVIII clotting activity** and are at risk for bleeding - **Prolonged bleeding after major injuries**
34
Describe the FMR1 gene and pathogenesis for disorder
- Located on **Xq27.3** - Encodes **Fragile X messenger ribonucleoprotein 1;** - a selective **RNA binding protein** for polyribosomes in translation (regulation and mRNA stability) - central role in neuronal development - Expanded **CCG repeats (>200)** = **excessive methylation of cytosines in promoter** of FMR1 gene = **failure to produce protein**
35
Describe Fragile X syndrome phenotype
- **Most common heritable form of intellectual disability** - Neurodevelopment disorders - Seizures - Sleep disorders - Scoliosis Penetrance in females = 50-60% range NOTE: increases within generations
36
Describe SLC6A8 gene and inheritance pattern for disorder
- Located on **Xq28** - Encodes **Solute carrier family 6 member 8** - **13 exons** (8.5 kb of genomic DNA) - X-linked recessive: **Hemizygous** or **heterozygous** (female) pathogenic variants in SLC6A8 = **creatine transporter deficiency**
37
Describe SLC6A8 phenotype
- creatine transporter deficiency - Epilepsy - Developmental delay - Hyperactivity
38
SLC6A8 variant diagnosis
- Heterozygous missense variant (c.1067G>T ) = (p.Gly356Val) in SLC6A8 - 50% of normal creatine uptake in fibroblasts - Chromosome analysis 46, XX - No skewed X-inactivation in peripheral blood
39
What is SLC6A8 protein?
Solute carrier family 6 member 8: - **NaCl dependent** - **Transports creatine** in/ out of cells - For high energy requiring organs (brain and muscle retina)