Final Review Sheet

From Iusmgenetics

Contents 1 Molecular and Biochemical Genetics 1.1 Objectives 1.2 Dominant Diseases 1.3 Recessive Diseases 1.4 Newborn Screening 1.5 General Molecular / Biochemical Pathogenesis Principles 2 Achondroplasia 2.1 General background information 2.2 Mode of inheritance 2.3 Single important gene 2.4 Etiology 2.5 Pathogenesis 2.6 Phenotypic information 2.7 Diagnosis 2.8 Treatment 2.8.1 Therapies to increase stature 2.9 5 important facts 2.10 Related Diseases / Not to be confused with 2.11 Questions and answers

Molecular and Biochemical Genetics

Objectives

• Important terms:
  • "Incomplete" dominance or "semi-dominant": homozygous individuals have a worse manifestation than heterozygous individuals (achondroplasia).
  • "Distinct disorder": consistent clinical and radiological findings.

• For individual diseases, know: clinical features, mode of inheritance, genes involved / gene defect, pathogenesis, treatment (sometimes)

Dominant Diseases

• Dominant disease are defined as those manifested when only one allele is mutated.
  • Recall that some diseases can be both dominant and negative because of allelic heterogeneity and locus heterogeneity.

Recessive Diseases

• Requires two mutant alleles to show the phenotype or disease state.

Newborn Screening

• The following criteria for newborn screening assure that our screening has analytic validity, clinical validity, and clinical utility:
  • The disorder must be well defined.
  • The disorder must be fairly high in population frequency (to justify the cost of newborn screening by the cost saved in care for the true-positives).
  • The disorder must be poorly clinically detected early in life (assymptomatic) (otherwise it is more cost effective to let physicians identify the disease at newborn checkups).
  • The disorder must be significant in morbidity / mortality if left untreated (otherwise we might start treating things that have little consequence).
  • The disorder must be treatable such that there is an improved condition (lest we start adding anxiety to insult).
  • The test must be rapid, inexpensive, specific AND sensitive over an entire population.
  • The test must be acceptable and cost-effective.
  • The test must be appropriately administered.

• In Indiana we screen for:
  • Metabolic disorders:
    • Phenylketonuria
    • Galactosemia
    • Maple Syrup Urine Disease
    • Homocysteinuria
    • Biotinidase Deficiency
  • Endocrine disorders:
    • Hypothyroidism
    • Congenital Adrenal Hyperplasia
  • Hemoglobinopathies:
    • Sickle Cell Disease
  • Cystic Fibrosis
• Using several methods: enzyme assays, radiommunoassays, electrophoresis, tandem mass spectrometry

General Molecular / Biochemical Pathogenesis Principles

• Dominant diseases usually result from:
  • Gain of abnormal function
  • Haploinsufficiency
  • Dominant-negative effects (think multi-subunit proteins)
• Recall that dominant disease can be "incomplete" and thus have more severe phenotypes when presented as homozygous conditions.
• Recessive diseases usually result from:
  • Loss of normal function
• Heterozygotes of recessive diseases (carriers) usually have enough wild-type gene product to function properly.

Achondroplasia

General background information

• Most common type of short limbed dwarfism
• Affects all ethnic groups equally
• 1912 - new cases were more often last-born than first-born children
  • Correlates with advanced paternal age having an influence.
  • The mutatnt FGFR3 sperm have a selective advantage over wildtype.
• 1994 - a group of scientists led by Dr. John Wasmuth discovered mutation of the receptor fibroblast growth factor receptor 3 (FGFR3) gene on human chromosome 4

Mode of inheritance

• Autosomal dominant, complete penetrance
  • It is considered "incomplete" dominance or "semi-dominant" because homozygous individuals have a worse manifestation than heterozygous individuals.
• No locus heterogenity and very little allelic heterogeneity (G1138A, G1138C; G380R; FGFR3)
• It is a distinct disorder because it has consistent clinical and radiological findings.
• 80% of cases are de novo (new mutations)
• Remaining cases due to inheritance of an altered FGFR3 gene from an affected parent(s)
• Only known risk is advanced paternal age (over 35 years)
• 100% penetrance; no modifying loci

• http://ideaforhealth.blogspot.com/2009/11/achondroplasia-causes-symptoms.html

Single important gene

• FGFR3 gene
• Chromosome 4p16.3
• 4.4-kb cDNA
• 2520 nucleotides
• 840-residue protein

Etiology

• Single gene disorder
• Mutations (missense) in FGFR3 gene on chromosome 4p16.3
• FGFR3 negatively regulates bone growth
• FGFR3 gene encodes the fibroblast growth factor receptor number 3 which signals for the production of a protein that limits the formation of bone from cartilage.
• FGFR3 mutations can be interpreted as gain of function mutations or constitutive activation
  • These are gain of function mutations that cause the receptor to be constitutively active
• Two mutations at nucleotide 1138 in the FGFR3 gene are responsible for Gly380Arg amino acid substitution
  • this mutation is in the transmembrane domain of the FGFR3 receptor
  • Recall that C->T is the most common single base mutation
  • note that this mutations does not occur in the wobble position but in the first position; "the code is not ambiguous but it is degenerate"

Pathogenesis

• Normally, ligand binds, cytosolic tyrosines on FGFR3 are phosed.
• Mutation causes ligand-independent activation which means the receptor is always active and always inhibiting chondrocytes and bone growth
• STAT signals inhibit chondrocyte proliferation
• MAPK signals negatively affect proliferation, terminal differentiation, and post-mitotic matrix synthesis via both the p38 and ERK pathways
• Increase in intrinsic receptor tyrosine kinase activity:
  • interfere with biosynthesis and transport of the receptor to the cell surface
  • delayed turnover of activated receptors leading to an increase in overall signal output
  • a defect in c-Cbl-mediated receptor ubiquitination that delays trafficking of mutant FGFR3 to lysosomes for degradation
  • SOC3 (suppressor of cytokine signaling 3) induced in response to exaggerated STAT1 signals might prolong the survival of mutant FGFR3

Phenotypic information

• Short stature with rhizomelic (proximal) shortening of the arms and legs and redundant skin folds on limbs
  • The proximal limb bones are shorter even than the already-short distal bones.
• Average height is about 4 feet, 2 inches.
• Short toes and fingers, as well as a trident configuration of the hands (third and fourth fingers are joined).
• Genu varum (bow legs)
• Lordosis and / or kyphosis
• Large head (macrocephaly) or abnormally large brain (megalencephaly)
  • Normal intelligence
• Characteristic facial features of frontal bossing (prominent forehead) and midface hypoplasia (depressed nasal bridge)

*Homozygous achondroplasia: die a few weeks to months after birth
**Generally due to respiratory failure.

• Heterozygous achondroplasia: delayed motor development but they have a normal life span and intelligence.
  • Average life expectancy decreased by 15 years
• Delayed motor development is due to spinal cord issues.
• Episodes where breathing slows or stops for short periods (apnea)
• Obesity
• Recurrent ear infections
• Lumbar spinal stenosis leads to compression of the spinal cord
• Hydrocephalus ("water on the brain")
• Normal fertility
  • Reduced reproduction because of social stigma
  • Many couples choose to adopt a child with short stature
• Non-affected parents should consider caesarean section when giving birth to a child with achondroplasia because of the large head size which is hard to get through the pelvis.
• Most women with achondroplasia need general rather than spinal or epidural anaesthesia to avoid problems related to spinal stenosis
• Women with achondroplasia also require caesarean section.

HERE 

Diagnosis

• Prenatal detection using ultrasound and/or DNA testing of amniocytes
• Affected fetuses recognized in third trimester of pregnancy
• Homozygous achondroplasia can be diagnosed prenatally, too.
• Newborns and children are generally diagnosed by bone measurement during a physical examination or by x-ray images
• DNA testing is rarely needed for diagnosis
• Diagnosis usually relies on PCR-RFLP method
• Melting curve analysis is a novel, accurate, rapid, and simple technique
• Technique can be applied to different single gene disorders
• G to C mutation could also be distinguished from wild type

Treatment

• Monitor height & weight, as well as head circumference for possible shunting
• Surgery to correct spinal stenosis

What is spinal stenosis?
*Narrowing of spinal column; puts pressure on the nerves and spinal cord.

• Significant spinal kyphosis may require a spinal fusion
• If bowing of legs interferes with walking, osteotomy may be performed

What is osteotomy?
*Break the legs and then allow them to heal in better anatomical position.

• Management of frequent middle-ear infections to prevent conductive hearing loss
• Speech evaluation by two years of age
• Careful monitoring of social adjustment

Therapies to increase stature

• Growth hormone therapy
  • Can increase limb length by up to 30 cm!
  • May have complications later in life.
• Surgical limb lengthening
  • Can add 15-30 cm to overall height.
  • Involves many surgeries.
  • Stretches nerves and vessels which may cause complications.
  • One must wear the device for a long time.
  • Breaking bones followed by slow stretching during the healing process using orthopaedic appliances
• Most clinical features are direct or indirect consequences of increased FGFR3 signaling on endochondral bone growth
• Possible future treatments include:
  • chemical inhibition of receptor signaling
  • antibody blockade of receptor activation
  • alteration of pathways that modulate the downstream propagation of FGFR3 signals

5 important facts

• Most common type of short limbed dwarfism
• Single gene disorder with gain of function mutation in the FGFR3 gene
• Autosomal dominant disease with 100% penetrance
• Homozygous achondroplasia - only live a few weeks to months; heterozygous achondroplasia live a normal life span
• High-resolution melting curve analysis is a novel, accurate, rapid, and simple technique for analyzing the human genome and detecting gene mutations
• http://www3.hi.is/nam/lifsam/Sameindadeild/Reynir/Acondroplasia.htm

Related Diseases / Not to be confused with

• Thanatophoric dysplasia
  • An excess production of FGFR3
  • Leads to perinatal death, severe bone deformation, excess skin folds, etc.
  • Thanatophoric dysplasia is similar to homozygous FGFR3.

• Hypochondroplasia is milder than achondroplasia.

• Don't need to memorize these:
  • SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans)
  • Muenke coronal craniosynostosis
  • Crouzon syndrome with acanthosis nigricans

• When looking at the gene fgfr3, there is a good amount of allelic variance that causes all these diseases (including achondroplasia).
  • There is a clear mapping between genotype and phenotype.
  • Achondroplasia mutations occur in the transmembrane domain.
  • There is an association between function / domain and disease.

Questions and answers

• http://howlifeismeasured.blogspot.com/2008/03/achondroplasia-facts.html
• What are five common phenotypes associated with achondroplasia?
• A man with achondroplasia marries a woman who does not have achondroplasia and they decide to start a family. Due to the myth, “little people only give birth to little people,” the man wants to adopt because he feels that he will pass on the altered FGFR3 gene to his child and cause the child to have achondroplasia. What would you tell this couple?