- Chromosomes.

 Chromosomes

Each cell contains a nucleus within which there are several thread-like structures known as chromosomes, so called because of their affinity for certain stains. (Chroma = color   soma = body (

Chromosome: is DNA thread-like double-helix tightly coiled around histones.

Human somatic cells have 23 pairs of chromosomes (diploid number), with one copy of each chromosome inherited from each parent:

ü  22 pairs of chromosomes are autosomes;

ü  The remaining pair is called the sex chromosomes (XX in female or XY in male).

Human genetic material contains 3.1 billion bases.

Each germ cell contains 23 single chromosomes (haploid number):

ü  22 autosomes +

ü  1 sex chromosome ( x in ovum - Y or X in sperm)

 The non-dividing chromosome appears as thread-like structure which has:

·       Short arm (p) (for ‘petit’ or small)

·       constriction (centromere): localize genes and categorize chromosomes.

·       Long arm (q) (as next letter in the alphabet!)   .

Gene-based therapies

The treatment of most genetic disorders is based on conventional therapeutic approaches.

Gene therapy is an umbrella term for a number of techniques aimed at treating or preventing genetic disease.

The mechanisms of gene-based therapies include:

·       replacing the mutated copy of a gene with a functional copy, restoring protein function.

·       inactivating a mutated gene that has impaired function (‘knocking out’)

·       introduction of a new gene for the production of a beneficial protein.

There are a variety of types of gene therapy products, including:

1.   Plasmid DNA: Circular DNA molecules can be genetically engineered to carry therapeutic genes into human cells.

2.  Viral vectors: Viruses have a natural ability to deliver genetic material into cells, and therefore some gene therapy products are derived from viruses. Once viruses have been modified to remove their ability to cause infectious disease, these modified viruses can be used as vectors (vehicles) to carry therapeutic genes into human cells.

3.  Bacterial vectors: Bacteria can be modified to prevent them from causing infectious disease and then used as vectors (vehicles) to carry therapeutic genes into human tissues.

4.    Human gene editing technology: The goals of gene editing are to disrupt harmful genes or to repair mutated genes.

5.   Patient-derived cellular gene therapy products: Cells are removed from the patient, genetically modified (often using a viral vector) and then returned to the patient.

- Human DNA and RNA.

Human DNA

All nucleic acids are polynucleotides. A nucleotide consists of three components:

1.     A nitrogenous base: (Adenosine and guanine which are based on purine rings, Thymine and cytosine which are based on pyrimidine rings).

2.     A pentose sugar (deoxyribose in DNA and Ribose in RNA) at 3′ end

3.     1-3 phosphate groups at 5′ end

 There are two kinds of nucleic acids:

o   Deoxyribonucleic acid (DNA)

o   Ribonucleic acid (RNA).

•Two polynucleotide chains are linear but antiparallel (i.e. one chain runs in a 5' to 3' direction; the other runs 3' to 5').

•   The sugar and phosphate groups form the linear backbone of the strands with the bases projecting inwards towards their partners held together by hydrogen bonds between the opposing bases:

ü  Adenine (A) pairs only with thymidine (T) (2 hydrogen bonds)

ü  Guanine (G) pairs only with cytosine (C) (3 hydrogen bonds).

· Multiple fragments that, together with a protein skeleton (chromatin), form chromosomes.

· Site: Nucleus and Mitochondria.

- Indications of genetic testing!

Medical applications (indications of genetic testing, karyotyping, chromosomal culture, analysis) :

1.     Multiple congenital anomalies:

Birth defects > one developmental regions of the body

A.    Confirmation of a clinical diagnosis

B.    Estimate recurrence risk of future sibling

2.     Dysmorphic features or well-defined syndrome e.g down syndrome

3.     Unexplained Mental retardation (chromosomal abnormalities are responsible for 1/3  50% of MR which is due to genetic factor)   

4.     Pubertal failure

5.     Ambiguous genitalia

Provide diagnosis e.g turner, klinfilter

If Normal karyotyping → other endocrinal disorders

6.     Infertility: at least 5% of infertile men are killfilter.

7.     Recurrent spontaneous Abortions and miscarriage:

High anion gap metabolic acidosis mnemonic.

High anion gap metabolic acidosis mnemonic.

(RUSH Kill ME)

Renal tubular acidosis (distal), Rhabdomyolysis (massive)

Uremia

Salicylate ingestion

Hypovolemia, Heart failure

 

Ketoacidosis (diabetic, starvation)

Inborn error of metabolism, Iron, Isoniazid ingestion

Lactic acidosis

Late metabolic acidosis of prematurity

 

Mitochondrial diseases, Methanol ingestion

Ethanol, Ethylene glycol ingestion

-Predictive (Presymptomatic) Genetic testing in pediatrics .

 

Predictive (Presymptomatic) Genetic testing in pediatrics

Children may be referred because they are at increased risk of developing a genetic disorder in childhood or adult life (i.e the individual is clinically normal now).

n  If the condition is likely to manifest in childhood (e.g. Duchenne muscular dystrophy) or if there are useful medical interventions available in childhood (e.g. screening by colonoscopy for colorectal tumours in children at risk of familial adenomatosis polyposis coli), then genetic testing is appropriate in childhood.

n  If the child is at risk of a late-onset and untreatable disorder (e.g. Huntington disease), then deferring genetic testing until the child becomes an adult, or at least sufficiently mature to be actively involved in seeking the test and can make the decision for himself/herself, is usually preferred.

n  If the child is not at risk of developing the condition but may be a carrier at risk of transmitting the disorder to their future children, then there is also a good case for deferring testing until the young person can participate actively in the decision. There may be less at stake with these reproductive carrier tests than with predictive tests for untreatable disorders, but there are still good grounds for caution and for careful discussion before proceeding with such tests.

These difficult issues are often best handled through a process of genetic counselling supporting open and sustained communication within the family and especially between parents and children.

Predictive testing is not usually offered without a formal process of genetic counselling over more than one consultation with time built in for reflection. Written consent for predictive testing is required by most laboratories.

So, Presymptomatic testing of disorders which manifest in adult life should not be performed until the individual can consent on their own behalf unless there is clear clinical benefit from testing earlier.