FACTORES QUE AFECTAN LAS PROPIEDADES DE LA CREMA

It is important to appreciate the different between somatic cell therapy and germ-line therapy.

• Somatic cell therapy means genetically altering specific body (or somatic) cells, such as trachea epithelial cells, bone marrow cells, pancreas cells, or whatever, in order to treat the disease. This therapy may treat the disease in the patient, but any genetic changes will not be passed on the offspring of the patient.

HGS Biology A-level notes NCM 8/09

• Germ-line therapy means genetically altering those cells (sperm cells, sperm precursor cells, ova, ova precursor cells, zygotes or early embryos) that will pass their genes down the “germ-line” to future generations. Alterations to any of these cells will affect every cell in the resulting human, and in all his or her descendants.

Germ-line therapy would be highly effective, but is also potentially dangerous (since the long-term effects of genetic alterations are not known), unethical (since it could easily lead to eugenics) and immoral (since it could involve altering and destroying human embryos). It is currently illegal in the UK and most other countries, and current research is focussing on somatic cell therapy only. All gene therapy trials in the UK must be approved by the Gene Therapy Advisory Committee (GTAC), a government body that reviews the medical and ethical grounds for a trial. Germ-line modification is allowed with animals, and indeed is the basis for producing GMOs.

HGS Biology A-level notes NCM 8/09

Genetic Screening and Counselling

There are around 25 000 genes in the human genome, and scientists have now identified and sequenced thousands of human genes. In many cases they also know exactly how the base sequence is different in the different versions (or alleles) of the genes that cause genetic diseases, susceptibility to disease or other clinically-important symptoms. This knowledge can be used to test, or screen, someone for the presence of these clinically-important alleles. The information gained from a screen can be used to select appropriate treatment at an early stage, even before symptoms appear. Even if a genetic disease is incurable, the screen may help in family planning and care of the patient.

Genetic screening is based on DNA hybridisation, and is similar to the Southern blot technique. Short lengths of DNA, 100-1000 nucleotides long, are synthesised with base sequences found only in the mutant alleles. These short lengths of DNA are called gene probes or hybridisation probes. Tiny quantities of these single-stranded gene probes are covalently fixed to a glass or silicon slide in a grid pattern. This slide is called a DNA microarray (or gene chip) and a single microarray a couple of centimetres long can hold 10 000 different gene probes.

The genetic screen procedure is:

1. A few cells are taken from a patient by biopsy.

2. DNA is extracted from the biopsy and if necessary amplified by PCR. 3. The DNA is labelled with a fluorescent chemical.

4. The DNA is denatured (i.e. separated into single strands) by heating or strong alkali.

5. The single-stranded DNA is added to the microarray and mixed for a few hours. DNA that has a complementary sequence to any of the probes fixed to the microarray will hybridise to the probes by complementary base pairing.

6. The microarray is washed with a buffer, washing away any loose fluorescent DNA that is not hybridised to the probes on the chip.

7. If the microarray is illuminated with ultra-violet light, any spots where the subject’s DNA has hybridised to a probe will fluoresce and can be seen, while the spots where the probes haven’t hybridised will remain dark. In practice the microarray is “read” with a laser, which illuminates each spot in turn, recording the amount of fluorescence in each case on a computer.

8. The locations of the fluorescent, “positive”, spots are matched with the name of that probe’s allele, giving a detailed genetic profile of the patient.

HGS Biology A-level notes NCM 8/09

Genetic screens can be carried out at different times for different reasons:

Adult screening is carried out to test adults for alleles of late-onset diseases (such as Huntington’s or breast cancer) before any symptoms appear. Adult screening is also used test for carriers of a genetic disease (such as cystic fibrosis). The sample is usually taken by a buccal smear – a scraping of cells from the inside of the cheek.

Newborn screening is carried out immediately after birth, when a blood sample is taken with a lancet from the baby’s heel. DNA is extracted from the baby’s cells and tested for alleles for conditions such as phenylketonuria and congenital hypothyroidism. These conditions benefit from being treated early in life.

Prenatal screening is carried out on fetal cells before birth. It is offered to parents when there is a risk that the fetus might have a serious genetic disability. Amniocentesis is used to collect fetal skin cells in the amniotic fluid between 14 to 20 weeks of gestation, and the DNA from these cells is tested for alleles for conditions such as Tay Sachs disease, sickle cell anaemia, thalassemia, cystic fibrosis, and fragile x syndrome. If the tests are positive the parents have the option of terminating the pregnancy or preparing themselves to care for a disabled child.

Pre-implantation screening (or preimplantation genetic diagnosis, PGD) is performed on human embryos created by in vitro fertilization. One or two cells are carefully removed from an 8-cell embryo and the DNA is tested. This procedure doesn’t harm the rest of the embryo, which continues to develop normally. Only embryos without genetic problems are used for implantation, so this avoids the need for terminations associated with prenatal screening.