Genetic Diseases and their treatment

Transcription

1 Genetic Diseases and their treatment A quick reminder DNA = deoxyribonucleic Acid, a double helix Strands linked via chemicals called nucleotide bases There are FOUR such bases Thymine, Adenosine, Guanine and Cytosine Thymine always links with Adenosine Guanine always links with Cytosine This is the genetic code CCGATTCGA Each set of 3 bases = codon

2 GENES Thousands of genes are collected together on CHROMOSOMES DNA strands in chromosome Humans have 23 pairs of chromosomes 22 pairs autosomal and one pair sex chromosomes (called X & Y because of shape) Genes represented by coloured bands in chromosome

3 GENES Part of a gene sequence could be...ccgattcga... This could mean... Mary had a little lamb A defect in the gene coding for the letter a (i.e. a POINT MUTATION) could result in Mary had a little limb Mary hid a little lamb Mary hid a little limb MAJOR change in output from apparently trivial error Each human cell has > 3 billion base pairs Although we have > 90 % genes in common with chimps and mice, a 1-2% difference = 10s millions combinations Genes are not necessarily contiguous sections of a chromosome. Coding bits (codons) are interspersed with non-functional DNA (so-called Junk DNA). Might provide protection against harmful mutations disputed. (humans = 1% of genome code for proteins)

4 GENES and INHERITANCE Mutations (errors in replication ; UV radiation etc) Single genes RARELY code for a single effect but they are much easier to study Between genes are codons that control START reading of a gene and STOP reading

5 INHERITANCE - Dominant and Recessive Genes Gene for eye colour Both copies code for BROWN eyes (homozygous) Both copies code for BLUE eyes (homozygous) BROWN eyes BLUE eyes BROWN, other copy codes BLUE One copy codes (heterzygous) BROWN eyes BROWN eye colour is DOMINANT

6 Genetic Diseases AUTOSOMAL RECESSIVE Have to have TWO aberrant genes i.e. be HOMOZYGOUS If only one mutated gene i.e. HETEROZYGOUS CARRIER AUTOSOMAL DOMINANT Only one mutated gene = HETEROZYGOUS Autosomal recessive inheritance X-LINKED RECESSIVE (Female is CARRIER but MALE is affected)

7 Autosomal RECESSIVE - Cystic Fibrosis Usually due to DELETION of 3 bases on gene 7 results in faulty protein. This results in secretions that should be thin being THICK (but ~ 300 mutations identified in CF) Signs &Symptoms Mucous build up in LUNGS (pancreas, liver, kidney, intestines) Breathing difficulties Repeated lung & sinus infections (prolonged antibiotic treatment) Poor food absorption Poor growth Treatment Most common in Northern Europeans (1 in 3000 births; 1 in 25 = carrier) Least common in African/Asians Life Expectancy yrs in developed world

8 Autosomal RECESSIVE - Sickle Cell Anaemia Distorted shape of Red Blood Cells Signs & Symptoms Impaired oxygen carrying Tendency to block blood vessels Pain Stroke Swollen hands & feet Infections In million people affected 43 million = CARRIERS 114,800 deaths TREATMENT 80% of cases in SubSaharan Africa Heterozygy confers resistance to malaria in early childhood

9 Autosomal DOMINANT Huntington's Disease (Chorea) Neurological disease Signs & Symptoms NONE (apart from occasional mood swings/unsociability) in 1st 30-40yrs ie until AFTER had children (hence inheritance) Twitches, jerks, spasms Objects hard to grasp Hands & legs uncontrollable writhing ( giant puppet show ) Finally deep cognitive decline with near complete loss motor function Malnourishment, dementia, infections - DEATH TREATMENT Woody Guthrie

10 Autosomal DOMINANT Marfan's Syndrome Connective Tissue disease Signs & Symptoms Defects in heart valves, lungs, eyes, bones & meninges People tend to be TALL, THIN with disproportionally LONG LEGS, TOES, ARMS,FINGERS Can have norm life span M and F equally affected 1 in 5000 (¾ = inherited' ¼ = spontaneous mutation) No ethnicity TREATMENT

11 X-linked RECESSIVE - Haemophilia Faulty blood clotting Queen Victoria's family tree inheritance of Haemophilia Signs & Symptoms Minor injuries (bruise) cause uncontrolled bleed under skin and into joint Pain Distortion of joint Death Treatment Haemophilia (due to missing clotting factor) is (usually) due to faulty, recessive gene on X chromosome so very rare in women (women have TWO X chromosomes) but they can be carriers (men rarely carriers as do not live long enough to have children)

12 X-linked RECESSIVE Duchenne's Muscular Dystrophy Progressive muscular weakness & wasting Signs & Symptoms Progressive leg weakness, baby boy using parents' legs to pull self up when learning to walk, not able to walk by 12 years Eventually all voluntary muscles affected, then heart and breathing later on Paralysed from neck down by 21st year ALSO Higher risk ADHD Learning Disorders (eg dyslexia) Short term memory impairment TREATMENT Life Expectancy ~ 25 years 1 in 7000 live births

13 Genetic therapies AIM Deliver functioning genes to adult (ie already affected) cells Deliver functioning genes to embryos then implant Delete aberrant genes from (preimplant) embryonic / foetal cells OR adult cells PROBLEM How to get editing system to target How to change ALL affected cells How to ensure correction is permanent (heritable) not just adding a functional gene

14 Genetic therapies DELIVERY SYSTEMS Direct injection into organ (eg functioning gene into lungs into CF patients) or into fertilised ovum (before 1st division) Nanoparticles (ie 1-100nm) silica, gold (injected into muscle of mice with MD, only 5% muscles repaired but enough to improve muscle strength) Fatty particles (used to treat hepatitis B in mice Viruses Bacteria Own Cells white blood cells (leukaemia) T- cells (cancer) red blood cells (sickle cell anaemia) Stem cells (vast potential applications) MANIPULATION SYSTEMS CRISPR cas9 Base editing (older systems (eg zinc-finger nucleases & TALENs, seem redundant now)

15 Side Step... Viruses Average virus is 1/100th size of average bacteria Packet of DNA or RNA in protein coat = VIRION Invade cells Commandeer cell's chemical machinery to keep virus going & replicate virus Viruses readily mutate in host cells Provide horizontal gene transfer ie movement of genetic material between unicellular and/or multicellular organisms (vertical is parent to offspring) Bacteria One celled organism lacking nucleus or membrane bound cellular organelles VIRUS particle = virion DNA or RNA strands in core Capsid = protein coat, sometimes enveloped in lipid

16 Gene Editing CRISPR Part of bacterial (Streptococcus pyogenes) defence against viruses Exposure to virus means bacterium copies a bit of viral DNA so it can recognise the virus in the future A repeat exposure to the virus means the bacterium can use a special RNA molecule to guide a protein (Cas 9) to that part of the viral DNA, and disable it by cutting For therapeutic use, guide RNA (grna) is created that matches the gene of interest. grna plus Cas9 are put into a plasmid, and the plasmid put into target cell Guide RNA grna locates target gene in the cell's DNA and Cas9 cuts both strands of DNA Cell repairs break by joining the cleaved DNA strands thus disabling target gene (non homologous end joining NHEJ After cleavage, a functional gene (=mutation)can be inserted (homology directed repair HMD

17 Gene Editing

18 CRISPR vs previous gene editing techniques Easier Cheaper Quicker (1-2 weeks vs 1-2 years) More efficient Some problems DNA repair mechanism not perfect may incorporate wrong bits of DNA as part of the patch CRISPR good at disabling gene (65% of HD gene in key areas animal brain disabled) Less good at repairing genes (repair is more useful in embryos) CRISPR targets a sequence of bases not individual ones (cf base editing) Mosaics : - embryos where only SOME of the cells are fixed. Adult MAY be Ok but faulty gene still present might become active; if in germ cells will pass to next generation Difficult to screen for in pre-implantation embryos (usual screen might not work) How to switch off CRISPR after desired change made and before potentially deleterious change happens (already achieved, kamikaze CRISPR)

19 Base Editing (modified version of CRISPR) Has been used experimentally to correct blood disease fault in non-viable human embryos

20 DISEASE VS DISORDER Criminal Behaviour Bipolar disorder Bonnie & Clyde Van Gogh Schizophrenia Asperger's Dwarfism Philip K Dick Elly Simmonds Steve Jobs

21 SOME ETHICAL CONSIDERATIONS Would-be parents who want unaffected children can have IVF with donor egg / sperm OR select from pre-implantation embryos Should genetic manipulation be only targeted at the post-natal (curing existing condition) not the preimplantation Are lives of people affected by genetic diseases less Are healthy normal people entitled to make decisions of behalf of those with life limiting diseases Is this EUGENICS by another route?

22 Sources Biologist Magazine The Gene (S. Mukherjee) New Scientist BBC.co.uk BBC R4 Royal Soc Biology Blogs and News Items Shutterstock Wikipedia THANK YOU!!