Important distinction 1 Nosce te ipsum the human genome 2 Part II genetics diagnostics and gene therapy of inherited disease 3 MCB 140 12 4 06 Monogenic disorders human diseases whose etiology can in some more or less linear fashion be traced to a single locus genetic lesion Diseases with a genetic component or a genetic predisposition disorders that mankind is known to be genetically polymorphic for in terms of susceptibility at multiple loci All other disease that may or may not be transcription based 1 Phenomena affecting ploidy e g aneuploidies such as Down Edwards Turner Klinefelter 2 Phenomena affecting chromosome structure e g translocations in leukemia 3 Phenomena affecting single loci genes or relatively small chromosomal segments 1 MCB 140 12 4 06 2 MCB 140 12 4 06 4 Archibald Garrod 1902 Higher frequency of children with alkaptonuria urine turns dark on standing and alkalinization from consanguineous marriages Why There is no reason to suppose that mere consanguinity of parents can originate such a condition as alkaptonuria in their offspring and we must rather seek an explanation in some peculiarity of the parents which may remain latent for generations It has recently been pointed out by Bateson that the law of heredity discovered by Mendel offers a reasonable account of such phenomena Garrod 1902 Lancet 2 116 http www esp org foundations genetics classical ag 02 pdf MCB 140 12 4 06 3 Garrod 1902 Lancet 2 116 Sickle cell anemia Sickle cell anemia a brief history NIH In the western literature the first description of sickle cell disease was by a Chicago physician James B Herrick who noted in 1910 that a patient of his from the West Indies had an anemia characterized by unusual red cells that were sickle shaped By 1923 it was realized the condition is hereditary In 1949 Neel realized that patients with SCA are homozygous and heterozygous carriers have a much milder condition sickle cell trait Sickle cell anemia is the most common inherited blood disorder in the United States affecting about 72 000 Americans or 1 in 500 African Americans SCA is characterized by episodes of pain chronic hemolytic anemia and severe infections usually beginning in early childhood MCB 140 12 4 06 5 MCB 140 12 4 06 6 1 Linus Pauling 1949 HbS has different charge V Ingram Nature 1956 On the existing evidence alone it is not possible to decide whether the difference between the proteins which is in any event small lies in the amino acid sequences of the polypeptide chains or whether it lies in the folding of these chains leading to the masking of some amino acid side chains Experiment 1 Digest Hb A and Hb S with trypsin protease cuts hemoglobin into 30 peptides 2 Separate resulting fragments by electrophoresis and then by chromatography 3 Trace the peptide map MCB 140 12 4 06 7 V Ingram 1956 Nature 178 792 MCB 140 12 4 06 8 One can now answer at least partly the question put earlier and say there there is a difference in the amino acid sequence in one small part of one of the polypeptide chains This is particularly interesting in view of the genetic evidence that the formation of hemoglobin S is due to a mutation in a single gene V Ingram 1956 Nature 178 792 MCB 140 12 4 06 9 V Ingram 1956 Nature 178 792 MCB 140 12 4 06 10 SCA 1 2 3 4 11 7 MCB 140 12 4 06 11 Fairly homogeneous genetic basis an A to T transversion in the sixth codon of the HBB gene that leads to a glu val substitution RFLP In North America heterozygosity for mutant allele is largely asymptomatic sickle cell trait because concentration of hemoglobin S is not high enough for the erythrocytes to sickle In areas with high incidence of malaria the fitness of heterozygotes is greater than of noncarriers or affected individuals overdominance because carriers are relatively malaria resistant explaining the high frequency of this allele Therapy hydroxyurea wakes up embryonic and fetal globin genes morphine for the pain and prophylactic penicillin Sickle cell anaemia A simple disease with no cure Nature 1989 MCB 140 12 4 06 12 2 Functional cloning Cystic fibrosis In the case of alkaptonuria sickle cell anemia and blood clotting disorders such as hemophilia the disease genes is identified based on some biochemical or other defect exhibited by the patient What if the defect cannot be traced in a simple way to a biochemical phenomenon Most common monogenic autosomal human genetic disorder 1 in every 2000 live births MCB 140 12 4 06 q2 0 05 q 2 2 p 97 8 2pq 4 carriers Complex dysfunction of the lungs and the pancreas MCB 140 12 4 06 13 14 Mapping by linkage positional cloning Life expectancy of CF patients If a given marker is linked is on the same chromosome as to the gene mutations in which cause a certain disease then one should be able to observe coinheritance of some allelic form of that marker to the occurrence of the disease Coinheritance occurrence in genotype of two loci with a frequency higher than Mendel s second law allows MCB 140 12 4 06 15 MCB 140 12 4 06 16 MCB 140 12 4 06 18 Mendel s second law 11 17 MCB 140 12 4 06 17 3 4 17 MCB 140 12 4 06 19 MCB 140 12 4 06 20 MCB 140 12 4 06 22 MCB 140 12 4 06 24 a Nanomouse Nano GENA50 b Dominant spotting KitW 39H GENA133 c A microphthalmia mutant GENA163 d e Batface a craniofacial mutant GENA123 Very simple and astonishingly influential consequence of all this stuff Two markers located on different chromosomes will segregate away from each other in one out of two meioses Fig 1 Nolan Two markers that are on the same chromosome will tend to stay together MCB 140 12 4 06 21 Nolan et al Nat Genet 2000 25 440 William Ernest Castle founder of mouse genetics 1 Inbreeding as a tool for making genetically uniform strains of mice that are homozygous for every allele in the genome 2 Brother sister matings makes 12 5 of all loci in the genome homozygous Clarence Little After 40 generations of brother sister mating 99 98 of genome is homozygous By F60 mice are considered genetically identical to one another 11 12 MCB 140 12 4 06 23 4 1 2 3 A B C D E F G H I J K L A B C D E F G H I J K L 1 3 A B C D E F G H I J K L A B C D E F G H I J K L x MCB 140 12 4 06 2 A B C D x E F G H 25 I J K L A B C D E F G H A B C D E F G H I J K L a b c d e f g h i x j k MCB 140 12 4 06 28 MCB 140 …
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