.. A Guide to the Genetics of ..
.. Leber's Hereditary Optic Neuropathy '
LHON ' .... a Mitochondrial Disorder of the Enzyme '
Complex I ' ..BASIC BIOLOGY ... A GLOSSARY OF TERMS
Cells Mitochondria Mitochondrial DNA - the mtDNA Mitochondrial Mutations Homoplasmy & Heteroplasmy The Mitochondrial Genes The Enzyme Complex I -----------------------------------------------------------------------------------------------CELLS The body is made up of cells, possibly as many as 75 trillion ! The outer part of the cell is the 'cell wall', and inside is the 'cytoplasm'. Suspended in the cytoplasm are various structures, most obviously a nucleus which contains the chromosomes, 22 pairs, and in a male X & Y, and in a female X & X. Also in the cytoplasm are the mitochondria whose function is to produce 'energy' for the cell.
A diagram taken from the Internet-----------------------------------------------------------------------------------------------
MITOCHONDRIA The mitochondria are small organelles that are suspended in the cytoplasm of cells. Their function is to produce 'energy' for the cell, mainly by converting ADP to ATP. In most cells there are possibly 50-100 mitochondria, but liver cells appear to have 200, or more, and an 'egg' cell has perhaps 2,000 mitochondria. One of the enzymes inside a mitochondrion is the COMPLEX I.
-----------------------------------------------------------------------------------------------MITOCHONDRIAL DNA - the mtDNA All mitochondria contain small amounts of DNA, as circular rings of about 16,569 bases; and, each mitochondrion may have several hundreds of these mtDNA molecules. A human mtDNA molecule contains the nucleotide bases Adenine, Cytosine, Guanine and Thymine, which are usually shown as 'A', 'C', 'G' & 'T'. mtDNA starts with bases: GATCACAGGTCTATCACCCTATTAACCA... and finishes with ...CGTTCCCCTTAAATAAGACATCACGATG The numbering of the bases is specified by the Cambridge Reference Sequence (CRS).
-----------------------------------------------------------------------------------------------MITOCHONDRIAL MUTATIONS The mtDNA is actually made up of two strands, with each numbered base being really a pair of nucleotide bases; the named base being on the MAJOR strand and a corresponding base on the MINOR strand. The Watson-Crick pairing system indicates that 'A' pairs with 'T', and 'C' pairs with 'G', and a mutation is said to have occurred when a nucleotide base becomes changed to another: such as 'A' changing to 'C', 'G' or 'T' A mutation can also be a deletion - when 1, or more, pairs of nucleotide pairs are lost, or an insertion when 1, or more, pairs of nucleotide pairs are gained. The following diagram shows the various types of mutations:
-----------------------------------------------------------------------------------------------A mutation is a rare event and is probably caused by error as a DNA strand is duplicated. In
LHON the main mutations are: G3460A - The 'G' at position 3460 has mutated to 'A' G11778A - The 'G' at position 11778 has mutated to 'A' T14484C - The 'T' at position 14484 has mutated to 'C'HOMOPLASMY & HETEROPLASMY In most people, any sample, whether taken for example, from the mouth or from the blood, will show the same sequence of nucleotide bases in the mtDNA - and the person is then said to show HOMOPLASMY for all the nucleotide bases. And, in persons with
-----------------------------------------------------------------------------------------------LHON there is usually homoplasmy for a main mutation whether it is the 'A' at 3460, the 'A' at 11778, or the 'C' at 14484. However, some persons withLHON show HETEROPLASMY, a state where their cells contain mtDNA with the original nucleotde base AND the mutation. So the results of testing a heteroplasmic person will show, for example: G3460R - where the 'R' indicates the presence of both 'G' and 'A' G11778R - where the 'R' indicates the presence of both 'G' and 'A' T14484Y - where the 'Y' indicates the presence of both 'T' and 'C' A simple explanation of 'heteroplasmy' is that it is a 'mutation in the making'. I.e. a mutation starts off in just one woman and she shows a low level of heteroplasmy Then in each of her children there is either, a higher level, or a lower level of heteroplasmy. And ... after several generations, a person might appear to be homoplasmic, or ... the opposite may happen and the mutation may disappear.THE MITOCHONDRIAL GENES The mtDNA is important as parts of the DNA molecule are actually genes used by the cell to manufacture peptides - a peptide is a short chain of amino acids that when 'assembled' with other peptides forms a protein. In the Human mtDNA there are 13 genes: ATP6, ATP8, CO1, CO2, CO3, CYB, ND1, ND2, ND3, ND4, ND4L, ND5, ND6 which are all involved in a biochemical process, OXPHOS, that produce 'energy' in the cell. In relation to
LHON it is the peptides ND1 (for the mutation G3460A), ND4 (G11778A) & ND6 (T14484C) that are important. Here are the amino acid lists for these peptides:ND1 - LHON mutation G3460A changes 52nd amino acid from 'A' to 'T' MPMANLLLLI VPILIAMAFL MLTERKILGY MQLRKGPNVV GPYGLLQPFA DA MKLFTKEP LKPATSTITL YITAPTLALT IALLLWTPLP MPNPLVNLNL GLLFILATSS LAVYSILWSG WASNSNYALI GALRAVAQTI SYEVTLAIIL LSTLLMSGSF NLSTLITTQE HLWLLLPSWP LAMMWFISTL AETNRTPFDL AEGESELVSG FNIEYAAGPF ALFFMAEYTN IIMMNTLTTT IFLGTTYDAL SPELYTTYFV TKTLLLTSLF LWIRTAYPRF RYDQLMHLLW KNFLPLTLAL LMWYVSMPIT ISSIPPQT ND4 -LHON mutation G11778A changes 340th amino acid from 'R' to 'H' MLKLIVPTIM LLPLTWLSKK HMIWINTTTH SLIISIIPLL FFNQINNNLF SCSPTFSSDP LTTPLLMLTT WLLPLTIMAS QRHLSSEPLS RKKLYLSMLI SLQISLIMTF TATELIMFYI FFETTLIPTL AIITRWGNQP ERLNAGTYFL FYTLVGSLPL LIALIYTHNT LGSLNILLLT LTAQELSNSW ANNLMWLAYT MAFMVKMPLY GLHLWLPKAH VEAPIAGSMV LAAVLLKLGG YGMMRLTLIL NPLTKHMAYP FLVLSLWGMI MTSSICLRQT DLKSLIAYSS ISHMALVVTA ILIQTPWSFT GAVILMIAHG LTSSLLFCLA NSNYERTHSR IMILSQGLQT LLPLMAFWWL LASLANLALP PTINLLGELS VLVTTFSWSN ITLLLTGLNM LVTALYSLYM FTTTQWGSLT HHINNMKPSF TRENTLMFMH LSPILLLSLN PDIITGFSS ND6 -LHON mutation T14484C changes 64th amino acid from 'M' to 'V' MMYALFLLSV GLVMGFVGFS SKPSPIYGGL VLIVSGVVGC VIILNFGGGY MGLMVFLIYL GGMM VVFGYT TAMAIEEYPE AWGSGVEVLV SVLVGLAMEV GLVLWVKEYD GVVVVVNFNS VGSWMIYEGE GSGLIREDPI GAGALYDYGR WLVVVTGWTL FVGVYIVIEI ARGN where the letter represent the amino acids: A - Alanine (Ala) C - Cysteine (Cys) D - Aspartic Acid (Asp) E - Glutamic Acid (Glu) F - Phenylalanine (Phe) G - Glycine (Gly) H - Histidine (His) I - Isoleucine (Ile) K - Lysine (Lys) L - Leucine (Leu) M - Methionine (Met) N - Asparagine (Asn) P - Proline (Pro) Q - Glutamine (Gln) R - Arginine (Arg) S - Serine (Ser) T - Threonine (Thr) V - Valine (Val) W - Tryptophan (Trp) Y - Tyrosine (Tyr)The following diagrams show suggested structures for these peptides - they have transmembrane helices and loops. The arrows point to where the
LHON mutations occur.
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The Enzyme Complex I (read as - 'complex one') The enzyme 'Complex I', or 'NADH:ubiquinone reductase' to give it a descriptive name, is an enzyme found in the inner membrane of mitochondria. It is a large protein involved with the pumping of H+ across the membrane so producing a potential difference which is used to change ADP to ATP. (ATP being the usual 'fuel' used in most cellular reactions).
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