Gene Variations for Duchenne
The genetic script for making the dystophin protein uses only four bases (A,G,T and C) and its words, the codons (in orange), are always three letters long. In Steve Wilton's (Perth Australia) analogy the three letter words in THE BAD AND OLD DOG ATE THE FAT CAT END represent the codons to make a sentence (dystrophin) that has a start and end and makes sense to us the reader. The letters of each word would represent one of the four bases.
So to make sense to us the reader all the letters (or bases) of each word have to be in the right order. Also all the words (codons) have to be correct for the whole sentence to have the right syntax. If any letters or words are missing or are in the wrong order then we will not be able to read the sentence. Genes are "read" in a similar way. If a mutation occurs due to a missing base or codon then the message can be lost.
Duchenne Muscular Dystrophy occurs through nonsense, premature stop signal, out of frame and duplication mutations of the dystophin gene causing no functional dystrophin protein to be produced in muscle cells. This leads to severe and progressive loss of muscle function.
Becker Muscular Dystrophy is caused by sense and in frame variations and leads to the production of dystrophin of reduced quantity and quality. Symptoms vary from borderline DMD to asymptomatic.
Single Point Variations
Point variations can occur by one letter or base being changed that will lead to loss of the genetic message.
Premature stop signal
If a variation occurs that leads to a codon sequence that represents a stop signal as in our analogy of an E for an A then the protein syntheis is terminated and no dystophin is produced leading to symptoms of Duchenne Muscular Dystrophy.
Deletion - out of frame
The deletion of an exon leading to an out of frame reading of the genetic message.
Duplication
The addition of a duplicate exon can also lead to an out of frame reading of the genetic message.
Sense variations
In some cases if one letter is changed - a point mutation - then the genetic code can still be read and still make some sense. If these changes only concern non essential structures of dystrophin then the more benign form of dystrophy Becker Muscular Dystrophy is more likely.
In frame deletions
The deletion of an exon leading to an inframe reading of the gentic message. This is likely to result in Becker Muscular Dystrophy.