Muscle type phosphofructokinase deficiency is an inherited glycogen storage disease.
It is caused by a nonsense mutation, which leads to a lack of phosphofructokinase subunits or activity. Without the PFK enzyme muscle cells and erythrocytes are not able to produce enough adequate energy for their needs. Therefore affected dogs display the following intermittent, clinical signs: weakness, lethargy, exercise intolerance, poor performance, muscle cramps, anaemia, jaundice and dark-coloured urine. Dark-coloured urine, a hallmark of this disorder, usually appears after strenuous exercise or after excessive barking, panting or heat exposure and is caused by the destruction of the erythrocytes.