Achondroplasia

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The Munchkin breed are classically bred for achondroplasia

Achondroplasia is a rare autosomal dominant genetic disorder of cats characterised by abnormally short or deformed limbs and an enlarged head. Dwarfism is commonly seen in this disease.

Dwarf or very smaller cats (pseudoachondroplasia) can arise for several reasons; genetic, hormonal or environmental. Some of these cats are very small but in normal proportion; these may either have a hormone deficiency (lack of growth hormone, congenital hypothyroidism[1], lysosomal storage disease[2]) or have been selectively breed from progressively smaller cats in the modern trend for miniature cats (esp, Munchkin and derivative breeds)[3].

Munchkin mutation

Pseudoachondroplasia is characterised by short limbs, but normal head proportions. The Munchkin mutation is often termed achondroplasia, but the head size is normal suggesting pseudachondroplasia. Problems associated with the Munchkin mutation are lordosis (spine dips downwards around the shoulder blades) and pectus (flattened ribcage). Small litter sizes when short-legged cats are bred together suggests the gene is lethal when 2 copies are inherited.

The most common and easily recognisable form is achondroplasia dwarfism which shortens the long bones of the limbs while leaving the trunk (body) unchanged. Achondroplasia dwarfism is characterised by abnormal body proportions. Achondroplasia dwarfism is the result of a dominant genetic trait affecting the hormones which control bone growth. Typically, the growth of the limbs is stunted, while the size of the trunk and mental capacity are normal.

The condition causes abnormally short and deformed limbs; this is most noticeable in short-legged dog breeds where the limbs are bowed or twisted. It also typically produces a large head with undershot (bulldog) jaw and crowded, misaligned teeth. Other cranial problems may occur due to the abnormal head shape. The limbs are frequently bowed which may result in poorly articulating joints. The vertebrae may also be affected. Although most affected cats are mentally normal, their abnormal body proportions may result in slow development in early kittenhood. The large head may result in kittens being delivered by caesarian section as they are unable to pass through the birth canal. Stuck kittens are likely to die and would likely cause the mother to die.

In the Munchkin; these short limbs are the distinguishing feature of the breed and the other deformities associated with achondroplasia are avoided (as far as possible) by careful selective breeding, avoiding breeding those individuals which have spine or chest deformities. Although achondroplasia is typically associated with a large or abnormal head, Munchkins do not seem to suffer this effect and therefore may have a different condition with similar physical effects rather than true achondroplasia. The Munchkin trait is more likely to be pseudochondroplasia or hypochondroplasia.

Achondroplasia also occurs at random in the feline population due to mutation hence the occasional report of short-legged cats (Kangaroo Cats and Squittens Revealed). It occurs in varying degrees, ranging from "nearly normal" to crippling with all legs severely deformed. The forelegs are usually more severely affected than the hindlegs. In animals this form of dwarfism ranges from mildly disabling to crippling or lethal (stillborn). In Munchkins, homozygous embryos apparently die early in the pregnancy and are reabsorbed, decreasing the litter size compared to litter size in most other breeds.

References

  1. Mooney CT (1998) Unusual endocrine disorders in the cat. In Practice 20:345-349
  2. Hegreberg, G.A., Norby, D.E., Hamilton, M.J. (1974) Lysosomal enzyme changes in an inherited dwarfism of cats. Federation Proceedings 33:598
  3. Gunn-Moore, D.A., Hagard, G., Turner, C., Duncan, A.W., Barr, F.J. (1996) Unusual metaphyseal disturbance in two kittens. Journal of Small Animal Practice 37:583-590