Causal Mutation for Leopard Complex Located
What is Leopard Complex
Leopard Complex (LP) is a white spotting pattern more commonly known as appaloosa or appaloosa spotting. It is characterized by areas of white centered over the hips. The extent to which leopard complex expresses can vary greatly and in part depends upon additional modifiers called Pattern (PATN). When LP is present without a Pattern Modifier, the horse will usually exhibit what is known as LP characteristics. These traits are sclera, mottled skin (pink skin with colored spots) and striped hooves. The horse may also roan or lose pigmentation as it ages. This is known as LP roaning and unlike gray, the horse will maintain pigment in bony areas including the nose and legs.
When LP is present with a Pattern Modifier, white spotting usually appears on the body of the horse in addition to the characteristics. It is currently theorized that there is one major pattern modifier and perhaps several smaller modifiers. Pattern 1 (PATN1) is thought to be responsible for leopard and near leopard type spotting as well as large blankets. The minor pattern modifiers are thought to be responsible for snowflaking and smaller blankets. Some overlap is possible between the two types of Patterns and a horse can carry both PATN1 and other minor pattern modifiers. It is even theoretically possible for a horse to be homozygous for PATN1 and carry smaller pattern modifier. Regular white markings such as face and leg white can also at times “boost” white LP pattern markings by enlarging areas of white on the hip and body and also reducing “spot” size in horses who are heterozygous for LP.
LP is inherited in a semi-dominant fashion with horses with a single copy of the mutation having “spots” within the white areas of their coats. Horses that are homozygous for LP and also have areas of white on their hips will NOT have areas of color within their white markings (aka spots). Horse that are heterozygous for LP WILL have spots of color inside their areas of white unless another white boosting pattern or modifier is present (happens rarily). Horses that are homozygous for LP also suffer from what is known as congenital stationary night blindness (CSNB). Which causes a reduction/absence of vision in low light/nighttime conditions.
The Mutation is Found
Although, a test for Leopard Complex in horses has been available for sometime via Animal Genetics (through cooperation with the Appaloosa Project). The research behind the test has only recently been published. The study, which was published in the October edition of Plos One, builds upon previous research by some of the same authors which had mapped Leopard Complex to the TRPM1 gene. This research was expanded to locate the actual causal mutation. This mutation was found to be “1378 bp insertion in intron 1 (ECA1g. 108,297,929_108,297,930 ins1378) of TRPM1.” This means that a fairly large piece of DNA has been added to the genetic code inside the TRPM1 gene which is found on horse Chromosome 1. Further more, the insertion was found to be retroviral in origin. A retrovirus is a virus that hijacks a cells DNA to reproduce itself. At times, strands of this viral DNA can be left behind inside cells. If the insertion affects reproductive cells which then are used to produce offspring, the DNA inserted by the virus becomes part of the offspring. This, like other types of mutation, can cause disruptions in the way a gene functions. In the case of Leopard Complex, the result is the Appaloosa Pattern and Congenital Stationary Night Blindness.
By studying skin biopsies of horses with LP the researchers found an absence of pigment producing cells in white areas of LP patterned horses. This suggests that the LP mutation has interrupted the production of these cells in the white areas. The pigment cells in the colored areas of LP/LP horses were also oddly shaped which, although not discussed by the researchers, could explain why some LP horses have odd coloring, often being lighter than comparable horses without LP. Why homozygous LP horses will have spots of color producing pigment within white areas was not clear to the researchers and work is ongoing to determine the cause of the “spots”. Researchers theorize “pigmentation could be disrupted by improper melanosome maturation. This may stimulate melanocytosis [pigment loss] resulting in white patterned areas during development and progressive roaning during the life of the animal”.
Congenital Stationary Night Blindness
Although the association of the LP color pattern with CSNB has been well established for many years this study puts to rest any lingering doubts that they are caused by the same mutation. Horses that are homozygous for LP are affected with CSNB and it is caused by the same mutation as the appaloosa color.
Ancient LP Horses
It has been reported previously that LP is an ancient mutation. This study confirmed that evidence of the LP mutation had been found in three ancient samples taken from Germany and Russia the oldest dated from 14000-15000 BC.
Evidence for a Retroviral Insertion in TRPM1 as the Cause of Congenital Stationary Night Blindness and Leopard Complex Spotting in the Horse