Amelogenesis imperfecta
Amelogenesis imperfecta (AI) is a congenital disorder which presents with a rare abnormal formation of the enamel or external layer of the crown of teeth, unrelated to any systemic or generalized conditions. Enamel is composed mostly of mineral, that is formed and regulated by the proteins in it. Amelogenesis imperfecta is due to the malfunction of the proteins in the enamel (ameloblastin, enamelin, tuftelin and amelogenin) as a result of abnormal enamel formation via amelogenesis. People afflicted with amelogenesis imperfecta may have teeth with abnormal color: yellow, brown or grey; this disorder can afflict any number of teeth  of both dentitions. Enamel hypoplasia manifests in a variety of ways depending on the type of AI an individual has (see below), with pitting and plane-form defects common. The teeth have a higher risk for dental cavities and are hypersensitive to temperature changes as well as rapid attrition, excessive calculus deposition, and gingival hyperplasia. The earliest known case of AI is in an extinct hominid species called "Paranthropus robustus", with over a third of individuals displaying this condition. Genetics. Several gene expression is needed for enamel formation where the relevant matrix proteins & proteinases are transcribed for regular crystal growth  & enamel mineralization. Mutations in the "AMELX", "ENAM", "MMP20", "KLK-4", "FAM83H", "WDR72", "C4orf26", "SLC24A4" "LAMB3" and "ITGB6" genes have been found to cause amelogenesis imperfecta (non-syndromic form). "AMELX" and "ENAM" encode extracellular matrix proteins of the developing tooth enamel and "KLK-4" and "MMP20" encode proteases that help degrade organic matter from the enamel matrix during the maturation stage of amelogenesis. "SLC24A4" encodes a calcium transporter that mediates calcium transport to developing enamel during tooth development. Less is known about the function of other genes implicated in amelogenesis imperfecta. Researchers expect that mutations in further genes are likely to be identified  as causes of amelogenesis imperfecta. Types include: Amelogenesis imperfecta can have different inheritance patterns depending on the gene that is altered. Mutations in the ENAM gene are the most frequent known cause and are most commonly inherited in an autosomal dominant pattern. This type of inheritance means one copy of the altered gene in each cell is sufficient to cause the disorder. Amelogenesis imperfecta is also inherited in an autosomal recessive pattern; this form of the disorder can result from mutations in the "ENAM", "MMP20", "KLK4", "FAM20A", "C4orf26" or "SLC24A4" genes. Autosomal recessive inheritance means two copies of the gene  in each cell are altered. About 5% of amelogenesis imperfecta cases are caused by mutations in the AMELX gene and are inherited in an X-linked pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In most cases, males with an X-linked form of this condition experience more severe dental abnormalities than affected females. Recent genetic studies suggest that the cause of a significant proportion of amelogenesis imperfecta cases remains to be discovered. Diagnosis. AI can be classified according to their  clinical appearances: Differential diagnosis would include dental fluorosis, molar-incisor hypomineralization, chronological disorders of tooth development. Treatment. Preventive and restorative dental care is very important as well as considerations for esthetic issues since the crown are yellow from exposure of dentin due to enamel loss. The main objectives of treatment is pain relief, preserving patient's remaining dentition, and to treat and preserve the patient's occlusal vertical height. Many factors are to be considered to decide on treatment options such as the classification and severity of AI, the patient's social history, clinical findings etc. There are many classifications  of AI but the general management of this condition is similar. Full-coverage crowns are sometimes being used to compensate for the abraded enamel in adults, tackling the sensitivity the patient experiences. Usually stainless steel crowns are used in children which may be replaced by porcelain once they reach adulthood. These aid with maintaining occlusal vertical dimension. Aesthetics may be addressed via placement of composite or porcelain veneers, depending on patient factors e.g. age. If the patient has primary or mixed dentition, lab-made composite veneers may be provided temporarily, to be replaced by permanent porcelain veneers once the patient  has stabilized permanent dentition. The patient's oral hygiene and diet should be controlled as well as they play a factor in the success of retaining future restorations. In the worst-case scenario, the teeth may have to be extracted and implants or dentures are required. Loss of nerves in the affected teeth may occur. Epidemiology. The exact incidence of amelogenesis imperfecta is uncertain. Estimates vary widely, from 1 in 700 people in northern Sweden to 1 in 14,000 people in the United States. The prevalence of amelogenesis imperfecta in non-human animals has not been explored, however its presence  has been noted. This condition is neither caused by nor the equivalent of dental fluorosis. A manifestation of amelogenesis imperfecta known as "snow capping" is confined to the outer prismless enamel layer. It may superficially resemble dental fluorosis, and indeed "snow capping" may be used as a descriptive term in some incidents of dental fluorosis.