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Burnside-Butler syndrome, also known as 15q11.2 BP1-BP2 microdeletion, is a congenital disorder caused by microdeletion of DNA sequences. It is associated with a number of developmental and psychiatric disorders.

History
Chromosome 15 has five common breakpoint sites along the proximal long arm; they are commonly referred to as BP1–BP5. Prader-Willi and Angelman syndromes are two classic disorders that are associated with a deletion on chromosome 15 within the q11 to q13 bands. There are two types of deletions that can occur within 15q11-q13, a longer type 1 deletion involving BP1 and BP3 or the shorter type 2 deletion involving BP2 and BP3. Both of these deletions will lead to either Prader- Willi syndrome or Angelman syndrome, depending on if the deletion occurs on the maternal or paternal inherited chromosome.

Studies have shown that individuals with type 1 deletion tend to have a more severe phenotype than those with the type 2 deletion. A study was then conducted with individuals who had Prader-Willi syndrome. It was established that 24% to 99% of the phenotypic variability, between individuals with type 1 and type 2 deletions could be attributed to alterations to four particular genes (i.e., NIPA1, NIPA2, CYFIP1, TUBGCP5), between BP1 and BP2 in the 15q11.2 chromosome band.

Dr. Rachel D. Burnside and colleagues then followed up and surveyed the first large cohort of patients presenting for genetic testing using high resolution microarrays. With their study they found that 0.86% of the approximate 17,000 individuals either had a deletion or duplication of the 15q11.2 BP1–BP2 region. They concluded that this genomic area was susceptible for alterations that would lead to neurological dysfunction and impaired development. Their ideas actually coincided with studies that were being conducted by Dr. Merlin G. Butler and colleagues. Combining both of these studies lead to the conclusion that a deletion involving this genomic region correlated with language or motor delays, behavioural problems, autism, seizures and occasionally mild dysmorphic features. These initial discoveries led to the discovery of the congenital disorder called Burnside- Butler syndrome, which entails the deletions to this chromosomal region and the associated phenotypic effects.

Genes
The BP1-BP2 region spans approximately 500 KB and contains four evolutionarily conserved genes: NIPA1, NIPA2, CYFIP1, and TUBGCP5. Each of these genes have specific roles. It is predicted that alterations to these genes may lead to the exhibited phenotypic effects.

The following descriptions explain the regular roles of each of these genes.

NIPA1 (Non- imprinted in Prader- Willi/ Angelman syndrome region 1) is associated with autosomal dominant hereditary spastic paraplegia, it mediates Mg2+ transport and is highly expressed in neuronal tissue.

NIPA2 is used in renal Mg2+ transport.

CYFIP1 plays an important role in the regulation of brain mRNAs. The gene product of CYFIP1 interacts with the protein product of the FMR1 gene- FMRP. This protein is responsible for a syndrome that causes familial intellectual disability primarily in males, called Fragile X syndrome.

Finally, TUBGCP5 is involved in neurobehavioural disorders including Attention deficit hyperactivity disorder (ADHD) and Obsessive compulsive disorder (OCD).

Consequences
Individuals with defects within the 15q11.2 band may not share a clinical phenotype or be clinically affected at all. This suggests that this region contains genetic material showing incomplete penetrance of pathogenicity. Incomplete penetrance means that this genetic mutation is not always associated with clinical features. Amongst current studies there are a few common clinical features that are recognized with this microdeletion syndrome.

Growth and Development
With this disorder it has been noted that deletions within the BP1–BP2 region have been susceptible to neurological dysfunction with general development and motor delays with speech. It has been proposed that these neurological disruptions may be attributed to the neuronal components of genes within this deleted region.

Dysmorphic Features
It is noted that individuals with Burnside Butler syndrome do not have a clear dysmorphic phenotype. There are a few general, non-specified dysmorphic features that have been observed. The most common one amongst individuals with Burnside Butler syndrome has been palatal abnormalities. This includes deformities to the region of the mouth that separates it from the nose cavity. There are other occasional dysmorphic features that may present themselves, including, broad foreheads, hypertelorism (increased distance between the orbits), slender fingers, pectus excavatum (a congenital chest wall deformity in which several ribs and the sternum grow abnormally, producing a concave, appearance in the anterior chest wall), plagiocephaly (asymmetrical flattening of one side of the skull), dysmorphic nose, dysmorphic teeth and contractures (shortening and hardening of muscles). Clinicians may look for certain attributes to diagnose individuals with Burnside Butler syndrome.

Behavioural and Psychiatric Problems
Studies have shown that individuals with the 15q11.2 BP1–BP2 microdeletion have shown certain behavioural and psychiatric problems. In a review of the behavioural and psychiatric features of individuals with Burnside Butler syndrome, more than half of the reported individuals had general behavioural problems. Some common behaviours can be associated with those observed amongst other disorders including, attention deficit disorder, autism spectrum disorder, obsessive compulsive disorder, self injurious behaviours, oppositional defiant disorder and schizophrenia. Studies have found a significant association between the microdeletion and schizophrenia.

Other related medical Concerns
Along with the above consequences, there are various miscellaneous medical conditions that have been reported to occur with Burnside Butler syndrome. Some of these include, abnormal brain imaging’s, clinical features of seizures or epilepsies, and coordination problems. Rarer consequences that may be seen include, congenital heart defects, genital abnormalities, recurrent infections, cataracts, hearing loss, tracheoesophageal fistula (an abnormal connection between the esophagus and trachea) and omphalocele (a rare abdominal wall defect where organs remain outside the abdomen).

Inheritance
It is still unclear as to how this deletion is inherited in an individual. Studies have shown that certain individuals have inherited the deletion from an affected mother, where as others inherited it from an affected father. Other studies have shown certain individuals that have inherited the deletion from an unaffected parent, making the 15q11.2 BP1–BP2 microdeletion a de novo mutation.