Chromosome 6, Partial Trisomy 6q is an extremely rare chromosomal disorder in which a portion of the 6th chromosome (6q) is present three times (trisomy) rather than twice in cells of the body. Associated symptoms and findings may vary in range and severity from case to case. However, many affected infants and children have slow physical development (growth retardation); mental retardation; malformations of the skull and facial (craniofacial) region; an unusually short, webbed neck; abnormal bending (flexion) or extension of certain joints in fixed postures (joint contractures); and/or other physical abnormalities. In most cases, Chromosome 6, Partial Trisomy 6q has been the result of a balanced translocation in one of the parents.
As noted above, the symptoms and physical findings associated with Chromosome 6, Trisomy 6q may be variable. However, in many cases, the disorder is characterized by growth delays before and after birth, severe to profound mental retardation, a delay in the acquisition of skills requiring coordination of muscular and mental activity (psychomotor retardation), distinctive malformations of the skull and facial (craniofacial) region, musculoskeletal abnormalities, and/or additional physical features.
Characteristic craniofacial abnormalities may include a small head (microcephaly); an abnormally flat face and back region of the head (occiput); “almond-shaped,” protruding, widely spaced eyes (ocular hypertelorism); and/or downwardly slanting eyelid folds (palpebral fissures). Affected individuals may also have a small, “bow-shaped” mouth with thin lips, a small jaw (micrognathia), incomplete closure of the roof of the mouth (cleft palate), a large, flat nose; malformed ears, and/or thin, arched eyebrows. In some cases, the fibrous joints (i.e., coronal and sagittal sutures) between certain bones in the front and the sides of the skull (frontal and parietal bones) may close prematurely (craniosynostosis), causing the head to grow upward (turricephaly). As a result, the head may appear unusually long, narrow, and pointed at the top, and the forehead may be abnormally prominent.
Many individuals with Chromosome 6, Partial Trisomy 6q also have distinctive abnormalities of the neck. The neck may be unusually short and wide, with abnormal webbing across the front (anterior) and/or side (lateral), potentially restricting movement of the jaw and neck. In addition, the hairline may be abnormally low on the back of the neck (nape).
Chromosome 6, Partial Trisomy 6q is also often associated with abnormal bending (flexion), extension, and fixation of certain joints (contractures), such as the fingers, wrists, and/or other regions (e.g., elbows, knees, hips), causing limitation of movement and abnormal postures. Affected individuals may also have webbing or fusion of certain fingers and/or toes (syndactyly), deformities in which the hands and/or feet are twisted out of shape or position (clubhands and/or clubfeet), abnormal curvature of the spine (scoliosis), reduced diameter of the chest; and/or widely spaced nipples.
Genital abnormalities may also be present. In affected females, there may be underdevelopment of the skin folds surrounding the vaginal opening (hypoplastic labia). In affected males, genital abnormalities may include an abnormally small penis (micropenis), underdevelopment of the scrotum; abnormal placement of the urinary opening (hypospadias), such as on the underside of the penis; and/or undescended testes (cryptorchidism).
In rare cases, individuals with Chromosome 6, Partial Trisomy 6q may also have various internal organ malformations. These may include heart (cardiac), intestinal, kidney (renal), and/or brain (cerebral) abnormalities.
In individuals with Chromosome 6, Partial Trisomy 6q, all or a portion of the end (distal) region of the long arm (q) of chromosome 6 is present three times (trisomy) rather than twice in cells of the body. Chromosomes are found in the nucleus of all body cells. They carry the genetic characteristics of each individual. Pairs of human chromosomes are numbered from 1 through 22, with an unequal 23rd pair of X and Y chromosomes for males and two X chromosomes for females. Each chromosome has a short arm designated as “p” and a long arm identified by the letter “q.” Chromosomes are further subdivided into regions and bands that are numbered. For example, the distal region of the long arm of chromosome 6, which is referred to as “6q2,” includes bands 6q21 to 6q27.
Reports indicate that, in those with the disorder, the duplicated portion of 6q2 has begun at various points (i.e., breakpoints) between bands 6q21 to 6q26 and may extend to the end (or “terminal”) of chromosome 6q (qter). It is possible that the range and severity of associated symptoms may depend on the specific length and location of the duplicated portion of 6q.
In most reported cases, Chromosome 6, Partial Trisomy 6q has resulted from a balanced chromosomal rearrangement in one of the parents, usually of maternal origin. However, paternal chromosomal rearrangements have been reported in rare instances. The parental chromosomal rearrangement has usually been a “balanced translocation”. Translocations occur when portions of certain chromosomes break off and are rearranged, resulting in shifting of genetic material and an altered set of chromosomes. If a chromosomal rearrangement is balanced, meaning that it consists of an altered but balanced set of chromosomes, it is usually harmless to the carrier. However, such a chromosomal rearrangement may be associated with an increased risk of abnormal chromosomal development in the carrier’s offspring.
Rare cases have also been reported in which the parental chromosomal rearrangement has been an inversion. An inversion is characterized by breakage of a chromosome in two places and reunion of the segment in the reverse order.
There have also been rare cases in which Chromosome 6, Partial Trisomy 6q has appeared to result from spontaneous (de novo) errors very early in embryonic development. In such de novo cases, the parents of the affected child usually have normal chromosomes and a relatively low risk of having another child with the chromosomal abnormality.
Chromosomal analysis and genetic counseling are typically recommended for parents of an affected child to help confirm or exclude the presence of a balanced translocation or other chromosomal rearrangement in one of the parents.
Chromosome 6, Partial Trisomy 6q is an extremely rare chromosomal disorder that appears to affect males and females equally. Approximately 30 cases have been reported in the medical literature.
In some cases, Chromosome 6, Partial Trisomy 6q may be suggested before birth (prenatally) by specialized tests such as ultrasound, amniocentesis, and/or chorionic villus sampling (CVS). During fetal ultrasonography, reflected sound waves create an image of the developing fetus, potentially revealing certain findings that suggest a chromosomal disorder or other abnormalities. With amniocentesis, a sample of fluid that surrounds the developing fetus is removed and analyzed, while CVS involves the removal of tissue samples from a portion of the placenta. Chromosomal studies performed on such fluid or tissue samples may reveal the presence o. Partial Trisomy 6q.
Chromosome 6, Partial Trisomy 6q may also be diagnosed and/or confirmed after birth (postnatally) by a thorough clinical evaluation, characteristic physical findings, and chromosomal analysis. Additional specialized tests may also be performed to help detect and/or characterize certain abnormalities that may be associated with the disorder.
The treatment of Chromosome 6, Partial Trisomy 6q is directed toward the specific symptoms that are apparent in each individual. Such treatment may require the coordinated efforts of a team of medical professionals, such as pediatricians, surgeons; physicians who specialize in disorders of the skeleton, muscles, joints, and related tissues (orthopedists), physical therapists, and/or other health care professionals.
For some affected individuals, physicians may recommend surgical correction of certain craniofacial, limb, genital, and/or internal organ malformations associated with the disorder. In addition, physical therapy, the use of certain orthopedic appliances, and/or additional orthopedic techniques, including surgery, may be advised to help manage musculoskeletal abnormalities, such as joint contractures and scoliosis. The surgical procedures performed will depend upon the severity of the anatomical abnormalities, their associated symptoms, and other factors.
Early intervention may be important in ensuring that affected children reach their potential. Special services that may be beneficial include special education and/or other medical, social, and/or vocational services. Genetic counseling will also be of benefit for the families of affected individuals. Other treatment for this disorder is symptomatic and supportive.
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