Pentasomy X

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Pentasomy X
Other names49,XXXXX
Foetal karyotype demonstrating pentasomy X.png
Karyotype of pentasomy X
Specialty Medical genetics   OOjs UI icon edit-ltr-progressive.svg
Symptoms Intellectual disability, facial dysmorphisms, heart defects
Usual onsetConception
DurationLifelong
Causes Nondisjunction
Diagnostic method Karyotype

Pentasomy X, also known as 49,XXXXX, is a chromosomal disorder in which a female has five, rather than two, copies of the X chromosome. Pentasomy X is associated with short stature, intellectual disability, characteristic facial features, heart defects, skeletal anomalies, and pubertal and reproductive abnormalities. The condition is exceptionally rare, with an estimated prevalence between 1 in 85,000 and 1 in 250,000.

Contents

The condition has a large variety of symptoms, and it is difficult to paint a conclusive portrait of its phenotypes. Though significant disability is characteristic, there are so few diagnosed cases that confident conclusions about the presentation and prognosis remain impossible. Pentasomy X may be mistaken for more common chromosomal disorders, such as Down syndrome or Turner syndrome, before a conclusive diagnosis is reached.

Pentasomy X is not inherited but rather occurs via nondisjunction, a random event in gamete development. In rare cases, it may be related to a parent's chromosomal mosaicism. The karyotype observed in pentasomy X is formally known as 49,XXXXX, which represents the 49 chromosomes observed in the disorder as compared to the 46 in typical human development.

Presentation

The major clinical features of pentasomy X are intellectual disability, short stature, facial and musculoskeletal abnormalities, and congenital heart defects. [1] Although one recorded case has been of low average intelligence, [2] all other known cases have been intellectually disabled, with an average IQ of 50. [1] The overall portrait is one of moderate intellectual disability, [3] defined by an adult cognitive capacity similar to that of a six- to eight-year-old and the ability to acquire basic living and employment skills with support. [4] Some girls with pentasomy X attend special education in mainstream schools through mainstreaming or inclusion, while some attend special schools. [3]

A 15-year-old girl with pentasomy X, demonstrating facial, limb, and skeletal features Pentasomy X phenotype.png
A 15-year-old girl with pentasomy X, demonstrating facial, limb, and skeletal features

Pentasomy X is associated with a number of physical anomalies, including short stature, clinodactyly (incurved pinky fingers), and distinctive facial features. Common findings include microcephaly, low-set ears, hypertelorism (wide-spaced eyes), and epicanthic folds. [5] The characteristic facies have been described as "coarse", [1] much like those of the related disorder tetrasomy X. [6] Pentasomy X is unique amongst X-chromosome polysomies for its association with short stature, when most related disorders are associated with tall stature; [7] the average height in pentasomy X is one standard deviation below the norm. [8] Hypotonia, often severe, is a frequent finding, as are related musculoskeletal issues such as hip dysplasia. [9] The severity of repeated joint dislocations may lead to a differential diagnosis of Larsen syndrome, as suggested in one reported case. [10] Bone maturation may be delayed. [11] Another skeletal finding is taurodontism, where the pulp of the teeth is enlarged into the roots; [5] other dental abnormalities, such as missing teeth and severe tooth decay, have also been reported. [12] These findings are not specific to pentasomy X, but rather common to sex chromosome aneuploidies in general and in particular show a strong resemblance to the male counterpart 49,XXXXY. [13] Epicanthic folds and hypertelorism are also observed in tetrasomy and trisomy X, [7] while clinodactyly and radioulnar synostosis are seen in all sex chromosome aneuploidies [1] [14] [15] and taurodontism is specifically common to X-chromosome polysomies. [16]

Heart defects are associated with the syndrome. Pentasomy X has one of the highest rates of congenital heart defects of any chromosomal disorder, with 56.5% of recorded patients having a heart defect of some kind. Patent ductus arteriosus is particularly frequent. [3] [17] The majority of such conditions resolve without surgical treatment, although a minority require it. [3] Ventricular septal defects are also frequent. [1] Other internal medical issues frequently recorded include kidney and urinary defects. [3] Epilepsy has been associated with the condition, [18] though seems to be rare. [3] In sex chromosome aneuploidies as a whole, epilepsy is usually mild and amenable to treatment, [19] and reports of epilepsy in pentasomy X have described it resolving with treatment and allowing antiepileptic drugs to eventually be stopped. [3]

Puberty is altered in pentasomy X, although as few adults with the condition have been reported, the full scope of such alterations is unclear. In the sister condition of tetrasomy X, half of all women undergo puberty normally, while half have no or incomplete puberty. [1] Some adolescents and adults with pentasomy X have been prepubertal, [1] while some have had premature ovarian failure (early menopause) [20] and some have had apparently non-noteworthy pubertal development. [21] [22] Though external genitalia is generally normal, underlying gonadal dysfunction is frequent, including ovarian dysfunction or an unusually small uterus. [17] No cases are known of women with pentasomy X having children, but although fertility is likely reduced, some may be able to. [23]

Little is understood about the psychological and behavioural phenotype of pentasomy X. Girls and women with the disorder are frequently described as shy and cooperative. [1] Such traits are common to other conditions involving extra copies of the X chromosome. [15] [24] Developmental delays can cause difficulty communicating, resulting in frustration and tantrums. Overall, the syndrome is not associated with severe behavioural issues. [3]

A number of disorders have been reported as comorbid with sex chromosome aneuploidies, including pentasomy X. In one case report, pentasomy X occurred alongside the similarly rare hyperimmunoglobulin E syndrome. [25] Other possibly coincidental associations have included cerebral palsy [22] and Dandy–Walker malformation. [26]

Causes

Maternal age in 21 cases of pentasomy X, showing the unclear relationship Pentasomy X maternal age.png
Maternal age in 21 cases of pentasomy X, showing the unclear relationship

Pentasomy X is caused by nondisjunction, a process through which gametes (eggs or sperm) with too many or too few chromosomes are produced. In nondisjunction, homologous chromosomes or sister chromatids fail to separate properly when producing gametes. [27] In sex chromosome tetrasomy and pentasomy, the extra chromosomes are consistently inherited from one parent. [1] In the specific case of pentasomy X, all known cases have inherited the additional chromosomes from the mother. This has been suggested to relate to genomic imprinting; specifically, it is hypothesized that specific loci on the sex chromosomes are affected by imprinting such that only maternal overimprinting is survivable, and cases of pentasomy X where the additional chromosomes were inherited from the father would be incompatible with life. [28] As well as during gamete development, nondisjunction can occur after conception, resulting in a mosaic karyotype. [29]

Nondisjunction is related to advanced maternal age, [30] although due to its rarity, the maternal age effect in pentasomy X is unclear. [31] More common aneuploidy syndromes, such as Down syndrome and Klinefelter's syndrome, have strong relationships with maternal age. [32] [33] Pentasomy X is not inherited [5] and is not caused by the actions of the parents. [34] However, in rare cases, pentasomy X may be related to chromosomal mosaicism in a parent. [3] [35]

X inactivation is a major factor in pentasomy X. X inactivation is the process through which genes in second (or higher) copies of the X chromosome are turned off, such that any cell has only one active copy of the chromosome. [36] However, X inactivation appears to be disrupted in pentasomy X, allowing up to half of the supposedly inactive genetic material to actually work. This is assumed to contribute to the severe phenotype of the condition compared to other sex chromosome aneuploidies. [21]

Diagnosis

Chromosome aneuploidies such as pentasomy X are diagnosed through the process of karyotyping, or chromosome testing. [37] Diagnosis cannot be made on the basis of phenotype alone, as multiple other conditions present similarly. [1]

The phenotype of pentasomy X is not specific to the disorder, and many other conditions can be differential diagnoses. One is tetrasomy X, a related disorder in which a girl or woman has four copies of the X chromosome. The general profiles of the conditions are similar, with developmental delays, mild dysmorphic features, and shared congenital anomalies such as clinodactyly and radioulnar synostosis. However, the phenotype of pentasomy X is more severe than that of tetrasomy X, with lower IQ and more severe dysmorphism. Pentasomy X also has additional characteristics uncommon in the tetrasomy, such as short stature. [1] [7] Mosaic karyotypes, with both 48,XXXX and 49,XXXXX cells, are also possible. Though very few mosaic cases have been reported, the phenotype appears intermediate in severity between tetrasomy and pentasomy X. [9]

Another potential differential diagnosis is Down syndrome. The features of the two conditions overlap, and some girls with pentasomy X may be assumed to have Down's before genetic ascertainment. [5] Some cases of pentasomy X have had family histories of Down syndrome, inciting speculation that the conditions may tend to recur in the same family lines; alternatively, it may suggest that some patients diagnosed with Down syndrome on the basis of phenotype may actually have pentasomy X. [17]

The phenotype of pentasomy X has also been compared to that of Turner syndrome, characterized by a female having one copy of the X chromosome. Both Turner's and pentasomy X are female-only disorders characterized by short stature, heart defects, and abnormal pubertal development. However, the intellectual disabilities observed in pentasomy X are rare in Turner syndrome. [5]

Prognosis

The long-term prognosis of pentasomy X is unclear, due to its low prevalence. [1] Though some reviews claim a poor prognosis due to the congenital defects observed in severe cases, [9] support groups report milder abnormalities than common in the medical literature, including adults with pentasomy X in fair health. [3] The spectrum of severity varies; long-term support is consistent, though some women have been reported as being able to work part-time and manage some of their affairs. [18] For sex chromosome tetrasomy and pentasomy disorders as a whole, good prognosis is linked to strong parental and personal support. Girls and women with pentasomy X whose caregivers have acted as advocates for their success have been reported as achieving at higher personal and social levels than the general portrait of the medical literature. [1]

Epidemiology

Pentasomy X is exceptionally rare. The disorder is estimated to occur in approximately 1 in 250,000 females. [38] Some higher estimates posit the condition may be as frequent as 1 in 85,000, as observed in the related 49,XXXXY syndrome. [21] Fewer than thirty cases of the disorder have been reported in the medical literature, although it is speculated that many more cases have gone undiagnosed. [2] Pentasomy X only occurs in females, as the Y chromosome is in most cases necessary for male sexual development. [5]

History

Pentasomy X was first diagnosed in 1963, in a two-year-old girl karyotyped for severe intellectual disability. At the time, four cases of XXXXY syndrome had already been recorded. [39] Pentasomy X was one of the later sex chromosome aneuploidies to be discovered, being preceded by Turner, [40] Klinefelter, [41] and trisomy X [42] in 1959, XXYY syndrome in 1960, [43] and XYY [44] and tetrasomy X [45] in 1961. By the time of Linden, Bender, and Robinson's seminal review of sex chromosome tetrasomy and pentasomy in 1995, only 25 cases had been recorded, the eldest in a girl of 16. [1] As late as 2011, reviews claimed no adult women with pentasomy X had been ascertained, [9] though chromosomal disorder organization Unique noted in 2005 its oldest member with pentasomy X was 29 years old. [3]

See also

Related Research Articles

<span class="mw-page-title-main">Autosome</span> Any chromosome other than a sex chromosome

An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosomal pairs, which may have different structures. The DNA in autosomes is collectively known as atDNA or auDNA.

<span class="mw-page-title-main">Trisomy</span> Abnormal presence of three copies of a particular chromosome

A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two. A trisomy is a type of aneuploidy.

<span class="mw-page-title-main">Aneuploidy</span> Presence of an abnormal number of chromosomes in a cell

Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human somatic cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any number of complete chromosome sets is called a euploid cell.

<span class="mw-page-title-main">Trisomy 18</span> Chromosomal disorder in which there are three copies of chromosome 18

Trisomy 18, also known as Edwards syndrome, is a genetic disorder caused by the presence of a third copy of all or part of chromosome 18. Many parts of the body are affected. Babies are often born small and have heart defects. Other features include a small head, small jaw, clenched fists with overlapping fingers, and severe intellectual disability.

<span class="mw-page-title-main">Nondisjunction</span> Failure to separate properly during cell division

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (mitosis/meiosis). There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

<span class="mw-page-title-main">Small supernumerary marker chromosome</span> Abnormal partial or mixed chromosome

A small supernumerary marker chromosome (sSMC) is an abnormal extra chromosome. It contains copies of parts of one or more normal chromosomes and like normal chromosomes is located in the cell's nucleus, is replicated and distributed into each daughter cell during cell division, and typically has genes which may be expressed. However, it may also be active in causing birth defects and neoplasms. The sSMC's small size makes it virtually undetectable using classical cytogenetic methods: the far larger DNA and gene content of the cell's normal chromosomes obscures those of the sSMC. Newer molecular techniques such as fluorescence in situ hybridization, next generation sequencing, comparative genomic hybridization, and highly specialized cytogenetic G banding analyses are required to study it. Using these methods, the DNA sequences and genes in sSMCs are identified and help define as well as explain any effect(s) it may have on individuals.

<span class="mw-page-title-main">Polysomy</span> Abnormal multiples of one or more chromosomes

Polysomy is a condition found in many species, including fungi, plants, insects, and mammals, in which an organism has at least one more chromosome than normal, i.e., there may be three or more copies of the chromosome rather than the expected two copies. Most eukaryotic species are diploid, meaning they have two sets of chromosomes, whereas prokaryotes are haploid, containing a single chromosome in each cell. Aneuploids possess chromosome numbers that are not exact multiples of the haploid number and polysomy is a type of aneuploidy. A karyotype is the set of chromosomes in an organism and the suffix -somy is used to name aneuploid karyotypes. This is not to be confused with the suffix -ploidy, referring to the number of complete sets of chromosomes.

<span class="mw-page-title-main">XXYY syndrome</span> Extra X and Y chromosome in males

XXYY syndrome is a sex chromosome anomaly in which males have two extra chromosomes, one X and one Y chromosome. Human cells usually contain two sex chromosomes, one from the mother and one from the father. Usually, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The appearance of at least one Y chromosome with a properly functioning SRY gene makes a male. Therefore, humans with XXYY are genotypically male. Males with XXYY syndrome have 48 chromosomes instead of the typical 46. This is why XXYY syndrome is sometimes written as 48, XXYY syndrome or 48, XXYY. It affects an estimated one in every 18,000–40,000 male births.

The Pallister–Killian syndrome (PKS), also termed tetrasomy 12p mosaicism or the Pallister mosaic aneuploidy syndrome, is an extremely rare and severe genetic disorder. PKS is due to the presence of an extra and abnormal chromosome termed a small supernumerary marker chromosome (sSMC). sSMCs contain copies of genetic material from parts of virtually any other chromosome and, depending on the genetic material they carry, can cause various genetic disorders and neoplasms. The sSMC in PKS consists of multiple copies of the short arm of chromosome 12. Consequently, the multiple copies of the genetic material in the sSMC plus the two copies of this genetic material in the two normal chromosome 12's are overexpressed and thereby cause the syndrome. Due to a form of genetic mosaicism, however, individuals with PKS differ in the tissue distributions of their sSMC and therefore show different syndrome-related birth defects and disease severities. For example, individuals with the sSMC in their heart tissue are likely to have cardiac structural abnormalities while those without this sSMC localization have a structurally normal heart.

<span class="mw-page-title-main">XXXXY syndrome</span> Chromosomal anomaly

49,XXXXY syndrome is an extremely rare aneuploidic sex chromosomal abnormality. It occurs in approximately 1 out of 85,000 to 100,000 males. This syndrome is the result of maternal non-disjunction during both meiosis I and II. It was first diagnosed in 1960 and was coined Fraccaro syndrome after the researcher.

A chromosomal abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where there is an atypical number of chromosomes, or as structural abnormalities, where one or more individual chromosomes are altered. Chromosome mutation was formerly used in a strict sense to mean a change in a chromosomal segment, involving more than one gene. Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis. Chromosome abnormalities may be detected or confirmed by comparing an individual's karyotype, or full set of chromosomes, to a typical karyotype for the species via genetic testing.

<span class="mw-page-title-main">Klinefelter syndrome</span> Human chromosomal condition

Klinefelter syndrome (KS), also known as 47,XXY, is a chromosome anomaly where a male has an extra X chromosome. These complications commonly include infertility and small, poorly functioning testicles. These symptoms are often noticed only at puberty, although this is one of the most common chromosomal disorders, occurring in one to two per 1,000 live births. It is named after American endocrinologist Harry Klinefelter, who identified the condition in the 1940s, along with his colleagues at Massachusetts General Hospital.

Sex chromosome anomalies belong to a group of genetic conditions that are caused or affected by the loss, damage or addition of one or both sex chromosomes.

The Focus Foundation, located in Davidsonville, Maryland, is a research foundation dedicated to identifying and assisting families and children who have X and Y Chromosomal Variations, dyslexia and/or developmental coordination disorder. These conditions can lead to language-based disabilities, motor planning deficits, reading dysfunction, and attention and behavioral disorders. The Focus Foundation believes that, through increased awareness, early identification, and specific and targeted treatment, children with these conditions can reach their full potential.

XXXYsyndrome is a genetic condition characterized by a sex chromosome aneuploidy, where individuals have two extra X chromosomes. People in most cases have two sex chromosomes: an X and a Y or two X chromosomes. The presence of one Y chromosome with a functioning SRY gene causes the expression of genes that determine maleness. Because of this, XXXY syndrome only affects males. The additional two X chromosomes in males with XXXY syndrome causes them to have 48 chromosomes, instead of the typical 46. XXXY syndrome is therefore often referred to as 48,XXXY. There is a wide variety of symptoms associated with this syndrome, including cognitive and behavioral problems, taurodontism, and infertility. This syndrome is usually inherited via a new mutation in one of the parents' gametes, as those affected by it are usually infertile. It is estimated that XXXY affects one in every 50,000 male births.

<span class="mw-page-title-main">XYYY syndrome</span> Chromosomal disorder

XYYY syndrome, also known as 48,XYYY, is a chromosomal disorder in which a male has two extra copies of the Y chromosome. The syndrome is exceptionally rare, with only twelve recorded cases. The presentation of the syndrome is heterogeneous, but appears to be more severe than its counterpart XYY syndrome. Common traits include borderline to mild intellectual disability, infertility, radioulnar synostosis, and in some cases tall stature.

<span class="mw-page-title-main">Tetrasomy X</span> Chromosomal disorder with 4 X chromosomes

Tetrasomy X, also known as 48,XXXX, is a chromosomal disorder in which a female has four, rather than two, copies of the X chromosome. It is associated with intellectual disability of varying severity, characteristic "coarse" facial features, heart defects, and skeletal anomalies such as increased height, clinodactyly, and radioulnar synostosis. Tetrasomy X is a rare condition, with few medically recognized cases; it is estimated to occur in approximately 1 in 50,000 females.

<span class="mw-page-title-main">XYYYY syndrome</span> Rare chromosomal disorder

XYYYY syndrome, also known as 49,XYYYY, is an exceptionally rare chromosomal disorder in which a male human has three additional copies of the Y chromosome. Only seven non-mosaic cases of the disorder have ever been recorded in the medical literature, as well as five mosaic cases, of which two had more 48,XYYY than 49,XYYYY cells. Due to the extreme rarity of the disorder, little is understood about it, and the phenotype appears to be variable.

<span class="mw-page-title-main">Trisomy X</span> Chromosome disorder in women

Trisomy X, also known as triple X syndrome and characterized by the karyotype 47,XXX, is a chromosome disorder in which a female has an extra copy of the X chromosome. It is relatively common and occurs in 1 in 1,000 females, but is rarely diagnosed; fewer than 10% of those with the condition know they have it.

<span class="mw-page-title-main">XXXYY syndrome</span> Chromosomal disorder

XXXYY syndrome, also known as 49,XXXYY, is a chromosomal disorder in which a male has three copies of the X chromosome and two copies of the Y chromosome. XXXYY syndrome is exceptionally rare, with only eight recorded cases. Little is known about its presentation, but associated characteristics include intellectual disability, anomalies of the external genitalia, and characteristic physical and facial features. It is not caused by characteristics of the parents, but rather occurs via nondisjunction, a random event in gamete development. The karyotype observed in the syndrome is formally known as 49,XXXYY, which represents the 49 chromosomes observed in the disorder as compared to the 46 in normal human development.

References

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