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Monosomy

Monosomy is a type of chromosomal aneuploidy characterized by the absence of one chromosome from a diploid pair, resulting in a total of 45 chromosomes in human cells instead of the typical 46.^[1] This numerical abnormality disrupts the balanced genetic complement essential for normal development and function, often leading to severe developmental issues or embryonic lethality.^[2] Monosomy typically arises from nondisjunction, a process where homologous chromosomes or sister chromatids fail to separate properly during meiosis I, meiosis II, or mitosis, producing gametes or cells with an abnormal chromosome number.^[3] In humans, such errors occur sporadically in parental germ cells, with maternal nondisjunction being more common due to age-related declines in meiotic fidelity, though paternal contributions also play a role.^[3] The resulting zygote inherits an unpaired chromosome, manifesting as monosomy across all or mosaic portions of the body.^[4] Full monosomies are exceedingly rare in live births because they are generally incompatible with life, causing profound gene dosage imbalances that halt embryonic development early.^[3] The only known viable full monosomy in humans is Turner syndrome (45,X), which affects approximately 1 in 2,000 to 2,500 female births and results from the loss of one X chromosome, leading to characteristic features such as short stature, ovarian dysgenesis, and cardiovascular anomalies.^[5] Partial monosomies, involving deletions of chromosome segments, can also occur and are survivable in some cases, exemplified by cri-du-chat syndrome due to partial monosomy of the short arm of chromosome 5 (5p-), which presents with intellectual disability, microcephaly, and a distinctive cat-like cry.^[3] These conditions highlight monosomy's role in a spectrum of genetic disorders, with diagnosis often relying on karyotyping or advanced genomic techniques.^[2]

Fundamentals

Definition

Monosomy is a type of chromosomal abnormality classified under aneuploidy, characterized by the absence of one chromosome from a homologous pair in diploid organisms, leading to a total chromosome count of 2n-1 rather than the typical 2n.^[6] This condition disrupts the normal genomic balance, as cells possess only a single copy of genes on the affected chromosome instead of the usual two.^[1] Aneuploidy encompasses deviations in chromosome number, including both monosomy and trisomy.^[7] Monosomy can be complete, involving the loss of an entire chromosome, or partial, where only a segment of the chromosome is deleted, leaving one full chromosome plus a portion of the second.^[1] Complete monosomy results in the total absence of genetic material from that chromosome pair, whereas partial monosomy retains some but not all of the expected material, often due to deletions.^[8] The concept of monosomy emerged in the early 20th century through foundational cytogenetic studies examining chromosome structures and numbers in various organisms.^[9] In human genetics, it gained formal recognition following the 1959 identification of the chromosomal basis for Turner syndrome, which confirmed monosomy X as a viable condition in live births.^[10] This breakthrough, building on earlier clinical descriptions from 1938, established monosomy as a key factor in genetic disorders.^[11] In karyotype notation, monosomy is typically represented by indicating the reduced chromosome number and specifying the missing chromosome, such as 45,X for complete monosomy of the X chromosome in humans.^[5] This standard nomenclature, part of the International System for Human Cytogenomic Nomenclature, facilitates precise communication of chromosomal configurations.^[12]

Types

Monosomy is classified based on the chromosomes affected and the extent of the genetic material loss, distinguishing between autosomal and sex chromosome forms as well as complete, partial, and mosaic variants.^[13] Autosomal monosomy involves the absence of one copy of an autosome, referring to chromosomes 1 through 22 in humans, and is typically incompatible with development in viable organisms due to the severe imbalance in gene dosage.^[14] In contrast, sex chromosome monosomy affects one of the sex chromosomes, such as monosomy X (resulting in a 45,X karyotype, as seen in Turner syndrome) or the rarer monosomy Y (45,Y karyotype), which is generally non-viable owing to the lack of essential genetic content on the Y chromosome alone.^[5] Monosomy can be further categorized as complete or partial, with complete monosomy denoting the full loss of one chromosome from a pair, leading to a haploid state for that entire chromosome.^[15] Partial monosomy, on the other hand, arises from the deletion of a specific segment of a chromosome, resulting in only one copy of that chromosomal region while the rest remains diploid.^[16] An example of partial autosomal monosomy is cri-du-chat syndrome, caused by a deletion on the short arm of chromosome 5 (5p-).^[17] Mosaic monosomy occurs when the chromosomal loss is present in only a subset of cells within an individual, arising from post-zygotic errors and resulting in two or more genetically distinct cell lineages from a single zygote.^[18] These classifications also align with broader distinctions between numerical and structural aneuploidies, where numerical aneuploidy refers to the gain or loss of whole chromosomes, such as in the aneuploidy spectrum that includes both monosomy and trisomy.^[2] Structural abnormalities, conversely, involve alterations to chromosome architecture, like interstitial deletions, without changing the overall chromosome count, often resulting in partial monosomies.^[19]

Genetic Mechanisms

Causes

Monosomy arises primarily from errors in chromosome segregation during cell division, with nondisjunction being the most common mechanism. Nondisjunction occurs when homologous chromosomes fail to separate properly during anaphase I of meiosis, resulting in gametes that contain either an extra or a missing chromosome.^[3] Similarly, nondisjunction in meiosis II involves the failure of sister chromatids to separate during anaphase II, again producing gametes with an abnormal chromosome number.^[3] When such a gamete (with n-1 chromosomes) is fertilized by a normal gamete, the zygote develops with monosomy for that chromosome.^[7] In addition to meiotic errors, mitotic nondisjunction can occur during early embryonic cell divisions, leading to somatic mosaicism where only a portion of the body's cells exhibit the monosomy.^[3] This post-zygotic event creates a mixture of euploid and aneuploid cell lines, potentially resulting in milder phenotypes depending on the affected tissues.^[18] Other mechanisms contributing to monosomy include chromosome loss during cell division, often due to anaphase lag, where a chromosome fails to align properly at the metaphase plate and is excluded from the daughter nuclei, forming a micronucleus or being degraded.^[20] Structural chromosomal abnormalities, such as translocations, can also predispose cells to unbalanced segregation and subsequent loss of genetic material, increasing the likelihood of monosomy.^[21] Advanced maternal age is a significant environmental risk factor that elevates the incidence of nondisjunction, particularly in meiosis I, with the risk increasing substantially after age 35 due to age-related declines in oocyte quality and cohesin function.^[22] These processes collectively result in aneuploidy, manifesting as monosomy when a single chromosome is lost.^[3]

Inheritance Patterns

Monosomy typically arises de novo during gametogenesis or early embryonic development, rather than being inherited from parents with normal karyotypes. In most cases, the chromosomal loss occurs sporadically in the parental gametes or through postzygotic mitotic errors, resulting in a low recurrence risk for future pregnancies in affected families, generally less than 1%. For instance, in non-mosaic monosomy X (Turner syndrome), the condition does not run in families and is not passed from parent to child, with empirical recurrence rates estimated at under 1% following prenatal counseling. This de novo origin is attributed to mechanisms such as nondisjunction during meiosis. The parental origin of monosomy often involves errors in paternal meiosis, particularly for sex chromosome monosomies. In approximately 70-80% of monosomy X cases, the retained X chromosome is maternal, indicating loss of the paternal sex chromosome during spermatogenesis or early zygotic division. While advanced maternal age is associated with increased overall aneuploidy risk due to meiotic spindle disruptions, the predominant mechanism for viable monosomy X traces to paternal contributions rather than maternal errors. Autosomal monosomies, though rarely viable beyond embryogenesis, similarly originate de novo in gametes without a strong parental bias documented in surviving cases. Mosaic forms of monosomy present a rare exception to de novo occurrence, as they can potentially be transmitted if a parent harbors germline mosaicism. In such scenarios, a subset of parental germ cells carries the monosomic lineage, allowing partial inheritance to offspring, though this affects fewer than 5% of cases overall and carries unpredictable transmission risks. Population-level incidence underscores the sporadic nature of monosomy; for example, complete monosomy X occurs in about 1 in 2,500 live female births, but rates are markedly higher in early pregnancies, comprising 10-20% of chromosomally abnormal spontaneous abortions. These elevated embryonic rates highlight monosomy's role in prenatal selection, with most cases failing to progress to term.

Biological Effects

In Humans

In humans, monosomy is viable only in the case of sex chromosome involvement, specifically the 45,X karyotype associated with Turner syndrome, as complete autosomal monosomies are incompatible with postnatal life.^[23] Turner syndrome affects approximately 1 in 2,500 live-born females and manifests with characteristic features including short stature, primary ovarian insufficiency leading to infertility, congenital heart defects such as bicuspid aortic valve or coarctation of the aorta, and neck webbing due to lymphatic dysplasia.^[21] Approximately 99% of 45,X conceptuses are spontaneously aborted in the first trimester, contributing to the condition's underrepresentation at birth relative to its conception frequency.^[24] Complete autosomal monosomies, such as those of chromosomes 21 or 18, invariably result in early embryonic lethality due to the profound gene dosage imbalance affecting critical developmental pathways.^[18] Rare mosaic forms of autosomal monosomy have been documented, but they lead to severe multisystem anomalies and typically do not support full-term survival without significant mosaicism for a normal cell line.^[25] In contrast, partial autosomal monosomies, such as the 5p deletion in cri-du-chat syndrome, allow survival but cause distinctive phenotypes including intellectual disability, microcephaly, hypertelorism, and a high-pitched, cat-like cry in infancy due to laryngeal abnormalities.^[26] Mosaic variants of monosomy exhibit variable severity depending on the proportion of affected cells and the chromosome involved; for instance, 45,X/46,XX mosaicism in Turner syndrome often presents with milder features, such as reduced short stature and lower incidence of cardiac anomalies compared to non-mosaic 45,X cases.^[27] These mosaics arise postzygotically and can result in a spectrum of phenotypes, from subclinical to classic syndromic presentations.^[21] Monosomy X is highly prevalent in early human pregnancy losses, accounting for approximately 6–11% of spontaneous abortions, and is detectable via prenatal screening methods that identify chromosomal imbalances in embryonic tissue.^[28] This high embryonic loss rate underscores the intolerance of human development to single-chromosome loss beyond sex chromosomes.^[29]

In Non-Human Organisms

In plants, monosomy is relatively common and often viable, particularly in polyploid species where multiple genome copies provide buffering against gene dosage imbalances. For instance, in common wheat (Triticum aestivum, 2n=42), all 21 possible monosomic lines (2n-1=41) have been established and are viable to maturity, though they exhibit reduced vigor compared to euploid plants, especially under suboptimal conditions.^[30] These monosomics typically show decreased fertility, with nullisomic offspring frequencies ranging from 0.9% to 7.6% depending on the missing chromosome, and partial sterility in both male and female gametes; however, some lines maintain sufficient fertility for breeding purposes.^[30] In diploid plants like tomato (Solanum lycopersicum, 2n=24), monosomy is less tolerated, with primary and tertiary monosomics displaying poor transmission to progeny (none observed in over 11,000 offspring from tested lines) and morphological abnormalities, but they remain viable for short-term studies.^[31] In animals, monosomy is generally rare and poorly viable outside of sex chromosomes, highlighting greater sensitivity to aneuploidy in diploid genomes. Autosomal monosomies in mice (Mus musculus) are lethal during early embryonic development, with onset of lethality detectable as early as the third or fourth day for some chromosomes, and all cases resulting in death by the pre- or peri-implantation stage due to severe developmental disruptions.^[14] For sex chromosome monosomy, XO individuals in fruit flies (Drosophila melanogaster) develop as viable but sterile males, as the Y chromosome is required for fertility but not for sex determination or basic viability, which is governed by the X-to-autosome ratio.00205-9) This contrasts with the lethality of autosomal monosomies in the same species, underscoring the specialized tolerance for sex-linked imbalances in certain invertebrates. Monosomy contributes to evolutionary processes in plants, particularly by driving speciation and hybrid sterility in polyploid contexts. In polyploid hybrids, unbalanced chromosome transmission can generate aneuploid gametes, leading to partial or complete sterility that reinforces reproductive isolation between nascent species and their progenitors; this mechanism is especially prevalent in allopolyploids, where monosomy exacerbates dosage conflicts between divergent homeologous chromosomes.^[32] Such instability promotes rapid karyotype evolution, facilitating the establishment of new polyploid lineages through compensatory adjustments or selection for balanced genotypes.^[32] Monosomic lines serve as key research models in plant cytogenetics for mapping genes and elucidating chromosome functions. In wheat, monosomic analysis has been instrumental in localizing genes to specific chromosomes by observing phenotypic effects in deficient backgrounds, enabling high-resolution genetic mapping and the development of substitution lines for crop improvement.^[30] Similarly, in tomato, monosomics and derived addition lines have been used to identify chromosomal regions controlling traits like morphology and fertility, with molecular cytogenetic tools such as GISH and RFLP accelerating the characterization of chromosome-specific functions despite transmission challenges.^[31]^[33] These approaches provide insights into gene dosage effects and genome organization without relying on full genome sequencing.

Clinical Considerations

Diagnosis

Diagnosis of monosomy typically involves cytogenetic and molecular techniques to detect the absence of one chromosome from the normal diploid set, resulting in a 45-chromosome karyotype. Karyotyping remains the gold standard for confirming complete monosomy, as it provides a visual representation of all chromosomes in metaphase spreads from cultured cells, such as those from peripheral blood, amniotic fluid, or chorionic villi.^[34] This conventional method can identify numerical abnormalities like monosomy X (Turner syndrome) with high accuracy but may miss low-level mosaicism or small deletions.^[35] Advanced diagnostic tools enhance detection of partial monosomies and mosaicism. Fluorescence in situ hybridization (FISH) uses fluorescent probes to target specific chromosomal regions, allowing rapid identification of partial monosomies or structural variants in interphase or metaphase cells, often as a complementary test to karyotyping.^[34] Chromosomal microarray (CMA), including array comparative genomic hybridization (aCGH) or single nucleotide polymorphism (SNP) arrays, offers higher resolution by detecting copy number variations as small as 50 kb across the genome, making it a first-tier test for individuals with developmental delays or congenital anomalies suggestive of monosomy.^[36] Next-generation sequencing (NGS)-based approaches, such as whole-genome sequencing or targeted panels, are particularly useful for identifying mosaic monosomy at low levels (e.g., <10% affected cells) by quantifying allele frequencies or read depths.^[37] Prenatal diagnosis is critical for early detection, especially in high-risk pregnancies. Non-invasive prenatal testing (NIPT) analyzes cell-free fetal DNA in maternal blood from as early as 10 weeks gestation, screening for aneuploidies with high sensitivity—approximately 99% for monosomy X—though it is not diagnostic and requires confirmation.^[38] If NIPT indicates risk, invasive procedures like chorionic villus sampling (CVS) at 10-13 weeks or amniocentesis at 15-16 weeks provide cells for karyotyping, CMA, or other analyses, with detection rates exceeding 99% for complete monosomies but potential for confined placental mosaicism.^[39] These methods balance fetal risk (e.g., 0.1-0.5% miscarriage rate for invasive tests) with diagnostic precision.^[40] Postnatally, diagnosis is often prompted by clinical or imaging findings. Ultrasound anomalies, such as cystic hygroma or increased nuchal translucency observed prenatally but evaluated postnatally, frequently lead to karyotyping in newborns with suspected Turner syndrome or other monosomies.^[21] In cases of mosaic monosomy, postnatal testing may involve multiple tissue samples (e.g., blood and skin fibroblasts) to assess variability.^[41]

Management and Prognosis

Management of monosomy in humans primarily focuses on supportive and symptomatic care, as there is no cure for the underlying chromosomal abnormality. For Turner syndrome (45,X), the most viable full monosomy, treatment begins early to address growth and developmental issues. Growth hormone therapy is typically initiated as early as age 2 to 6 years if growth failure is evident, with height monitored every 3-4 months in the first year and every 4-6 months thereafter, leading to an average adult height gain of 5-10 cm when combined with appropriate dosing.^[42]^[43]^[44]^[45] Estrogen replacement therapy, often starting with low-dose transdermal estradiol around age 11-12 to mimic natural puberty, is essential for those with ovarian failure to support secondary sexual characteristics, bone health, and cardiovascular protection; it continues lifelong and may be paired with progestin to induce menstruation.^[46]^[47] Recent guidelines also recommend discussing fertility preservation options, such as oocyte cryopreservation, for post-menarcheal individuals with potential fertility, and establishing a structured transition to adult care in early adolescence.^[45] Cardiac management includes regular monitoring with echocardiography or MRI for aortic dilatation and coarctation of the aorta, with surgical intervention as needed to correct structural defects and reduce complication risks.^[48]^[49] Partial monosomies, such as those in cri-du-chat syndrome (5p deletion), require multidisciplinary approaches centered on symptom alleviation rather than reversal of the genetic loss. Care involves coordinated input from physical therapists to improve motor skills, speech therapists to address communication delays, occupational therapists for daily functioning, and genetic counselors for family support; early intervention programs enhance developmental outcomes.^[50]^[51]^[52] No specific pharmacological treatments target the monosomy itself, so management emphasizes nutritional support, behavioral therapies, and monitoring for associated issues like scoliosis or feeding difficulties. Prognosis varies by type and timely intervention but has improved with modern care. In treated Turner syndrome, life expectancy is reduced compared to the general population (by approximately 10 years), primarily due to cardiovascular complications, though modern management mitigates some risks.^[53]^[48]^[45] Growth hormone therapy not only boosts stature but also supports overall health without increasing aortic enlargement. For autosomal partial monosomies like cri-du-chat, survival into adulthood is common with supportive care—over 80% reach age 20—despite persistent intellectual disabilities and potential need for lifelong assistance.^[54] Genetic counseling plays a key role post-diagnosis to assess recurrence risks and reproductive options. For de novo monosomies, the recurrence risk in future pregnancies is low, typically less than 1%, though parental karyotyping is recommended to rule out balanced translocations.^[55] Couples may pursue in vitro fertilization with preimplantation genetic testing (PGT-A) to screen embryos for aneuploidies like monosomy, enabling selection of chromosomally normal ones for transfer.^[56]^[57]

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