Chromosome 22 is one of the smallest human chromosomes, carrying roughly 52,000 kilobases of DNA, yet it is disproportionately important in human health.1PubMed Central. Human chromosome 22 It was the first human chromosome to have its DNA sequence reported, back in 1999, revealing at least 545 genes packed into about 33.4 megabases of usable sequence.2PubMed. The DNA sequence of human chromosome 22 Those genes participate in immune defense, brain development, heart formation, and tumor suppression, and when things go wrong on this chromosome, the consequences range from leukemia to intellectual disability to congenital heart defects.
Structure and Size
Chromosome 22 is an acrocentric chromosome, meaning its centromere sits near one end rather than in the middle. The short arm is mostly made up of heterochromatin and contains ribosomal RNA genes, which help build the molecular machinery cells use to make proteins.1PubMed Central. Human chromosome 22 Staining studies have identified two active regions on the short arm that code for ribosomal RNA, though the presence of extra gene copies there does not seem to increase pairing with other acrocentric chromosomes during cell division.3PubMed. Human inherited marker chromosome 22 short-arm enlargement: investigation of rDNA gene multiplicity, Ag-band size, and acrocentric association
The long arm, labeled “q,” is where the action is. It holds the protein-coding genes, pseudogenes, and regulatory sequences that underlie most of the disorders discussed below. For decades, the short arms of acrocentric chromosomes like 22 remained poorly characterized because they are packed with highly repetitive sequences that traditional sequencing technology could not resolve. The Telomere-to-Telomere (T2T) Consortium changed that by producing a gapless sequence of the entire human genome, including the short arms of all five acrocentric chromosomes, unlocking nearly 200 million base pairs of previously unresolved sequence across the genome.4PubMed Central. The complete sequence of a human genome
Why Chromosome 22 Is Prone to Rearrangements
A distinctive feature of chromosome 22’s long arm is the presence of large blocks of nearly identical DNA called low copy repeats, known as LCR22s. These repeated stretches are unusually big and structurally variable, and they sit in several locations along the 22q11 region.5PubMed Central. The 22q11 low copy repeats are characterized by unprecedented size and structural variability Because they look so similar to each other, the cell’s DNA repair machinery sometimes lines them up incorrectly during cell division, a process called non-allelic homologous recombination. When this happens, chunks of the chromosome can be deleted, duplicated, or rearranged.
Optical mapping studies have shown that within these repeat blocks, certain sequences containing members of the FAM230 gene family appear to serve as preferred sites where these misaligned recombination events occur.6Scientific Reports. Optical mapping of the 22q11.2DS region reveals complex repeat structures and preferred locations for non-allelic homologous recombination (NAHR) This structural vulnerability is the reason chromosome 22 gives rise to an unusually wide array of deletion and duplication syndromes relative to its size. The repeat architecture essentially sets a trap: the more identical the flanking sequences, the higher the chance the cell will make a copying error during meiosis.
22q11.2 Deletion Syndrome
The most common disorder linked to these rearrangements is 22q11.2 deletion syndrome, historically known under several names including DiGeorge syndrome and velocardiofacial syndrome. It results from losing a stretch of DNA on the long arm of chromosome 22, typically about 3 megabases, and it is among the most frequent chromosomal microdeletions in humans. The hallmark features include heart defects, underdevelopment of the thymus and parathyroid glands, distinctive facial features, and dental abnormalities.7PubMed Central. Understanding the role of Tbx1 as a candidate gene for 22q11.2 deletion syndrome
Most of the developmental problems trace back to abnormal formation of the pharyngeal apparatus, the embryonic structures that give rise to parts of the face, heart, and neck. Research in mice has identified a gene called TBX1 as a major driver. Deleting just one copy of TBX1 in mice disrupts the fourth pharyngeal arch arteries, which contribute to the aortic arch, while losing both copies devastates the entire pharyngeal arch artery system.8PubMed. Tbx1 haploinsufficieny in the DiGeorge syndrome region causes aortic arch defects in mice The fact that losing a single copy is enough to cause problems explains why people carrying one deleted chromosome 22 still develop symptoms: the remaining copy alone cannot supply enough TBX1 protein for normal development.
The severity of the syndrome varies enormously from person to person, even among family members carrying the same deletion. Recent research has begun to explain why. A study of patients with congenital heart defects found that roughly 8.5% of those with 22q11.2 deletion syndrome also carried rare damaging variants in genes that regulate chromatin, the packaging around DNA. These chromatin-regulating genes interact with the TBX1 network, and when they are disrupted alongside TBX1 itself, heart defects become more likely.9npj Genomic Medicine. Chromatin regulators in the TBX1 network confer risk for conotruncal heart defects in 22q11.2DS In other words, the deletion sets the stage, but the genetic background determines how the scene plays out.
When the Same Region Is Duplicated Instead
The same repeat-driven mechanism that causes deletions at 22q11.2 can also produce duplications. People with 22q11.2 microduplication syndrome carry an extra copy of the same region that is lost in the deletion syndrome. The duplication is frequently inherited from a parent who may show no obvious symptoms, and its effects are highly variable and generally milder than those seen with the deletion.10Genetics in Medicine. Microduplications of 22q11.2 are frequently inherited and are associated with variable phenotypes
Some people with the duplication have developmental delays, speech problems, or subtle facial differences such as unusual ear shape or squared fingertips. Others have little to no apparent effect. Velopharyngeal insufficiency, where the soft palate does not close properly during speech, has been noted in several individuals, which is interesting because it also appears in the deletion syndrome.11The American Journal of Human Genetics. Microduplication 22q11.2, an Emerging Syndrome: Clinical, Cytogenetic, and Molecular Analysis of Thirteen Patients In rare cases the presentation can be severe, including cleft palate and significant jaw underdevelopment.12PubMed Central. 22q11.2 Microduplication: An Enigmatic Genetic Disorder The wide phenotypic range and frequent inheritance from seemingly unaffected parents make the duplication harder to diagnose and counsel about than the deletion.
Cat-Eye Syndrome
A different kind of duplication at the proximal end of 22q produces cat-eye syndrome. In this condition, a person carries extra copies of a small region at 22q11.1-q11.21, typically on a small extra marker chromosome. The name comes from the distinctive eye coloboma, a gap or notch in the iris, though not everyone with the syndrome has it. Other features can include anal malformations and, in some cases, hemifacial microsomia, where one side of the face is underdeveloped.13PubMed. Hemifacial microsomia in cat-eye syndrome: 22q11.1-q11.21 as candidate loci for facial symmetry Like most chromosome 22 disorders, the severity is unpredictable, and some carriers lead essentially normal lives while others require significant medical support.
Phelan-McDermid Syndrome
At the other end of chromosome 22’s long arm sits the SHANK3 gene, and its loss causes Phelan-McDermid syndrome. The deletion occurs at 22q13.3 and can result from a straightforward deletion, a translocation, or the formation of a ring chromosome.14PubMed Central. Deletion 22q13.3 syndrome SHANK3 codes for a scaffolding protein that holds together the structure of excitatory synapses in the brain, connecting surface receptors to the internal skeleton of the cell. When one copy is lost, the resulting shortage of SHANK3 protein leads to fewer dendrites and impaired communication between neurons.15PubMed Central. Phelan-McDermid Syndrome and SHANK3: Implications for Treatment
Children with Phelan-McDermid syndrome typically show global developmental delay, intellectual disability, severe speech delays, and low muscle tone. Autism spectrum disorder is common. Mouse models in which SHANK3 has been knocked out reproduce many of these features and confirm that SHANK3 is central to the syndrome, though other genes in the deleted region may contribute as well.16Molecular Syndromology. The 22q13.3 Deletion Syndrome (Phelan-McDermid Syndrome) Because the diagnosis depends on detecting the deletion or mutation rather than recognizing a consistent facial appearance, Phelan-McDermid syndrome is likely underdiagnosed, particularly in adults who received an autism or intellectual disability diagnosis before genetic testing became routine.
The Philadelphia Chromosome and Leukemia
Chromosome 22 plays a starring role in cancer biology through the Philadelphia chromosome, an abnormality discovered in the 1960s that became the first genetic alteration reliably linked to a specific human cancer. It arises from a reciprocal translocation between chromosomes 9 and 22, written as t(9;22)(q34;q11). The swap fuses part of the BCR gene on chromosome 22 to the ABL1 gene from chromosome 9, creating a hybrid BCR-ABL1 gene that produces an always-on signaling protein.17Cytokine & Growth Factor Reviews. BCR-ABL: The molecular mastermind behind chronic myeloid leukemia This drives the uncontrolled proliferation of white blood cells in chronic myeloid leukemia (CML).
The Philadelphia chromosome also appears in a subset of acute lymphoblastic leukemia cases. The development of imatinib, the first drug designed to block BCR-ABL1, transformed CML from a nearly always fatal diagnosis into a manageable chronic condition, with patients now often achieving near-normal life expectancy.18PubMed Central. Response and Resistance to BCR-ABL1-Targeted Therapies When resistance to imatinib arises, second-generation drugs like dasatinib, nilotinib, and bosutinib offer alternatives. For patients whose tumors harbor a particularly stubborn mutation called T315I, the third-generation drug ponatinib and the mechanistically distinct drug asciminib, which binds to a completely different site on the protein, provide additional options.19Pharmacological Research. Targeting BCR-Abl in the treatment of Philadelphia-chromosome positive chronic myelogenous leukemia CML treatment has become the textbook example of how understanding a single genetic event on a single chromosome can reshape a disease.
Tumor Suppressors on Chromosome 22
Beyond its role in leukemia, chromosome 22 carries tumor suppressor genes whose loss drives other cancers. The NF2 gene encodes a protein called Merlin that normally restrains cell growth. When both copies are knocked out, the result is neurofibromatosis type 2, a condition defined by the growth of tumors along nerves, most characteristically bilateral vestibular schwannomas, benign tumors on the nerves that control hearing and balance.20PubMed Central. Role of Merlin/NF2 inactivation in tumor biology
Meningiomas, tumors that grow from the membranes surrounding the brain and spinal cord, also have a deep connection to chromosome 22. Losing an entire copy of the chromosome, a state called monosomy 22, is the single most frequent genetic alteration found in meningiomas.21PubMed Central. Genetic/molecular alterations of meningiomas and the signaling pathways targeted Studies of sporadic meningiomas have found that about half show monosomy for the whole chromosome, while others carry terminal deletions of the long arm that include the NF2 region.22PubMed. Deletions on chromosome 22 in sporadic meningioma Mapping studies that compared tumor DNA to normal DNA in the same patients found that about 40% of meningiomas had lost one copy at every chromosome 22 marker tested, while an additional 14% showed partial losses consistent with terminal deletions, suggesting that this rearrangement is an early, driving event in meningioma formation.23PubMed Central. Deletion mapping of a locus on human chromosome 22 involved in the oncogenesis of meningioma
Another tumor suppressor on chromosome 22, SMARCB1, is linked to malignant rhabdoid tumors, aggressive cancers that typically appear in very young children. In one documented case, a child carried a germline deletion of about 2.8 megabases at 22q11.2 that included SMARCB1, and the tumor arose when the second copy of the gene was also knocked out by a separate deletion.24PubMed Central. Congenital anomalies and rhabdoid tumor associated with 22q11 germline deletion and somatic inactivation of the SMARCB1 tumor suppressor When SMARCB1 is lost, the regulatory landscape of the genome reshapes dramatically, activating distant enhancer sequences that switch on the MYC oncogene in patient-specific patterns.25Nature Communications. SMARCB1 loss activates patient-specific distal oncogenic enhancers in malignant rhabdoid tumors The fact that different tumors activate MYC through different enhancers may help explain why rhabdoid tumors vary in behavior and could eventually guide more personalized treatment.
Immune System Genes
Chromosome 22 is home to the immunoglobulin lambda (IGL) light-chain locus, one of three genomic regions that encode antibodies. The locus spans roughly 1,100 kilobases and contains dozens of variable (V) gene segments organized into three clusters, along with joining (J) and constant (C) gene segments.26PubMed. Organization of the human immunoglobulin lambda light-chain locus on chromosome 22q11.2 When your body encounters a pathogen, B cells recombine these segments to produce a vast repertoire of different antibodies, each tailored to recognize a particular invader.
Long-read sequencing studies across ethnically diverse populations have shown that this locus is far more variable between people than previously appreciated. Researchers identified 36 previously unknown alleles of IGL variable genes and found that the rate of single-nucleotide variation within these genes is far higher than the genome-wide background.27Genes & Immunity. Characterization of the immunoglobulin lambda chain locus from diverse populations reveals extensive genetic variation Older short-read sequencing data produced a high rate of false positives when analyzing this region, meaning earlier estimates of IGL diversity were unreliable. Understanding this diversity matters because variation in antibody genes can influence how effectively someone responds to vaccines and infections.
COMT, PRODH, and Psychiatric Risk
Two genes on chromosome 22 sit at the intersection of metabolism and mental health. The COMT gene encodes an enzyme that breaks down dopamine and other catecholamines in the brain’s prefrontal cortex. It contains a well-studied variation where one version of the protein (Val) works faster than the other (Met), meaning people with the Val form clear dopamine more quickly.28PubMed Central. Is COMT a susceptibility gene for schizophrenia? Because COMT sits within the region deleted in 22q11.2 deletion syndrome, people with that syndrome have only one working copy. Studies have found that which version they retain, Val or Met, influences prefrontal cognitive function.29PubMed. Effects of a functional COMT polymorphism on prefrontal cognitive function in patients with 22q11.2 deletion syndrome
The relationship between COMT and schizophrenia risk in 22q11.2 deletion syndrome turns out to be unexpectedly complex, with interactions involving sex and additional factors. In one study, patients with the deletion who developed schizophrenia were more often Val hemizygous (carrying the faster-acting enzyme), while those without schizophrenia were more often Met hemizygous, and significant interactions between the COMT variant and sex were observed on dopamine-related markers.30PubMed. Dopamine metabolism in adults with 22q11 deletion syndrome, with and without schizophrenia–relationship with COMT Val¹⁰⁸/¹⁵⁸Met polymorphism, gender and symptomatology
Nearby on chromosome 22 sits PRODH, which encodes the first enzyme in the breakdown of the amino acid proline. Loss-of-function variants in PRODH lead to elevated blood proline levels, a state called hyperprolinemia. In a screen of schizophrenia patients, researchers found a heterozygous deletion of the entire PRODH gene in one family that included two members with schizophrenia, and identified additional missense mutations in other patients, all associated with elevated proline.31PubMed. PRODH mutations and hyperprolinemia in a subset of schizophrenic patients Multiple human, mouse, and fly studies now support the idea that certain loss-of-function PRODH variants contribute to schizophrenia risk in some individuals.32PubMed. PRODH variants and risk for schizophrenia
What makes this even more interesting is that COMT and PRODH appear to interact. In patients with the Val/Val COMT genotype, higher proline levels were associated with fewer negative symptoms of schizophrenia, while in Met carriers the opposite was true: rising proline was linked to worsening negative symptoms.33Translational Psychiatry. Evidence that COMT genotype and proline interact on negative-symptom outcomes in schizophrenia and bipolar disorder These two genes, sitting close together on the same chromosome and often deleted together in 22q11.2 deletion syndrome, thus have intertwined effects on brain chemistry and psychiatric outcomes.
Lessons from Comparing Species
One way to understand which parts of chromosome 22 matter most is to compare it with its counterpart in other species. When researchers sequenced chimpanzee chromosome 22, which corresponds to human chromosome 21 due to a chromosome numbering difference between the species, they found that about 1.44% of the sequence consisted of single-base changes, along with nearly 68,000 insertions or deletions. These differences were enough to alter the protein sequences in 83% of the 231 coding genes examined.34PubMed. DNA sequence and comparative analysis of chimpanzee chromosome 22 Different families of jumping gene elements had expanded to different degrees in each lineage, suggesting that these mobile DNA sequences have played distinct roles in human and chimpanzee evolution. Comparing which genes have changed rapidly and which have stayed nearly identical across species helps researchers identify which chromosome 22 genes are under strong evolutionary pressure and which tolerate more variation, providing clues about which genes are most essential to normal development.