Pathology (genetic diseases 2)

• First, they function in the acidic milieu of the lysosomes.
• Second, these enzymes constitute a special category of secretory proteins that are destined not for the extracellular fluids but for intracellular organelles. This requires special processing within the Golgi apparatus

• Similar to all other secretory proteins, lysosomal enzymes (or acid hydrolases, as they are sometimes called) are synthesized in the endoplasmic reticulum and transported to the Golgi apparatus.
• Within the Golgi complex they undergo post-translational modifications.
• This modification involves the attachment of terminal mannose6-phosphate groups to some of the oligosaccharide side chains.
• The phosphorylated mannose residues serve as an “address label” that is recognized by specific receptors found on the inner surface of the Golgi membrane.
• Lysosomal enzymes bind these receptors and are thereby segregated from the numerous other secretory proteins within the Golgi.
• Subsequently, small transport vesicles containing the receptor-bound enzymes are pinched off from the Golgi and proceed to fuse with the lysosomes.
• Thus, the enzymes are targeted to their intracellular abode, and the vesicles are shuttled back to the Golgi.
• 

• Acid hydrolase deficiency
• Lack of an enzyme activator or protector protein.  
• Lack of a substrate activator protein. In some instances, proteins that react with the substrate to facilitate its hydrolysis may be missing or defective.  
• Lack of a transport protein required for egress of the digested material from the lysosomes

Incomplete digestion of material by lysosome

• Enzyme replacement therapy
• Substrate reduction therapy is based on the premise that if the substrate to be degraded by the lysosomal enzyme can be reduced, the residual enzyme activity may be sufficient to catabolize it and prevent accumulation.
• In many disorders, exemplified by Gaucher disease, the enzyme activity is low because the mutant proteins are unstable and prone to misfolding, and hence degraded in the endoplasmic reticulum. In such diseases an exogenous competitive inhibitor of the enzyme can, paradoxically, bind to the mutant enzyme and act as the “folding template” that assists proper folding of the enzyme and thus prevents its degradation. Such molecular chaperone therapy is under active investigation

• Glycogenosis--> Type 2 (Pompe): α-1,4-Glucosidase (lysosomal glucosidase)
• SPHINGOLIPIDOSES--> GM1(GM1 ganglioside β-galactosidase)/ GM2--> Tay Sachs (Hexosaminidase, α subunit)
• SULFATIDOSES--> Metachromatic leukodystrophy (Arylsulfatase A)/Krabbe disease (Galactosylceramidase)/ Fabry diseaseα-(Galactosidase A)/ Gaucher disease (Glucocerebrosidase)/Niemann-Pick disease: types A and B (Sphingomyelinase)
• MPS--> MPS I H (Hurler)α-L-Iduronidase/ MPS II (Hunter)l-Iduronosulfate sulfatase
• MUCOLIPIDOSES --> I-cell disease (Deficiency of phosphorylating enzymes essential for the formation of mannose-6-phosphate recognition marker)

• brain is rich in gangliosides, and hence defective hydrolysis of gangliosides, as occurs in GM1 and GM2 gangliosidoses, results primarily in accumulation within neurons and consequent neurologic symptoms.
• Defects in degradation of mucopolysaccharides affect virtually every organ, because mucopolysaccharides are widely distributed in the body.
• Because cells of the mononuclear phagocyte system are especially rich in lysosomes and are involved in the degradation of a variety of substrates, organs rich in phagocytic cells, such as the spleen and liver, are frequently enlarged in several forms of lysosomal storage disorders

Because cells of the mononuclear phagocyte system are especially rich in lysosomes and are involved in the degradation of a variety of substrates, organs rich in phagocytic cells, such as the spleen and liver, are frequently enlarged in several forms of lysosomal storage disorders

• Type 2—Pompe disease
• α-1,4-Glucosidase (lysosomal glucosidase)
• Results in accumulation of glycogen

• GM1
• GM2--> Tay-Sachs disease (Hexosaminidase, α subunit accumulated GM2 ganglioside)
• Sandhoff disease (Hexosaminidase, β subunit)

• Metachromatic leukodystrophy--> Arylsulfatase A--> Sulfatide
• Krabbe disease--> Galactosylceramidase--> Galactocerebroside
• Fabry disease--> α-Galactosidase A--> Ceramide trihexoside
• Gaucher disease--> Glucocerebrosidase--> Glucocerebroside
• Niemann-Pick disease: types A and B--> Sphingomyelinase--> Sphingomyelin

α-Galactosidase A deficiency (Fabry)

Galactosylceramidase deficiency (Krabbe)

Sulfatide accumulation (metachromatic LD)

• MPS I H (Hurler)-->α-L-Iduronidase
• MPS II (Hunter)l--->Iduronosulfate sulfatase
• Dermatan sulfate, heparan sulfate accumulated

• Deficiency of phosphorylating enzymes essential for the formation of mannose-6-phosphate recognition marker;
• acid hydrolases lacking the recognition marker cannot be targeted to the lysosomes but are secreted extracellularly
• Mucopolysaccharide, glycolipid accumulated

Tay-Sachs disease

• GM2 gangliosidosis, results from mutations in the α-subunit locus on chromosome 15 that cause a severe deficiency of hexosaminidase A.
• This disease is especially prevalent among Jews, particularly among those of Eastern European (Ashkenazic) origin

neurons in the central and autonomic nervous systems and retina

• On histologic examination, the neurons are ballooned with cytoplasmic vacuoles, each representing a markedly distended lysosome filled with gangliosides.
• Stains for fat such as oil red O and Sudan black B are positive.
• With the electron microscope whorled configurations within lysosomes composed of onion-skin layers of membranes are observed.
• In time there is progressive destruction of neurons, proliferation of microglia, and accumulation of complex lipids in phagocytes within the brain substance.
• A similar process occurs in the cerebellum as well as in neurons throughout the basal ganglia, brain stem, spinal cord, and dorsal root ganglia and in the neurons of the autonomic nervous system.
• The ganglion cells in the retina are similarly swollen with GM2 ganglioside, particularly at the margins of the macula.
• A cherry-red spot thus appears in the macula, representing accentuation of the normal color of the macular choroid contrasted with the pallor produced by the swollen ganglion cells in the remainder of the retina.
• This finding is characteristic of Tay-Sachs disease and other storage disorders affecting the neurons
• 

• The affected infants appear normal at birth but begin to manifest signs and symptoms at about age 6 months.
• There is relentless motor and mental deterioration, beginning with motor incoordination, mental obtundation leading to muscular flaccidity, blindness, and increasing dementia.
• Sometime during the early course of the disease, the characteristic, but not pathognomonic, cherry-red spot appears in the macula of the eye in almost all patients.
• Over the span of 1 or 2 years a complete vegetative state is reached, followed by death at age 2 to 3 years

• most affect protein folding.
• Such misfolded proteins trigger the “unfolded protein” response leading to apoptosis.
• These findings have given rise to the possibility of chaperone therapy of Tay-Sachs disease

lysosomal accumulation of sphingomyelin due to an inherited deficiency of sphingomyelinase

• Type A is a severe infantile form with extensive neurologic involvement, marked visceral accumulations of sphingomyelin, and progressive wasting and early death within the first 3 years of life.
• In contrast, type B disease patients have organomegaly but generally no central nervous system involvement. They usually survive into adulthood.
• As with Tay-Sachs disease, Niemann-Pick disease types A and B are common in Ashkenazi Jews

The acid sphingomyelinase gene maps to chromosome 11p and is one of the imprinted genes that is preferentially expressed from the maternal chromosome as a result of epigenetic silencing of the paternal gene

• Mononuclear phagocyte system cells become enlarged due to the distention of lysosomes with sphingomyelin and cholesterol.
• Innumerable small vacuoles of relatively uniform size are created, imparting foaminess to the cytoplasm .
• EM --> vacuoles are engorged secondary lysosomes that often contain membranous cytoplasmic bodies resembling concentric lamellated myelin figures, sometimes called “zebra” bodies
• The lipid-laden phagocytic foam cells are widely distributed in the phagocytic system. 
• Massive splenomegaly without prominent hepatomegaly
• Generalized LAP
• In the brain the gyri are shrunken and the sulci widened. The neuronal involvement is diffuse, affecting all parts of the nervous system. Vacuolation and ballooning of neurons constitute the dominant histologic change, which in time leads to cell death and loss of brain substance.
• Retinal cherry-red spot 
• 
•  The hepatocytes and Kupffer cells have a foamy, vacuolated appearance due to deposition of lipids
• 

• Clinical manifestations in type A disease may be present at birth and almost invariably become evident by age 6 months. Infants typically have a protuberant abdomen because of the hepatosplenomegaly.
• Once the manifestations appear, they are followed by progressive failure to thrive, vomiting, fever, and generalized lymphadenopathy as well as progressive deterioration of psychomotor function.
• Death comes, usually within the first or second year of life.

• Different from two other types (more common than type A and B)
• Mutations in two related genes, NPC1 and NPC2, can give rise to it, with NPC1 being responsible for 95% of cases.
• Unlike most other lysosomal storage diseases, NPC is due to a primary defect in lipid transport.
• Affected cells accumulate cholesterol as well as gangliosides such as GM1 and GM2.
• Both NPC1 and NPC2 are involved in the transport of free cholesterol from the lysosomes to the cytoplasm.
• NPC is clinically heterogeneous. It may present as hydrops fetalis and stillbirth, as neonatal hepatitis, or as a chronic form characterized by progressive neurologic damage.
• The most common form presents in childhood and is marked by ataxia, vertical supranuclear gaze palsy, dystonia, dysarthria, and psychomotor regression

primary defect in lipid transport.

• NPC is clinically heterogeneous. 
• It may present as hydrops fetalis and stillbirth, as neonatal hepatitis, or as a chronic form characterized by progressive neurologic damage. 
• The most common form presents in childhood and is marked by ataxia, vertical supranuclear gaze palsy, dystonia, dysarthria, and psychomotor regression

Gaucher disease

• A cluster of AR disorders resulting from mutations in the gene encoding glucocerebrosidase.
• MC lysosomal storage disorder.
• The affected gene encodes glucocerebrosidase, an enzyme that normally cleaves the glucose residue from ceramide.
• As a result of the enzyme defect, glucocerebroside accumulates principally in phagocytes but in some subtypes also in the central nervous system.
• Glucocerebrosides are continually formed from the catabolism of glycolipids derived mainly from the cell membranes of senescent leukocytes and erythrocytes

cleaves the glucose residue from ceramide.

Pathologic changes in Gaucher disease are caused not just by the burden of storage material but also by activation of macrophages and the consequent secretion of cytokines such as IL-1, IL-6, and tumor necrosis factor (TNF).

• I-->99% the chronic non-neuronopathic form. / storage of glucocerebrosides is limited to the mononuclear phagocytes throughout the body without involving the brain. Splenic and skeletal involvements dominate this pattern of the disease. It is found principally in Jews of European stock ( reduced but detectable levels of glucocerebrosidase activity). Longevity is shortened but not markedly. 
• Type II, or acute neuronopathic Gaucher disease, is the infantile acute cerebral pattern. This form has no predilection for Jews. In these patients there is virtually no detectable glucocerebrosidase activity in the tissues. Hepatosplenomegaly is also seen in this form of Gaucher disease, but the clinical picture is dominated by progressive central nervous system involvement, leading to death at an early age.
• A third pattern, type III, is intermediate between types I and II. These patients have the systemic involvement characteristic of type I but have progressive central nervous system disease that usually begins in adolescence or early adulthood.

• storage of glucocerebrosides is limited to the mononuclear phagocytes throughout the body without involving the brain.
• Splenic and skeletal involvements dominate this pattern of the disease

In contrast to other lipid storage diseases, Gaucher cells rarely appear vacuolated but instead have a fibrillary type of cytoplasm likened to crumpled tissue paper

• 1) Glucocerebrosides accumulate in massive amounts within phagocytic cells (Gaucher cells), are found in all the lymphophagocytic system, alveolar septa and the air spaces in the lung.
• 2) In contrast to other lipid storage diseases, Gaucher cells rarely appear vacuolated but instead have a fibrillary type of cytoplasm likened to crumpled tissue paper.
• 3) Gaucher cells are often enlarged, sometimes up to 100 μm in diameter, and have one or more dark, eccentrically placed nuclei. Periodic acid–Schiff staining is usually intensely positive.
• 4) With the electron microscope the fibrillary cytoplasm can be resolved as elongated, distended lysosomes, containing the stored lipid in stacks of bilayers
• 5) Massive splenomegaly
• 6) Generalized LAP
• 7) areas of bone erosion that may cause pathologic fractures. Bone destruction occurs due to the secretion of cytokines by activated macrophages.
• 8) In patients with cerebral involvement, Gaucher cells are seen in the Virchow-Robin spaces, and arterioles are surrounded by swollen adventitial cells.
• 9) There is no storage of lipids in the neurons, yet neurons appear shriveled and are progressively destroyed. It is suspected that the lipids that accumulate in the phagocytic cells around blood vessels secrete cytokines that damage nearby neurons.
• 

secretion of cytokines by activated macrophages

• There is no storage of lipids in the neurons, yet neurons appear shriveled and are progressively destroyed.
• It is suspected that the lipids that accumulate in the phagocytic cells around blood vessels secrete cytokines that damage nearby neurons

True

Most commonly there is pancytopenia or thrombocytopenia secondary to hypersplenism

CNS

• Bone pain
• Pathologic Fx

• Deficiencies of lysosomal enzymes involved in the degradation of mucopolysaccharides (glycosaminoglycans).
• Chemically, mucopolysaccharides are long-chain complex carbohydrates that are linked with proteins to form proteoglycans. They are abundant in the ground substance of connective tissue. The glycosaminoglycans that accumulate in MPSs are dermatan sulfate, heparan sulfate, keratan sulfate, and chondroitin sulfate.
• The enzymes involved in the degradation of these molecules cleave terminal sugars from the polysaccharide chains disposed along a polypeptide or core protein.
• All the MPSs except one are inherited as autosomal recessive traits; the exception, called Hunter syndrome, is an X-linked recessive trait.
• coarse facial features, clouding of the cornea, joint stiffness, and mental retardation.
• Urinary excretion of the accumulated mucopolysaccharides is often increased

• The accumulated mucopolysaccharides are generally found in mononuclear phagocytic cells, endothelial cells, intimal smooth muscle cells, and fibroblasts throughout the body.
• Common sites of involvement are thus the spleen, liver, bone marrow, lymph nodes, blood vessels, and heart

• Microscopically, affected cells are distended and have apparent clearing of the cytoplasm to create so-called balloon cells (minute vacuoles)
• These are swollen lysosomes containing a finely granular PAS–positive material (MPS).
• Similar lysosomal changes are found in the neurons of those syndromes characterized by CNS involvement. In addition, however, some of the lysosomes in neurons are replaced by lamellated zebra bodies similar to those seen in Niemann-Pick disease.
• Hepatosplenomegaly, skeletal deformities, valvular lesions, and subendothelial arterial deposits, particularly in the coronary arteries, and lesions in the brain, are common threads that run through all of the MPSs.
• In many of the more protracted syndromes, coronary subendothelial lesions lead to myocardial ischemia. Thus, myocardial infarction and cardiac decompensation are important causes of death.

Hepatosplenomegaly, skeletal deformities, valvular lesions, and subendothelial arterial deposits, particularly in the coronary arteries, and lesions in the brain, are common threads that run through all of the MPSs.

• Hurler syndrome, also called MPS I-H, results from a deficiency of α-l-iduronidase. It is one of the most severe forms of MPS. Affected children appear normal at birth but develop hepatosplenomegaly by age 6 to 24 months. Their growth is retarded, and, as in other forms of MPS, they develop coarse facial features and skeletal deformities. Death occurs by age 6 to 10 years and is often due to cardiovascular complications. 
• Hunter syndrome, also called MPS II, differs from Hurler syndrome in mode of inheritance (X-linked), absence of corneal clouding, and milder clinical course

• Glycogen synthesis begins with the conversion of glucose to glucose-6-phosphate by the action of a hexokinase (glucokinase).
• A phosphoglucomutase then transforms the glucose-6-phosphate to glucose-1-phosphate, which, in turn, is converted to uridine diphosphoglucose.
• A highly branched, large polymer is then built (molecular weight as high as 100 million), containing as many as 10,000 glucose molecules linked together by α-1,4-glucoside bonds.
• The glycogen chain and branches continue to be elongated by the addition of glucose molecules mediated by glycogen synthetases.
• During degradation, distinct phosphorylases in the liver and muscle split glucose-1-phosphate from the glycogen until about four glucose residues remain on each branch, leaving a branched oligosaccharide called limit dextrin.
• This can be further degraded only by the debranching enzyme.
• In addition to these major pathways, glycogen is also degraded in the lysosomes by acid maltase. If the lysosomes are deficient in this enzyme, the glycogen contained within them is not accessible to degradation by cytoplasmic enzymes such as phosphorylases
• 

• Hepatic forms-->von Gierke disease, or type I glycogenosis, liver phosphorylase and debranching enzyme (hepatic enlargement and hypoglycemia dominate the clinical picture)
• Myopathic forms-->muscle phosphorylase (McArdle disease, or type V glycogenosis), muscle phosphofructokinase (type VII glycogen storage disease): muscle cramps after exercise and lactate levels in the blood fail to rise after exercise due to a block in glycolysis
• Glycogen storage diseases associated with (1) deficiency of α-glucosidase (acid maltase) and (2) lack of branching enzyme do not fit into the hepatic or myopathic categories. They are associated with glycogen storage in many organs and death early in life. Acid maltase is a lysosomal enzyme, and hence its deficiency leads to lysosomal storage of glycogen (type II glycogenosis, or Pompe disease) in all organs, but cardiomegaly is the most prominent feature

• The liver is a key player in glycogen metabolism. It contains enzymes that synthesize glycogen for storage and ultimately break it down into free glucose, which is then released into the blood.
• An inherited deficiency of hepatic enzymes that are involved in glycogen degradation therefore leads not only to the storage of glycogen in the liver but also to a reduction in blood glucose concentrations (hypoglycemia) 

• In the skeletal muscles, as opposed to the liver, glycogen is used predominantly as a source of energy during physical activity. ATP is generated by glycolysis, which leads ultimately to the formation of lactate.
• If the enzymes that fuel the glycolytic pathway are deficient, glycogen storage occurs in the muscles and is associated with muscular weakness due to impaired energy production.

• Glucose-6-phosphatase
• Hepatomegaly—intracytoplasmic accumulations of glycogen and small amounts of lipid; intranuclear glycogen  
• Renomegaly—intracytoplasmic accumulations of glycogen in cortical tubular epithelial cells
• In untreated patients: Failure to thrive, stunted growth, hepatomegaly, and renomegaly  
• Hypoglycemia due to failure of glucose mobilization, often leading to convulsions  
• Hyperlipidemia and hyperuricemia resulting from deranged glucose metabolism; many patients develop gout and skin xanthomas  
• Bleeding tendency due to platelet dysfunction  
• With treatment: Most survive and develop late complications (e.g., hepatic adenomas)
• Neutropenia
• 

All positive

Lactic acid, uric acid, lipids elevated in GSD 1

Type I (glucose 6 phosphatase deficiency=Von Gierke)

• Muscle phosphorylase
• Skeletal muscle only—accumulations of glycogen predominant in subsarcolemmal location
• Painful cramps associated with strenuous exercise;
• myoglobinuria occurs in 50% of cases;
• onset in adulthood (>20 years);
• muscular exercise fails to raise lactate level in venous blood;
• serum creatine kinase always elevated;
• compatible with normal longevity

• Serum CK always elevated
• Exercise fail to elevate lactate in venous blood

• Debrancher deficiency
• Hepatomegaly
• Onset in infancy
• Mild fasting hypoglycemia
• May have cardiac or skeletal muscle manifestations (eg, elevated CK)
• May have elevated RBC glycogen

Cardiomegaly

Acid maltase is a lysosomal enzyme, and hence its deficiency leads to lysosomal storage of glycogen (type II glycogenosis, or Pompe disease) in all organs

• Generalized glycogenosis
• Lysosomal glucosidase (acid maltase)  
• Mild hepatomegaly—ballooning of lysosomes with glycogen, creating lacy cytoplasmic pattern  
• Cardiomegaly—glycogen within sarcoplasm as well as membrane-bound  
• Skeletal muscle—similar to changes in heart
• Massive cardiomegaly (HCMP), muscle hypotonia, and cardiorespiratory failure within 2 years; a milder adult form with only skeletal muscle involvement, presenting with chronic myopathy
• 
• Normal myocardium with abundant eosinophilic cytoplasm. B, Patient with Pompe disease (same magnification) showing the myocardial fibers full of glycogen seen as clear spaces

• AR
• Lack of homogentisic oxidase, an enzyme that converts homogentisic acid to methylacetoacetic acid in the tyrosine degradation pathway
• homogentisic acid accumulates in the body. A large amount is excreted, imparting a black color to the urine if allowed to stand and undergo oxidation

• The retained homogentisic acid binds to collagen in connective tissues, tendons, and cartilage, imparting to these tissues a blue-black pigmentation(ochronosis) most evident in the ears, nose, and cheeks. 
• The most serious consequences of ochronosis, however, stem from deposits of the pigment in the articular cartilages of the joints.
• The pigment accumulation causes the cartilage to lose its normal resiliency and become brittle and fibrillated.
• Wear-and-tear erosion of this abnormal cartilage leads to denudation of the subchondral bone, and often tiny fragments of the fibrillated cartilage are driven into the underlying bone, worsening the damage.
• The vertebral column, particularly the intervertebral disc, is the prime site of attack, but later the knees, shoulders, and hips may be affected.
• The small joints of the hands and feet are usually spared

IV disks

• The metabolic defect is present from birth, but the degenerative arthropathy develops slowly and usually does not become clinically evident until the 30s.
• Although it is not life-threatening, it may be severely crippling. The arthropathy may be as extreme as that encountered in the severe forms of osteoarthritis

• According to the common disease/common variant hypothesis, complex genetic disorders occur when many polymorphisms, each with a modest effect and low penetrance, are inherited
• While complex disorders result from the collective inheritance of many polymorphisms, different polymorphisms vary in significance
• Some polymorphisms are common to multiple diseases of the same type, while others are disease specific

prophase

Total number of chromosomes is given first, followed by the sex chromosome complement, and finally the description of abnormalities in ascending numerical order. For example, a male with trisomy 21 is designated 47,XY,+21.

• In a banded karyotype, each arm of the chromosome is divided into two or more regions bordered by prominent bands. The regions are numbered (e.g., 1, 2, 3) from the centromere outward. Each region is further subdivided into bands and sub-bands, and these are ordered numerically as well.
• Thus, the notation Xp21.2 refers to a chromosomal segment located on the short arm of the X chromosome, in region 2, band 1, and sub-band 2.

• Any exact multiple of the haploid number is called euploid.
• If an error occurs in meiosis or mitosis and a cell acquires a chromosome complement that is not an exact multiple of 23, it is referred to as aneuploidy

nondisjunction and anaphase lag

When nondisjunction occurs during gametogenesis, the gametes formed have either an extra chromosome (n + 1) or one less chromosome (n - 1). Fertilization of such gametes by normal gametes results in two types of zygotes—trisomic (2n + 1) or monosomic (2n - 1).

• In anaphase lag, one homologous chromosome in meiosis or one chromatid in mitosis lags behind and is left out of the cell nucleus.
• The result is one normal cell and one cell with monosomy.

• monosomy or trisomy involving the sex chromosomes, or even more bizarre aberrations, are compatible with life and are usually associated with variable degrees of phenotypic abnormalities. 
• Monosomy involving an autosome generally causes loss of too much genetic information to permit live birth or even embryogenesis, but several autosomal trisomies do permit survival.
• With the exception of trisomy 21, all yield severely handicapped infants who almost invariably die at an early age

Mosaicism can result from mitotic errors during the cleavage of the fertilized ovum or in somatic cells

Occasionally, mitotic errors in early development give rise to two or more populations of cells with different chromosomal complement, in the same individual, a condition referred to as mosaicism

True

In the division of the fertilized ovum, an error may lead to one of the daughter cells receiving three sex chromosomes, whereas the other receives only one, yielding, for example, a 45,X/47,XXX mosaic. All descendent cells derived from each of these precursors thus have either a 47,XXX complement or a 45,X complement. Such a patient is a mosaic variant of Turner syndrome, with the extent of phenotypic expression dependent on the number and distribution of the 45,X cells.

• Less common than sex chromosome
• An error in an early mitotic division affecting the autosomes usually leads to a nonviable mosaic due to autosomal monosomy.
• Rarely, the nonviable cell population is lost during embryogenesis, yielding a viable mosaic (e.g., 46,XY/47,XY,+21). Such a patient is a trisomy 21 mosaic with variable expression of Down syndrome, depending on the proportion of cells containing the trisomy

• a fairly large amount of DNA (approximately 2–4 million base pairs)
• The resolution is much higher with fluorescence in situ hybridization (FISH), which can detect changes as small as kilobases

• Deletion refers to loss of a portion of a chromosome.
• Most deletions are interstitial, but rarely terminal deletions may occur.
• Interstitial deletions occur when there are two breaks within a chromosome arm, followed by loss of the chromosomal material between the breaks and fusion of the broken ends.
• One can specify in which region(s) and at what bands the breaks have occurred. For example, 46,XY, del(16) (p11.2p13.1) describes breakpoints in the short arm of chromosome 16 at 16p11.2 and 16p13.1 with loss of material between breaks.
• Terminal deletions result from a single break in a chromosome arm, producing a fragment with no centromere, which is then lost at the next cell division, and a chromosome bearing a deletion. The end of the chromosome is protected by acquiring telomeric sequences

• A ring chromosome is a special form of deletion. It is produced when a break occurs at both ends of a chromosome with fusion of the damaged ends.
• If significant genetic material is lost, phenotypic abnormalities result.
• This might be expressed as 46,XY,r(14).
• Ring chromosomes do not behave normally in meiosis or mitosis and usually result in serious consequences.

• Inversion refers to a rearrangement that involves two breaks within a single chromosome with reincorporation of the inverted, intervening segment.
• An inversion involving only one arm of the chromosome is known as paracentric.
• If the breaks are on opposite sides of the centromere, it is known as pericentric.
• Inversions are often fully compatible with normal development

inversion

• Isochromosome formation results when one arm of a chromosome is lost and the remaining arm is duplicated, resulting in a chromosome consisting of two short arms only or of two long arms.
• An isochromosome has morphologically identical genetic information in both arms.
• The most common isochromosome present in live births involves the long arm of the X and is designated i(X)(q10).
• The Xq isochromosome is associated with monosomy for genes on the short arm of X and with trisomy for genes on the long arm of X

the long arm of the X and is designated i(X)(q10).

Transolcation

• In balanced reciprocal translocation, there are single breaks in each of two chromosomes, with exchange of material.
• A balanced reciprocal translocation between the long arm of chromosome 2 and the short arm of chromosome 5 would be written 46,XX,t(2;5)(q31;p14). This individual has 46 chromosomes with altered morphology of one of the chromosomes 2 and one of the chromosomes 5.
• Because there has been no loss of genetic material, the individual is likely to be phenotypically normal.
• A balanced translocation carrier, however, is at increased risk for producing abnormal gametes.
• For example, in the case cited above, a gamete containing one normal chromosome 2 and a translocated chromosome 5 may be formed.
• Such a gamete would be unbalanced because it would not contain the normal complement of genetic material. Subsequent fertilization by a normal gamete would lead to the formation of an abnormal (unbalanced) zygote, resulting in spontaneous abortion or birth of a malformed child

• The other important pattern of translocation is called a robertsonian translocation (or centric fusion), a translocation between two acrocentric chromosomes.
• Typically the breaks occur close to the centromeres of each chromosome.
• Transfer of the segments then leads to one very large chromosome and one extremely small one.
• Usually the small product is lost; however, because it carries only highly redundant genes (e.g., ribosomal RNA genes), this loss is compatible with a normal phenotype.
• Robertsonian translocation between two chromosomes is encountered in 1 in 1000 apparently normal individuals.
• The significance of this form of translocation also lies in the production of abnormal progeny