FIGURE 3-2. Fluorescent in situ hybridization (FISH) technique. Left: (A) Idiogram of chromosomes 4 and 8 demonstrates a translocation with suspected breakpoints at 4p15.32 and 8p22 (arrows). (B) Partial karyotype of normal and abnormal (arrow) chromosomes 4. An example of a normal chromosome 8 with the suspected breakpoint of 8p22 (arrow) is below. This karyotype described as 46,XY,-4, + der(4t)(4;8)(p15.32;p22) de novo is from a patient with Wolf-Hirschorn syndrome and represents a deletion of the chromosome 4p and, presumably, a duplication of chromosome 8p (partial trisomy). (Courtesy of Laurel Estabrooks, Ph.D.) Right: The 4;8 translocation illustrated by chromosome 8-specific paint probes. Specific chromosome 8 paint probes hybridize to the two copies of normal chromosome 8 (entire chromosome has hybridized to the paint probes) and to an additional segment on the distal short arm of chromosome 4 (arrow). (Courtesy of Dr. L. Estabrooks.)

FIGURE 3-3. FISH technique. Cosmid probes specific for the distal region of chromosome 4p illustrate a deletion of chromosome 4p; one arrow indicates the hybridized region of the normal chromosome 4 and the other arrow indicates the region of the short arm of chromosome 4 with the deletion and without hybridization. A "repeat probe" specific for the centromeric region of chromosome 4 is used to identify the chromosome 4 pair (hybridized region at site of constriction of chromosome). (Courtesy of Dr. L. Estabrooks, with permission.)

believed to be 48. The normal human chromosome complement consists of 22 pairs of autosomes and 1 pair of gender-determining chromosomes (XY for male and XX for female), divided into groups on the basis of length and centromeric position. The short arm of a chromosome is termed "p" and the long arm "q," and any portion may be duplicated or deleted. In 1960, the initial Denver classification was developed at a meeting in Colorado, based on the overall length and centromeric position; seven groups, labeled A through G, were created. In 1971, the Paris nomenclature was created and banding further identified each chromosome. Chromosomes 1, 2, and 3 constitute group A; 4 and 5, group B; 6 to 12 and X, group C; 13 to 15, group D; 16 to 18, group E; 19 and 20; group F; and 21, 22, and Y, group G.

In the late 1950s, the first chromosomal aberrations were identified as an abnormal number of chromosomes. The basis of

Turner's syndrome (monosomy X),97 Klinefelter's syndrome (a male with one Y chromosome and more than one X chromosome),165 and Down's syndrome (trisomy 21)194 (Fig. 3-2) was established shortly after the publication of Tjio and Levan.351 Many other chromosomal diseases have since been delineated. Chromosomal studies are now a major diagnostic tool in the evaluation of children with congenital malformations and developmental delay. Approximately 1 in every 200 liveborn children and more than half of spontaneous abortions carry a chromosomal abnormality.

Most numerical chromosomal anomalies originate during gametogenesis and are caused by nondisjunction or anaphase lag. During the first meiotic division, homologous, duplicated chromosomes pair and then segregate with each migrating to opposite poles independently; two cells, each with 23 randomly duplicated chromosomes, are the result. A second division of the duplicated chromosomes follows. Failure of separation of homologous chromosomes may occur in the first division or failure of chromatid separation of the duplicated chromosomes may occur in the second. In either case, complementary gametes with 24 chromosomes (1 present in duplicate) and 22 chromosomes (1 missing) may result. If the former is fertilized by a normal gamete (23 chromosomes), the zygote would have 47 chromosomes, 1 being present in triplicate (trisomy); if the latter, the zygote would have 45 chromosomes with 1 missing (monosomy). An abnormal number of chromosomes is termed aneuploidy. The autosomal trisomies compatible with term gestation are those of chromosomes 13, 18, and 21, all of which are associated with mental retardation, dysmorphic features, and malformations. Trisomies of other chromosomes that are usually lethal in utero are identified in spontaneous abortions or may result in a live birth if occurring in a mosaic form. Autosomal monosomy is usually lethal, although monosomies of chromosomes 21 and 22 have been reported.

Nondisjunction of sex-determining chromosomes results in less severe phenotypic consequences. Monosomy X is the basis of Turner's syndrome, and females with XXX and XXXX have been identified. Males with XXY and XYY are not uncommon, and increasing numbers of X and Y to XXXXY or XXYY have been reported (Klinefelter's syndrome). If nondisjunction occurs after fertilization, mosaicism results. Cell lines with trisomies or monosomies may persist in the fetus or individual. The diagnosis of mosaicism requires chromosomal analysis of more than one tissue type. In general, cells with autosomal monosomies are nonviable, but monosomy of chromosomes 9, 21, 22, and X may persist.

The second group of clinically recognizable chromosomal syndromes is deletions or missing portions of a chromosome, called partial monosomies; the total number of chromosomes is normal. Most result from a break in a chromosome, usually of a terminal portion with subsequent loss from the cell. If the deletion was de novo in an egg or sperm, only a single child in a family is expected to have the chromosomal abnormality. Deletions also may be inherited as the unbalanced form of a translocation for which a normal parent has an abnormal but balanced chromosome constitution (see following) or may occur sporadically as a result of an abnormality of one of the germ cells (Fig. 3-4). Many partial deletions occur frequently enough that each is considered a syndrome. Ring chromosomes are formed when the ends of chromosome break and form a ring; clinical variation is considerable. There is loss of chromatin (DNA) from both

FIGURE 3-4. Deletion 13q. Karyotype of a patient with mental retardation and retinoblastoma caused by an interstitial deletion of the long arm of chromosome 13. The deleted material extends from bands q14 through q22 (46,XX,13q —) as depicted by smaîî arrows on the normal left chromosome 13. The large arrow identifies the deleted chromosome 13. (Courtesy of Dr. R.S. Sparkes.)

FIGURE 3-4. Deletion 13q. Karyotype of a patient with mental retardation and retinoblastoma caused by an interstitial deletion of the long arm of chromosome 13. The deleted material extends from bands q14 through q22 (46,XX,13q —) as depicted by smaîî arrows on the normal left chromosome 13. The large arrow identifies the deleted chromosome 13. (Courtesy of Dr. R.S. Sparkes.)

FIGURE 3-5. Ring chromosome 11. Karyotype of a patient with mild mental retardation, short stature, and behavioral problems. (Courtesy of Dr. R.S. Sparkes.)

ends, and affected children may resemble those with simple deletions of the same chromosome (Fig. 3-5).

The third general group is defined by a duplication of a portion of a chromosome, called partial trisomy. The FISH technique may provide the source; if small, the origin of the "extra" chromosomal DNA may be difficult to determine. Partial duplications may be caused by an extra piece of a chromosome, resulting in an increase in the total number of chromosomes (aneuploidy), or by the attachment of duplicated material to another chromosome.

The fourth general category is translocation. DNA may be transferred from one chromosome to another, a translocation; if no genetic material is lost, the translocation is balanced (Fig. 3-6). The first identified human translocations were centric fusions between two acrocentric chromosomes (centromere near one end of the chromosome), which reduced the chromosome count by one; nonessential DNA on the short arms was lost, and the affected individual was apparently normal. Some chromosomes are predisposed to this type of translocation in which the two long arms of a chromosome are fused at or near the centromere with loss of all or a portion of the two short arms; such a translocation between bi-armed chromosomes is termed Robertsonian (Fig. 3-7). Most of the translocations causing

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