Chromosome Number and Structure: Aneuploidy and Rearrangements

In 1959 Jérôme Lejeune, Marthe Gautier and Raymond Turpin in Paris counted 47 chromosomes, not the usual 46, in cultured cells from nine children with Down syndrome.

That one extra chromosome, later numbered 21, explained a condition that had been described for nearly a century. In the same year, other laboratories tied an extra X to Klinefelter syndrome and a missing one to Turner syndrome. This lesson asks how a cell ends up with the wrong number of chromosomes, why some wrong numbers are tolerated and others are not, and what happens when a chromosome breaks and rejoins in the wrong way.

The idea: a missed separation leaves one cell with two copies and another with none

A few words first. Aneuploidy is the gain or loss of a single chromosome. Trisomy means three copies of one chromosome instead of two; monosomy means one. Nondisjunction is a failure of chromosomes to part when a cell divides.

You met the two meiotic divisions in the lesson on chromosomes and meiosis. At meiosis I the two homologues of each pair separate. At meiosis II the two sister chromatids of each chromosome separate. Think of two coats on a hook: meiosis I hands the two coats to two people, and meiosis II splits each coat’s matching gloves. Fail at the first step and one person gets both coats. Fail at the second and one person gets both gloves of a single pair.

Worked problem, part 1: which gametes?

Follow chromosome 21 in one human spermatocyte. Call the father’s two homologues 21a and 21b. Assume only one error, otherwise normal divisions, and that all four products are recovered.

1. Error at meiosis I. 21a and 21b go to the same secondary spermatocyte. Meiosis II then runs normally. That cell gives two sperm, each with one chromatid of 21a and one of 21b: two disomic sperm (n + 1). The other secondary spermatocyte had no chromosome 21: two nullisomic sperm (n − 1).
2. Error at meiosis II. Meiosis I is normal. In one secondary spermatocyte, say the one with 21a, the sisters fail to part: one sperm with two copies of 21a, one with none. The other secondary spermatocyte divides normally: two normal sperm.
3. Fertilize a normal egg (one chromosome 21). A disomic sperm gives a trisomic zygote, a nullisomic sperm a monosomic one, a normal sperm a normal one.

disomic gametes per meiosis = 2 after a meiosis I error, 1 after a meiosis II error

Now scale up. Take 100 meiocytes: 70 normal, 10 with a meiosis I error, 20 with a meiosis II error. Disomic products are 10 × 2 + 20 × 1 = 40. Nullisomic products are also 40. Normal products are 70 × 4 + 20 × 2 = 320. Check: 320 + 40 + 40 = 400 = 100 × 4. With normal partners, that gives 80 percent euploid, 10 percent trisomic and 10 percent monosomic zygotes. These numbers come from an invented exercise, not from measured human rates.

Worked problem, part 2: which parent, which division?

The two errors leave different fingerprints near the centromere, because homologues differ there and sisters do not. Take a DNA marker tightly linked to the chromosome 21 centromere. The father carries alleles 1 and 2; the mother carries 3 and 4.

• Child A types 1, 3, 4. One paternal allele, two different maternal alleles. The extra chromosome is maternal, and since it carries both of her homologues, the error was at maternal meiosis I.
• Child B types 1, 3, 3. Two copies of one maternal allele: sisters stayed together, consistent with maternal meiosis II.

The caveat is crossing over. A crossover between the marker and the centromere can make sisters differ at the marker, so a meiosis II error would look like a meiosis I error, and the reverse. Markers as close to the centromere as possible reduce that risk.

Human eggs add errors the two-step cartoon leaves out. Sister chromatids can separate early, at meiosis I, and in reverse segregation both pairs of sisters split at meiosis I.

Too many or too few: trisomy and monosomy in people

Most aneuploid human embryos do not survive pregnancy. Of the autosomal trisomies, only 21, 18 and 13 are seen regularly at birth, and trisomy 21 is by far the commonest of these. Down syndrome occurs in about 1 in 700 births in widely cited estimates for the United States, a figure that varies by population and strongly by maternal age. Most cases are free trisomy 21, a separate extra chromosome. Studies that typed DNA markers in families found that most free trisomy 21 is maternal in origin, and that most maternal errors arise at meiosis I. The risk rises with the mother’s age. In donated human eggs, Gruhn and colleagues found that aneuploidy follows a U-shaped curve with donor age, higher in the youngest and the oldest donors.

The sex chromosomes are different. Turner syndrome occurs in females; about half have monosomy X, a 45,X karyotype, and the rest have mosaicism (a mix of cell lines) or a partly lost or rearranged X. Most 45,X conceptions are thought to be lost in pregnancy, and many survivors are mosaic. Klinefelter syndrome is most often 47,XXY, a male with an extra X. Do not confuse it with 47,XYY.

Why are these tolerated when an extra 21 is so consequential? As the lesson on sex linkage explained, mammalian cells silence every X beyond one, so extra X chromosomes add little active dose. The Y carries few genes. The features that remain are thought to come largely from genes that escape X inactivation, which are present in one active copy in 45,X and in extra active copies in 47,XXY.

Whole extra sets: polyploidy

A polyploid has complete extra sets of chromosomes. An autopolyploid has extra sets from one species, often from a gamete that never reduced its chromosome number. An allopolyploid combines sets from two or more species, usually through a hybrid whose chromosome sets then doubled. Bread wheat is a hexaploid allopolyploid: six sets, 42 chromosomes, two sets from each of three ancestral grasses. Because each set has its own partner, meiosis still forms pairs.

Odd numbers of sets cause trouble. Cultivated seedless bananas are triploid, and three sets cannot be split evenly at meiosis, so nearly every gamete is unbalanced and seeds rarely form.

Polyploidy is common in flowering plants and rare in animals. Plants often self-fertilize or spread without sex, so a new polyploid does not need a matching partner, and plants appear to tolerate shifts in dose more readily. Some animals, such as the fruit fly, set sex by a balance of sex chromosomes and autosomes, which an extra set may disturb. Exceptions exist among fish and frogs. In humans, triploidy (69 chromosomes) is almost always lost in pregnancy.

Broken and rejoined: deletions, duplications, inversions, translocations

• A deletion removes a segment. Loss of part of the short arm of chromosome 5 causes cri du chat syndrome; a small deletion at 7q11.23 that includes the elastin gene underlies Williams syndrome.
• A duplication adds a copy of a segment. In 1991 Lupski and colleagues associated a duplication on 17p with Charcot-Marie-Tooth disease type 1A.
• An inversion reverses a segment. A pericentric inversion includes the centromere; a paracentric one does not. In a heterozygote the chromosomes pair by forming a loop. A single crossover inside a paracentric loop gives one dicentric chromatid, which breaks at anaphase, and one acentric fragment, which is lost. A crossover inside a pericentric loop gives chromatids that each carry a duplication and a deletion. Either way, recombinant gametes are unbalanced, so few recombinants appear among the offspring.
• A reciprocal translocation swaps segments between two chromosomes. A carrier’s four chromosomes pair as a cross-shaped group of four. Alternate segregation gives normal or balanced gametes; adjacent segregation gives unbalanced ones.
• A Robertsonian translocation fuses the long arms of two acrocentric chromosomes (in humans 13, 14, 15, 21 and 22), losing their short arms. Those short arms carry mainly repeated ribosomal RNA genes, with copies on the other acrocentrics, so a carrier with 45 chromosomes is usually unaffected. If a carrier passes on the fused chromosome plus a free 21, the child has translocation Down syndrome with 46 chromosomes.

The Philadelphia chromosome is a reciprocal translocation in blood cells, acquired rather than inherited. Nowell and Hungerford saw the small chromosome in 1960. Rowley showed in 1973 that its missing piece had moved to chromosome 9. In the 1980s, other laboratories found that the swap joins two genes, BCR and ABL, and the fusion protein is a tyrosine kinase that stays switched on (see the lesson on oncogenes).

Seeing it: from karyotype to microarray

A karyotype is a picture of a cell’s chromosomes. Banded with trypsin and Giemsa, it shows number and large changes, about 5 to 10 million base pairs in routine work, and it is the one routine test that sees balanced rearrangements directly. FISH uses a fluorescent probe for one region, about 100 thousand base pairs, and works in nuclei that are not dividing, though it reports only on the probe’s target. A chromosomal microarray measures the dose of segments across the genome, down to about 20 to 200 thousand base pairs depending on the array, but it is blind to balanced changes, and a dose-only array can miss triploidy. In one large prenatal study, microarray found an extra clinically relevant deletion or duplication in 6.0 percent of karyotypically normal samples from pregnancies with a structural anomaly. Sequencing can place breakpoints to the base, and it has shown that many rearrangements that look balanced under the microscope break genes or hide small losses.

Design problem

A man with typical features has 45 chromosomes, including a Robertsonian translocation that joins 14 and 21. His child has Down syndrome. Explain why he is unaffected, list the gametes he can make, and say which zygotes you expect to survive with a normal egg. How would you confirm the explanation?