Recessive disease

In patients with ataxia-telangiectasia, an autosomal recessive disease in humans resulting in the development of cerebellar ataxia and lymphoreticular neoplasms, there appears to exist an increased sensitivity to damage by x-ray. Patients with Fanconi s anemia, an autosomal recessive anemia characterized also by an increased frequency of cancer and by chromosomal instability, probably have defective repair of cross-linking damage. 

Hereditary hemochromatosis is an autosomal recessive disease of increased intestinal iron absorption and deposition in hepatic, cardiac, and pancreatic tissue. Hepatic iron overload results in the development of fibrosis, hepatic scarring, cirrhosis, and hepatocellular carcinoma. Hemochromatosis can also be caused by repeated blood transfusions, but this mechanism rarely leads to cirrhosis. 

Wilson s disease is another autosomal recessive disease leading to cirrhosis. Protein abnormalities result in excessive copper deposition in body tissues. The faulty protein is responsible for facilitating copper excretion in the bile, so copper accumulates in hepatic tissue. High copper levels within hepatocytes are toxic, and fibrosis and cirrhosis may develop in untreated patients. Those with Wilson s disease usually present with symptoms of liver or neurologic disease while still in their teens. 

MF effects on FA relatives and healthy donors. (Fanconi anemia is an autosomal recessive disease associated with the overproduction of free radicals, Chapter 31.) It has been shown earlier [215] that FA leukocytes produce the enhanced amount of hydroxyl or hydroxyl-like free radicals, which are probably formed by the Fenton reaction. It was suggested that MF would be able to accelerate hydroxyl radical production by FA leukocytes. Indeed, we found that MF significantly enhanced luminol-amplified CL produced by non-stimulated and PMA-stimulated FA leukocytes but did not affect at all oxygen radical production by leukocytes from FA relatives and healthy donors (Table 21.3). It is interesting that MF did not also affect the calcium ionophore A23187-stimulated CL by FA leukocytes, indicating the absence of the calcium-mediated mechanism of MF activity, at least for FA leukocytes. 

Cystic fibrosis is the most common lethal autosomal-recessive disease, in which oxidative stress takes place at the airway surface [274]. This disease is characterized by chronic infection and inflammation. Enhanced free radical formation in cystic fibrosis has been shown as early as 1989 [275] and was confirmed in many following studies (see references in Ref. [274]). Contemporary studies also confirm the importance of oxidative stress in the development of cystic fibrosis. Ciabattoni et al. [276] demonstrated the enhanced in vivo lipid peroxidation and platelet activation in this disease. These authors found that urinary excretion of the products of nonenzymatic lipid peroxidation PGF2 and TXB2 was significantly higher in cystic fibrotic patients than in control subjects. It is of importance that vitamin E supplementation resulted in the reduction of the levels of these products of peroxidation. Exhaled ethane, a noninvasive marker of oxidative stress, has also been shown to increase in cystic fibrosis patients [277]. 

Phosphoglycerate kinase deficiency is an X-linked recessive disease (type IX, Fig. 42-1). The most common clinical presentation includes hemolytic anemia with or without CNS involvement (see below). Thus far, ten patients have been described with a purely myopathic... 

This is a rare, autosomal recessive disease that was first described when giant cytoplasmic granules were observed in neutrophils, monocytes and... 

Cystic fibrosis (CF) is the most common potentially lethal autosomal recessive disease among Caucasians. The incidence is estimated to be approximately 1 in 2000 births (Bl). Since it is inherited as an autosomal recessive condition, screening to identify couples at risk has been suggested. However, CF screening is complicated because many mutations of the CF gene exist. Thus, the feasibility of screening a population for carriers of cystic fibrosis gene mutations is primarily dependent upon the frequency of the common mutation in that population. 

Answer C. CP is an autosomal recessive disease. Choices A, B, and D best illustrate AD, XR and mitochondrial pedigrees, respectively. 

In contrast to autosomal dominant diseases, autosomal recessive diseases are typically seen in only one generation of a pedigree (Fig II-1-5). 

Consanguinity (the mating of related individuals) is sometimes seen in recessive pedigrees because individuals who share common ancestors are more likely to carry the same recessive disease-causing alleles. 

Because males have only one copy of the X chromosome, they are said to be hemizygous (hemi = half ) for the X chromosome. If a recessive disease-causing mutation occurs on the X chromosome, a male will be affected with the disease. 

Because males require only one copy of the mutation to express the disease, whereas females require two copies, X-Iinked recessive diseases are seen much more commonly in males than in... 

Figure II-1-7 shows the recurrence risks for X-linked recessive disease.

Figure 11-1-7. Recurrence Risks for X-Linked Recessive Diseases...

Environmental influences (e.g., the autosomal recessive disease xeroderma pigmento-siun will be expressed more severely in individuals who are exposed more frequently to ultraviolet radiation)... 

The Hardy-Weinberg principle can be applied to estimate the prevalence of heterozygous carriers in populations when we know only the prevalence of the recessive disease. 

For autosomal recessive diseases, such as PKU, the prevalence of heterozygous carriers is much higher than the prevalence of affected homozygotes. In effect, the vast majority of recessive genes are hidden in the heterozygotes. 

Because males have only one X chromosome, gene frequency estimation for X-Hnked traits differs from that of autosomal traits. Consider hemophilia A (Chapter 1), which is an X-linked recessive disease. If a male s X chromosome has a factor VIII mutation, he will have hemophilia A. If his X chromosome does not have the mutation, he will not develop the disease. Thus, the gene frequency for hemophilia A is obtained simply by counting the proportion of affected males in the population (i.e., the proportion of X chromosomes containing the mutation). Approximately one in 10,000 males has hemophilia A. Thus, the gene frequency for this disease, q, is 1/10,000. 

C. The couple has an increased risk of producing a child with an autosomal recessive disease. 

A man is a known heterozygous carrier of a mutation that causes hemochromatosis (autosomal recessive disease). Suppose that 1% of the general population consists of homozygotes for this mutation. If the man mates with somebody from the general population, what is the probability that he and his mate will produce a child who is an affected homozygote ... 

Answer C. Because the couple shares conunon ancestors (i.e., one set of grandparents), they are more likely to be heterozygous carriers of the same autosomal recessive disease-causing mutations. Thus, their risk of producing a child with an autosomal recessive disease is elevated above that of the general population. 

Consanguinity (choice A) could elevate the incidence of this autosomal recessive disease in a specific family, but it does not account for the elevated incidence of this specific disease in the African American population in general. 

Answer A. Because males have only a single X chromosome, each affected male has one copy of the disease-causing recessive mutation. Thus, the incidence of an X-linked recessive disease in the male portion of a population is a direct estimate of the gene frequency in the population. 

Individuals with a family history of an autosomal or X-linked recessive disease may wish to know whether they are a heterozygous carrier of the disease. This can be established by genetic diagnosis (e.g., for cystic fibrosis, hemochromatosis, PKU, or albinism). In some specific cases, a population at high risk for a specific disease may be screened for carrier status using genetic diagnosis (e.g., Tay-Sachs disease in the Jewish population [see Clinical Correlate]). 

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