Biochemical genetics deals not only with enzymatic defects but also with proteins of all functions, including cytoskeletal and extracellular structure, regulation, and signaling. The principal functions of the biochemical genetics laboratory are to determine the presence or absence of proteins, to assess the qualitative characteristics of proteins, to assay metabolites, and to verify the effectiveness of proteins in vitro. The key elements from the referring clinician's perspective are (1) to indicate what the suspected clinical diagnoses are and (2) to make certain that the proper specimen is obtained and transported to the laboratory in a timely manner.
INDICATIONS FOR BIOCHEMICAL INVESTIGATIONS
Some inborn errors are relatively common in the general population, eg, hemochromatosis, defects of the low-density lipoprotein receptor, and cystic fibrosis (eTable 40–3). Others, although rare across the entire population, are common in certain ethnic groups, such as Tay-Sachs disease in Ashkenazic Jews, sickle cell disease in people of African origin, and thalassemias in populations from around the Mediterranean basin and Asia. Many of these disorders are autosomal recessive, and the frequency of heterozygotes is many times that of the fully expressed disease. Screening for carrier status can be effective if certain requirements are satisfied (eTable 40–4). For example, all of the United States and the District of Columbia require screening of newborns for phenylketonuria and often other metabolic diseases. Such programs are cost-effective even for rare conditions such as phenylketonuria, which occurs in only one of every 11,000 births. Unfortunately, not all disorders that meet the requirements in eTable 40–4 are screened for in every state. Furthermore, compliance is highly variable among programs, and follow-up diagnostic tests, management, and counseling are in some cases inadequate. Babies most likely to be missed are those born at home and those discharged before they have digested much milk or formula. In some states, parents can refuse to have their infants studied. Several commercial laboratories have marketed screening for over 35 inborn errors of metabolism to hospitals. This supplemental newborn screening involves tandem mass spectroscopic analysis of the same blood spots used in state-mandated programs.
eTable 40–3.Representative inborn errors of metabolism. ||Download (.pdf) eTable 40–3. Representative inborn errors of metabolism.
|General Class of Defect ||Example ||Biochemical Defect ||Inheritance |
|Aminoacidopathy ||Phenylketonuria ||Phenylalanine hydroxylase ||AR |
|Connective tissue ||Osteogenesis imperfecta type II ||Alpha-1(I) and alpha-2(I) procollagen ||AD |
|Gangliosidosis ||Tay-Sachs disease ||Hexosaminidase A ||AR |
|Glycogen storage disease ||Type I ||Glucose-6-phosphatase ||AR |
|Immune function ||Chronic granulomatous disease ||Cytochrome b, beta-chain ||XL |
|Lipid metabolism ||Familial hypercholesterolemia ||Low-density lipoprotein receptor ||AD |
|Mucopolysaccharidosis (MPS) ||MPS II (Hunter syndrome) ||Iduronate sulfatase ||XL |
|Porphyria ||Acute intermittent ||Porphobilinogen deaminase ||AD |
|Transport ||Cystic fibrosis (CF) ||CF transmembrane conductance regulator ||AR |
|Urea cycle ||Citrullinemia ||Arginosuccinate synthetase ||AR |
eTable 40–4.Requirements for effective population screening for inborn errors of metabolism.