Anand Panambulam, MD
Barry Wershil, MD
Montefiore Medical Center
Albert Einstein College of Medicine
Avrahom Gurwitz, MPH
SUNY-Downstate Medical Center
Pretest (detailed explanations at the bottom):
Q1. True or False: Metabolic disorders in neonates can be diagnosed by its specific signs and symptoms.
Q2. True or false: Unrecognized metabolic disorder can cause lasting neurological damage in a neonate.
Q3. True or false: Newborn screening reports are available to the clinician for all inborn errors of metabolism before a metabolic disorder presents.
Q4. Best response: Metabolic disorders are caused by one of the following mechanisms:
A. Deficient enzyme.
B. Accumulation of substrate
C. Accumulation of the toxic metabolites of the substrate.
D. Deficient end product.
Q5. True or false: Inborn errors of metabolism can present after the first year of life.
On completion of this module, residents and physicians will be able to:
1. Recognize the signs and symptoms of metabolic disorders, especially in a neonate.
2. Understand the metabolic pathway and the mechanisms of disease presentation.
3. Approach to a metabolic disorder and the tests to order.
4. Appreciate principles of neonatal screening and treatment.
1. Chuang DT and Vivian E. Shih. The Metabolic and molecular basis of inherited disease. Edited by Scriver et al McGraw-Hill; 2001 1971-2005
2. Saudubray JM, et al: Clinical approach to inherited metabolic disorders in neonates: An overview. Semin Neonatol 2002; 7: 3-15
3. Scriver C. Human genetics: lessons from Quebec populations.
Annu Rev Genomics Hum Genet. 2001;2:69-101
Inborn errors of metabolism are hereditary disorders in which gene defects cause clinically significant blocks in metabolic pathways. Usually they are inherited as autosomal recessive disorders. Following a "one gene: one protein" theory, the protein affected may be an enzyme, a receptor, a transport vehicle or be membrane-based. These are complex diseases with varied manifestations. Although most of the clinically significant disorders present in the neonates, inborn errors can present at any age, from neonatal age group to adolescence to adulthood.
Because of non-recognition after infancy, these disorders may not be diagnosed or misdiagnosed. Undiagnosed or late diagnosis will result in permanent neurological damage. Although neonatal screening is available for certain - but not all - metabolic disorders, they can present before the screening reports are available to the clinician. Therefore most will remain undiagnosed until clinical signs and symptoms appear. For some metabolic disorders, prenatal diagnosis is possible, either by amniocentesis or by chorionic villi tissue sampling (CVS). However, amniocentesis and CVS are not routinely done due to the risk of fetal demise and is only used when there is a high clinical suspicion during the pregnancy. Most importantly, an accurate diagnosis of all inborn errors is needed for the purposes of genetic counseling and to guide parents future pregnancy planning.
The goal of this chapter is to enhance a physician's ability to recognize a metabolic disorder at an early stage so that appropriate diagnostic tests can be ordered and appropriate treatment started. This will prevent neurological damage, organ failure and help in the normal development of the child. This section focuses on a disorder that occurs in the neonatal period. The case study presented below is on maple syrup urine disease (branched chain ketoaciduria). This will generally serve as a model for inborn errors of intermediary metabolism.
Billy is an 8-day-old male child born to a 26-year-old mother, presenting with feeding difficulties. He was born full-term, with a normal vaginal delivery. The Apgar score was 8 at 1 minute and 9 at 5 minutes. The physical examination was normal at birth. Billy was started on breast feedings and he tolerated the feedings well. On the 8th day of life, Billy began to suck poorly and he was lethargic. He was brought to the family pediatrician who's first thought was sepsis. Billy was admitted to the hospital and full sepsis work up was done. As part of his work-up he got blood, urine and CSF cultures and was started on antibiotics. He was continued on breast feeds for nutrition. However, his condition worsened, with increasing lethargy. By the second day he began to have seizures.
On examination the child had lost more weight than expected. His limbs were hypertonic, and the diapers had a peculiar smell that the pediatrician recognized as maple syrup. A metabolic disorder of branched chain aminoacidemia (known as "maple syrup urine disease") was considered. Breast-feeding was stopped, and appropriate tests were ordered.
Q1. What in a family history suggest an increased risk for an inborn error of metabolism?
A1. The first question is always, "Has an earlier born child been affected by a known or unknown illness with a similar history?" and "Has there been an unexplained neonatal death in the family?" On further history taking, Billy's family reported that an earlier infant had died in the newborn period with a presumed diagnosis of sepsis. An older brother, Jack, is healthy and has no medical problems whatsoever.
The second concern is whether there is consanguinity (due to the recessive gene effects) or a "colony effect" due to the parents coming from a closely related group. In, Billy's case there was no history of consanguinity, but the families do come from a similar Amish Mennonite background. Asking about ethnicity is important since certain diseases occur at an increased rate in different groups. For example Maple Serum Urine Disease (MSUD) is a defect in the de-branching of branched chain amino acids. MSUD occurs at a frequency of 1 in 185,000 in the general population compared to 1 in 176 in the Mennonites of Lancaster County in Pennsylvania. This is due to the "colony effect", small groups keep marrying one another without identifiable consanguinity.
We can use the Hardy-Weinberg equation
[p + q = 1 and p² + 2pr + q² = 1]
whereby "p" is the diseased gene frequency, "p²" is the disease frequency, and "2pq" is the carrier rate. (Since "q," the normal gene frequency can be assumed to be "1" then q² is also "1" and 2pq is simply twice the gene frequency).
A simple technique: Always leave the given information, either p², the disease frequency, or 2p, the carrier rate, in the form of a fraction. This allows us to use the disease frequency (p²) to calculate the gene frequency (p) and the carrier rate (2p). For example, the disease frequency (p²) for Cystic Fibrosis (CF) in European ancestry populations is 1/1,600. The gene frequency (p) is 1/40 (square root of disease frequency) the gene frequency of the normal European ancestry (q) population is 39/40 (essentially 1). Therefore, the carrier rate (2pq) is 2 x 1/40 x 1 = 1/20. With such a high carrier rate there is a very high probability of two people of Europeans ancestry, both having CF (1/20 x 1/20 = 1/400), having a child with CF.
Q2: Billy's parents are Mennonites and Billy has an older brother, Jack, who does not have MSUD. If Jack was to marry a Mennonite woman, what is the probability that their child would have MSUD? (Hint: Use the Table 1 and the Hardy-Weinberg equation.)
A2. Since Billy has MSUD and neither his parent's have the disease, the parents must be heterozygous carriers. As MSUD is an autosomal recessive disease, before Jack was born he had a ¼ chance of inheriting two bad genes, ½ chance of inheriting a good and bad gene (carrier), and a ¼ chance of inheriting both good genes. However, Jack is now healthy and therefore cannot have both bad genes. He now has a 2/3 chance of having one good and one bad gene (carrier) and 1/3 chance of having two good genes. The carrier rate for MSUD in the Mennonite population is 1/15. Therefore, the probability that Jack is a carrier for MSUD is 2/3 and the probability that the Mennonite girl he is about to marry is a carrier for MSUD is 1/15. This makes the probability that they both are carriers for MSUD 2/3 x 1/15 = 2/45. The probability that two carriers of an autosomal recessive will have a child with the disease is ¼. Therefore the probability that Jack and his wife, if they are carriers, will have a child with MSUD is also ¼. This makes the probability of Jack marrying an Amish girl and having a child with MSUD 2/45 x ¼ = 1/90.
If he were to marry a non-Mennonite woman, the calculations for her risk would be for MSUD in the general population [2/3 x ¼ x 1/215 = 1/1290]
In summary, you can calculate the likelihood of a sibling without identifiable disease marrying a carrier of the disease and both being affected as 2/3 X 2pq (carrier rate). Each of their offspring has a ¼ chance of being affected by disease. Thus for CF, a disease with a 1/1,600 disease frequency (p2), the carrier rate of 2pq = 1/20. An unaffected white sib of a person with CF marrying another unrelated white person would have a 1/30 chance of a match with a heterozygotic carrier and a 1/120 chance of having an affected child. Once the match has been established, however, the probability remains ¼.
The Case Continues
Billy's complete blood count, serum glucose, electrolytes, bicarbonate, calcium, blood gas analysis and blood ammonia level were all normal. Blood, urine and cerebrospinal fluid did not grow any organisms. His head CT scan showed generalized edema.
His urine was positive for ketoacids and serum levels for branched-chain amino acids were elevated. He was diagnosed with branched chain amino academia - (MSUD) and appropriate treatment was started.
Q3: What are the pathways of a substrate breakdown and how metabolic diseases arise?
A2: The common element of an inborn error is failure in production of an enzyme structure - one gene/one protein. When one enzyme is missing (heterozygous carrier) the other enzyme is adequate to cover the function and there is no disease. When both enzymes are missing (homozygous) there is no function of that enzyme and disease becomes clinically apparent.
A substrate is transported with the help of specific receptors through the membrane into the cytosol or the mitochondria of the cell. There, it is broken down by the enzymes into an end product. In some of the reactions, vitamins act as co-enzymes. As mentioned previously, there can be abnormalities at different steps (see Figure 1). The cause for the disorder is almost always the deficient enzyme. Many consequences follow.
For example, when there is a deficiency of the enzyme at step 3, then the following can occur:
1. Accumulation of the substrate or toxic intermediaries. These may be byproducts of accumulated substrate metabolism.
2. Deficiency of the end product.
3. Repair can be affected by inducing enzyme activity or
4. Using alternative pathways
This phenomenon can be illustrated by considering a blockage in the function of a dam (see Figure 2).
Figure 2: Figure of girl fishing on side of dam.
CAPTION: The blocked dam leads to accumulation of substrate and potentially toxic metabolites (#1). There is an inadequate supply of product downstream (#2). One can "lubricate" the dam mechanism (#3). For example in branched chain aminoacidemia (MSUD) some (about 30% of cases) respond to Thiamin. One can dig an alternative pathway from upstream to downstream (#4). For example, adding argenine in urea cycle abnormalities.
Q4: What are the causes the clinical of the presentation?
A4: The symptoms may arise due to the accumulation of the substrates, toxic metabolites or due to deficiency of the end product. The clinical picture also depends on the penetrance of the disease. The enzyme activity in MSUD can vary from less than 2% when the disease is severe to about 40% when the disease can be intermediate or intermittent brought about by stress such as infection. In severe cases MSUD can present as an encephalopathy.
Accumulation of the substrate: In MSUD the substrates are the branched-chain amino acids leucine, isoleucine and valine. These are essential amino acids which cannot be synthesized by the body and constitute about 35% of essential amino acids in the muscle. In Figure 3, steps 1 and 2 are the common pathways in the metabolism of these amino acids. In MSUD there is a block at step 2 with resultant accumulation of BCCA and its keto-acids. Thiamine acts as a co-enzyme at step 2. The accumulating substrate is the BCAA. Of the three substrates, increased leucine is most toxic and is associated with most symptoms including the neurological symptoms. Increased isoleucine leads to maple syrup odor, whereas increased valine does not result in any clinical symptomatology.
Accumulation of toxic intermediaries: As described above a block in step 2 results in the accumulation of the ketoacids, which are not normally detected in the body. These are toxic, especially the keto-acid leucine. These a-ketoacids are excreted in the urine and can be detected by the dinitrophenylhydrazine (DNPH) test. Both leucine and its keto-acid readily cross the blood brain barrier and their concentration is increased in the brain. They alter the neurotransmitter pathways with resultant encephalopathy.
Deficiency of the end product: In MSUD, the end product is acyl-CoA. Since numerous pathways produce it, there is no deficiency. This is therefore not a cause of any symptoms and hence need not be supplemented. This is an example of the body having redundant pathways and the failure of one pathway does not cause a shortage of end product. In some endocrine disorders, enzyme deficiency can lead to the deficient hormone production, which needs to be supplemented.
For example, in another inborn error, congenital adrenal hyperplasia due to 21-hydroxylase deficiency, the end products cortisol and aldosterone are deficient, resulting in severe salt wasting. The substrate prior to the block is shunted into androgen production resulting in virilization. Here it is necessary to replace the end products. This also functions to limit the shunting into the androgen pathway.
Q5: When do in born errors of metabolism present and what are the signs and symptoms?
A5: Inborn errors of metabolism can present with multitude of signs and symptoms. Metabolic disorders occurs more frequently in consanguineous marriages and can present anywhere from birth to adolescent period to adulthood depending on the type of metabolic defect and the penetrance. Also, the disease can occur due to new mutations. There are many different types of presentation. It can present as an acute episode, late-onset intermittent episode or as a chronic progressive disorder.
It is important to emphasize that the early sings and symptoms of a metabolic disorder in the neonatal period are nonspecific. The earliest symptoms are indistinguishable from sepsis, such as poor feeding and lethargy. Acute encephalopathy is the most common presentation of aminoacidopathies, organic acidurias, urea cycle defects and galactosemia. It is associated with high anion gap acidosis, ketosis or hyperammonemia.
Mitochondrial disorders and fatty acid oxidation defects can have symptoms both due to accumulation of toxic metabolites and deficient energy affecting the heart, brain and liver.
MSUD usually presents at one week of age. The child is normal at birth and symptoms develop 4-7 days later. It is noteworthy to realize that the disease presents before the results of the newborn screening results are available. Breast-feeding may delay the symptoms to the second week. The most common presentations are lethargy and poor feeding. This is followed by neurological signs such as hypertonia, dystonia and if left untreated seizures and coma. There is abnormal odor of the urine, that of maple syrup.
Q6: How do you approach a suspected metabolic disorder?
A6: The single most important key to diagnosis is the clinician suspecting a metabolic disorder. Without clinical suspicion the diagnosis can be easily missed. Routine blood tests may reveal evidence of acidosis and hypoglycemia. Although sepsis can produce these symptoms, a persistent acidosis (especially with a high anion gap), a lack of response to antibiotics or a clinical deterioration should lead to a suspicion of a metabolic disorder and the following basic investigations should be instituted (see Table 2).
Ketosis in the presence of metabolic acidosis is a strong clue to the presence of inborn errors of metabolism and is almost pathognomonic. Absence of ketonuria in a hypoglycemic infant should raise the suspicion of fatty acid oxidation defects. Ammonia is elevated in urea cycle disorders. When the suspicion of metabolic disorder is confirmed, more tests can be ordered to identify the disorder. Tests such as plasma and urine amino acid levels and urine organic acid analysis should be collected before stopping the feeds. Enzyme assays on cultured leukocytes or fibroblasts confirms the diagnosis. DNA probes are also available for molecular diagnosis.
*MCAD: Medium-chain acyl-CoA dehydrogenase deficiency
Of note, while collecting samples for ammonia, lactate and pyruvate, it is important that tourniquet should not be applied and the samples are transported immediately on ice.
Q7: What are the disorders that can be diagnosed with newborn screening?
A7: Each state in the United States has its own list depending on the prevalence of the disorder, the availability and sensitivity of the test, and whether some benefit is to be had from providing the screening early. The cost/benefit ratio is calculated independently for each state and the question they try to answer is that is testing of every infant cost effective with respect to the cost of missing the diagnosis on a relatively small number of affected individuals. New York State tests for phenylketonuria, congenital hypothyroidism, congenital adrenal hyperplasia, galactosemia, sickle cell disease, maple syrup urine disease, homocystinuria, biotinidase deficiency, cystic fibrosis, medium chain acyl-CoA dehydrogenase deficiency (MCAD), and HIV. As of 2002, the list of testable inborn errors has become too long to list. There are over 40 items and the list grows longer each year.
Q8: What are the principles of treatment of inborn errors of metabolism?
A8: The metabolic disorders can be treated in a number of ways, including a combination of the following:
1. Avoiding or decreasing the substrate. This will limit the accumulation of the substrate and its intermediary metabolites. However, if the substrate is essential, like BCAA, then this is not an option.
2. Removing the toxic accumulates, especially in the acute stage.
3. Adding a co-enzyme in pharmacological doses. This helps in certain types of disorders. Thiamine, for example stimulates the debrancher enzyme in approximately 1/3 of children with MSUD.
4. Replacement of the enzyme by liver transplantation or by gene therapy. This is the definite therapy and is curative. However, we are a long way from having this as routine therapy.
5. Supplementing the deficient end product if it is essential for the body. This may also inhibit the enzyme and prevent the formation of by products. Attempts have been made to create feed-back loops suppressing substrate formation in several diseases including smith-lemli-opitz syndrome. This is a disease of cholesterol metabolism and attempts at supplementing the end-product (cholesterol) to suppress the substrate (7DHC) have failed.
6. By-pass circuits can be stimulated. Hippurate and benzoate have been used in urea cycle defects for this purpose
The following treatment steps are taken with MSUD:
1. Since the branched chain amino acids are essential, they have to be provided in the diet. Therefore in MSUD it is important to provide only the daily requirements of leucine, isoleucine and valine, once the acute stage is over and to avoid excessive amounts. The treatment is life long, unless the enzyme deficiency is corrected by gene therapy.
2. During the acute stage, the toxic intermediaries, which are the respective ketoacids, should be removed by insulin and glucose infusion. In rare case dialysis may be necessary.
3. Co-enzyme B1 (thiamine) responsive MSUD: In this condition (about 33% of cases) the disease does not present in the neonatal period, but later in infancy. Usually the child is evaluated for developmental delay and mental retardation. The enzyme activity ranges from 3 to 30%. They respond to a combination of thiamine with a dose range of 10 mg to 1000 mg and moderate dietary restriction.
Monitoring: The dietary restriction is life long and monitoring of the BCAA is important. The key is to maintain the levels of BCAA within the normal range. The treatment should be individualized and the amount of restriction depends on the BCAA levels. Initially the levels should be monitored every week until the child is 6-12 months and then can be extended. The disease can exacerbate anytime in life due to stresses such as infection, but it becomes less likely as the child becomes older.
The Case Continues
Billy is immediately given an infusion of insulin and glucose and sent to be dialyzed. He is also immediately put on a BCAA restricted diet. He is started on 100 mg of thiamine daily. He rapidly improves. He is discharged on hospital day 20 with instructions to the parents on his disease and the lifelong dietary modifications and thiamine requirement.
Ask the residents to use the case to reiterate the learning objectives.
1. Recognize the signs and symptoms of metabolic disorders, especially in a neonate:
Billy presented with feeding difficulties, lethargy, and seizures. These were the non-specific findings and these signs mimic sepsis. The physician kept a high suspicion for the diagnosis of inborn error of metabolism which became more evident when Billy did not improve on antibiotics. The million-dollar clue was the maple syrup smell in the urine. However, without keeping high clinical suspicion that sign might be missed.
2. Understand the metabolic pathway and the mechanisms of disease presentation:
Metabolic pathways are a series of steps in which compounds are changed in our bodies to either become active or inactive. Inborn errors of metabolism are a deficiency of one of these steps in which substrate and toxic by-products accumulate and end-product is not made. In Billy's case he was missing the enzyme a-keto acid dehydrogenase and there was a buildup of BCAA (substrate) and ketoacids (toxic by-products). The toxic ketoacids were causing clinical symptoms.
3. Approach to a metabolic disorder and the tests to order:
Never forget the ABC's; all patients must first be stabilized regardless of the cause. Once stabilized, first rule out common causes such as sepsis and do routine labs. This was done in Billy's case and they all came back normal. This helped steer the physicians away from the original diagnosis of sepsis and to do more focused tests for an inborn error of metabolism. When they sent blood for BCAA and ketoacids levels and it was elevated the diagnosis was confirmed. Different diseases will require different tests.
4. Appreciate principles of neonatal screening and treatment:
Neonatal screenings allow us to pick up a disease before it manifests clinically and to provide early treatment so that permanent damage does not occur. In Billy's case the neonatal screening test results did not return yet. Billy had a severe form (very low enzyme activity) and had clinical symptoms right away. This allowed the clinician to pick up the disease and treat it right away. Had Billy had a less severe form of the disease he would have gone home and had slow build up of toxic by-products would lead to permanent neurological defects. This is where screening would save Billy. After about two to three weeks the physician would get the results of the newborn screening test for Billy and it would show decreased activity in the enzyme. Billy's physician could then immediately treat Billy and permanent damage could be averted.
Billy is now 4 years old. He is reaching all his developmental milestones and is quite healthy.
Q9: It seems Billy is doing well. What can we expect?
A9: Billy will need to keep following up with a biochemical geneticist familiar with the management of MSUD at regular intervals (at least once every 6-12 mo). He should avoid consuming branched-chain amino acids above his daily allowance. Billy is also at risk for metabolic decompensation during periods of increased catabolism, such as stress and infection. Most importantly, as Billy gets older he needs to be educated about the principles of dietary treatment, calculation of his leucine requirement, proper dosage of his thiamine, and his intake and initial management of acute episodes. He should carry with him at all times a written emergency regimen and emergency card stating his disease. Careful dietary compliance is necessary to prevent developmental delay and neurological symptoms. If Billy and his parents keep with his regimen, Billy has an excellent long-term prognosis.
References and recommended reading:
1. Chuang DT and Vivian E. Shih. The Metabolic and molecular basis of inherited disease. Edited by Scriver et al McGraw-Hill; 2001 1971-2005
2. Saudubray JM, et al. Clinical approach to inherited metabolic disorders in neonates: an overview. Semin Neonatol 2002; 7: 3-15
Post 1: True or false. Clinical suspicion is the most important first step in the diagnosis of inborn errors of metabolism.
Post 2: True or false. The presentation of inborn errors of metabolism in a neonate can be indistinguishable from that of sepsis.
Post 3: True or false. Use of vitamins in large doses may be useful in the treatment.
Post 4: True or false. When ketosis is associated with metabolic acidosis, it is unlikely to be due to a metabolic disorder.
Post 5: Choose the best response. In the treatment of metabolic disorder:
a. The substrate should be restricted and the levels should be monitored to have a good result.
b. Treatment can be weaned and then stopped once the child is normal
c. Metabolic disorders cannot be cured.
Post 6: True or false. Metabolic disorders always present at birth or in the first year of life
Annotated Pre-test Answers
A1. False: Unfortunately, there are minimal signatures to in-born errors. Some have characteristic smells or findings on physical examination, but for the most part, signs and symptoms are similar to other forms of childhood illness.
A2. True: One reason for the new born screening for diseases like PKU is that unrecognized this genetic defect produces a severely handicapped adult. Therapy early on in life will prevent most of the sequelae.
A3. False: Not at present, but "stay tuned in!" The science of in born errors is progressing rapidly and new methods for early detection are being devised. Currently, it takes about two weeks before the newborn screening report is available to the clinician, and by then the damage may be irreversible. Also, not every type of error can be detected. High clinical suspicion must always be present.
A4. The answer is A: For the most part, inborn errors reflect lost of a structural or operator gene for a single enzyme. The other findings listed are a consequence of the enzyme defect.
A5. True: Many of the disorders have variable penetrance and may present with varying severities. As such, they can present in adult life with a much different clinical picture. Also, dietary self-modifications early on in life may mask a disorder until adulthood. For example, a child with hereditary fructose intolerance will learn quickly that taking in sweet foods is likely to cause hypoglycemia and will avoid them. A proper work-up for hepatomegaly in a child would include looking for this disorder carefully. A fructose tolerance test will send the blood sugar to zero. Be prepared with a glucose infusion in hand.